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military and commercial temperature ranges may 1997 1997 integrated device technology, inc dsc-3037/7 1 integrated device technology, inc. syncfifo is a trademark and the idt logo is a registered trademark of integrated device technology, inc. functional block diagram input register output register ram array 8,192 x 18 16,384 x 18 flag logic ff / ir paf ef / or pae hf read pointer read control logic write control logic write pointer reset logic wen wclk d 0 -d 17 ld mrs ren rclk oe q 0 -q 17 ? ? ? ? timing fs offset register ? prs fwft/si sen rt 3037 drw 01 idt72255 idt72265 cmos supersync fifo ? 8,192 x 18, 16,384 x 18 features: 8,192 x 18-bit storage capacity (idt72255) 16,384 x 18-bit storage capacity (idt72265) 10ns read/write cycle time (8ns access time) retransmit capability auto power down reduces power consumption master reset clears entire fifo, partial reset clears data, but retains programmable settings empty, full and half-full flags signal fifo status programmable almost empty and almost full flags, each flag can default to one of two preselected offsets program partial flags by either serial or parallel means select idt standard timing (using ef and ff flags) or first word fall through timing (using or and ir flags) ? easily expandable in depth and width ? independent read and write clocks (permit simultaneous reading and writing with one clock signal) ? available in the 64-pin thin quad flat pack (tqfp), 64- pin slim thin quad flat pack (stqfp) and the 68-pin pin grid array (pga) ? output enable puts data outputs into high impedance ? high-performance submicron cmos technology ? industrial temperature range (-40 o c to +85 o c) is avail- able, tested to military electrical specifications description: the idt72255/72265 are monolithic, cmos, high capac- ity, high speed, low power first-in, first-out (fifo) memories with clocked read and write controls. these fifos are appli- cable for a wide variety of data buffering needs, such as optical disk controllers, local area networks (lans), and inter-proces- sor communication. both fifos have an 18-bit input port (d n ) and an 18-bit output port (q n ). the input port is controlled by a free-running clock (wclk) and a data input enable pin ( wen ). data is written into the synchronous fifo on every clock when wen is asserted. the output port is controlled by another clock pin (rclk) and enable pin ( ren ). the read clock can be tied to the write clock for single clock operation or the two clocks can run asynchronously for dual clock operation. an output enable pin ( oe ) is provided on the read port for three-state control of the outputs. the idt72255/72265 have two modes of operation: in the idt standard mode , the first word written to the fifo is deposited into the memory array. a read operation is required to access that word. in the first word fall through mode (fwft), the first word written to an empty fifo appears for latest information contact idt's web site at www.idt.com or fax-on-demand at 408-492-8391.
2 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 automatically on the outputs, no read operation required. the state of the fwft/si pin during master reset determines the mode in use. the idt72255/72265 fifos have five flag functions, ef / or (empty flag or output ready), ff / ir (full flag or input ready), and hf (half-full flag). the ef and ff functions are selected in the idt standard mode. the ir and or functions are selected in the first word fall through mode. ir indicates that the fifo has free space to receive data. or indicates that data contained in the fifo is available for reading. hf is a flag whose threshold is fixed at the half-way point in memory. this flag can always be used irrespective of mode. pae , paf can be programmed independantly to any point in memory. they, also, can be used irrespective of mode. programmable offsets determine the flag threshold and can be loaded by two methods: parallel or serial. two default offset settings are also provided, such that pae can be set at 127 or 1023 locations from the empty boundary and the paf threshold can be set at 127 or 1023 locations from the full boundary. all these choices are made with ld during master reset . in the serial method, sen together with ld are used to load the offset registers via the serial input (si). in the parallel method, wen together with ld can be used to load the offset registers via d n . ren together with ld can be used to read the offsets in parallel from q n regardless of whether serial or parallel offset loading is selected. during master reset ( mrs ), the read and write pointers are set to the first location of the fifo. the fwft line selects idt standard mode or fwft mode. the ld pin selects one of two partial flag default settings (127 or 1023) and, also, serial or parallel programming. the flags are updated accordingly. the partial reset ( prs ) also sets the read and write pointers to the first location of the memory. however, the mode setting, programming method, and partial flag offsets are not altered. the flags are updated accordingly. prs is useful for resetting a device in mid-operation, when repro- tqfp (pn64-1, order code: pf) stqfp (pp64-1, order code: tf) top view pin 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 fs v cc gnd d17 d16 d15 d14 d13 d12 d11 d10 d9 d8 d7 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 q17 q16 gnd q15 q14 v cc q13 q12 q11 gnd q10 q9 q8 q7 q6 gnd wclk fwft/si gnd / v cc / rclk q5 q4 v cc q3 q2 gnd q1 q0 gnd d0 d1 d2 d3 d4 d5 d6 3037 drw 02 pin configurations
3 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges gramming offset registers may not be convenient. the retransmit function allows the read pointer to be reset to the first location in the ram array. it is synchronized to rclk when rt is low. this feature is convenient for sending the same data more than once. if, at any time, the fifo is not actively performing a function, the chip will automatically power down. this occurs if neither a read nor a write occurs within 10 cycles of the faster clock, rclk or wclk. during the power down state, supply current consumption (i cc2 ) is at a minimum. initiating any operation (by activating control inputs) will immediately take the device out of the power down state. the idt72255/72265 are depth expandable. the addition of external components is unnecessary. the ir and or functions, together with ren and wen , are used to extend the total fifo memory capacity. the fs line ensures optimal data flow through the fifo. it is tied to gnd if the rclk frequency is higher than the wclk frequency or to vcc if the rclk frequency is lower than the wclk frequency the idt72255/72265 is fabricated using idts high speed submicron cmos technology. pga (g68-1, order code: g) top view notes : 1. dnc = do not connect d 8 pin 1 designator abcdef ghj kl q 0 d 2 paf dnc v cc rclk ren oe gnd dnc mrs ld wclk prs v cc wen d 17 gnd d 15 d 16 d 11 d 14 d 12 d 10 d 9 d 7 d 6 pae d 4 d 3 d 1 d 0 q 1 q 2 gnd q 3 q 4 gnd q 8 q 7 q 10 dnc gnd v cc q 17 q 16 q 15 q 14 q 13 q 12 q 9 hf ef / or ff / ir 11 10 09 08 07 06 05 04 03 02 01 rt fwft/ si d 13 d 5 gnd v cc gnd q 11 q 6 q 5 3037 drw 03 sen fs gnd pin configurations (cont.)
4 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 symbol name i/o description d 0 Cd 17 data inputs i data inputs for a 18-bit bus. mrs master reset i mrs initializes the read and write pointers to zero and sets the output register to all zeroes. during master reset, the fifo is configured for either fwft or idt standard mode, one of two programmable flag default settings, and serial or parallel programming of the offset settings. prs partial reset i prs initializes the read and write pointers to zero and sets the output register to all zeroes. during partial reset,the existing mode (idt or fwft), programming method (serial or parallel), and programmable flag settings are all retained. rt retransmit i allows data to be resent starting with the first location of fifo memory. fwft/si first word fall i during master reset, selects first word fall through or idt standard mode. through/serial in after master reset, this pin functions as a serial input for loading offset registers wclk write clock i when enabled by wen , the rising edge of wclk writes data into the fifo and offsets into the programmable registers. wen write enable i wen enables wclk for writing data into the fifo memory and offset registers. rclk read clock i when enabled by ren , the rising edge of rclk reads data from the fifo memory and offsets from the programmable registers. ren read enable i ren enables rclk for reading data from the fifo memory and offset registers. oe output enable i oe controls the output impedance of q n sen serial enable i sen enables serial loading of programmable flag offsets ld load i during master reset, ld selects one of two partial flag default offsets (127 and 1023) and determines programming method, serial or parallel. after master reset, this pin enables writing to and reading from the offset registers. fs frequency select i the fs setting optimizes data flow through the fifo. ff / ir full flag/ o in the idt standard mode, the ff function is selected. ff indicates whether or input ready not the fifo memory is full. in the fwft mode, the ir function is selected. ir indicates whether or not there is space available for writing to the fifo memory. ef / or empty flag/ o in the idt standard mode, the ef function is selected. ef indicates whether or output ready not the fifo memory is empty. in fwft mode, the or function is selected. or indicates whether or not there is valid data available at the outputs. paf programmable o paf goes high if the number of free locations in the fifo memory is more than almost full flag offset m which is stored in the full offset register. paf goes low if the num- ber of free locations in the fifo memory is less than m. pae programmable o pae goes low if the number of words in the fifo memory is less than offset n almost empty which is stored in theempty offset register. pae goes high if the number of flag words in the fifo memory is greater than offset n. hf half-full flag o hf indicates whether the fifo memory is more or less than half-full. q 0 Cq 17 data outputs o data outputs for a 18-bit bus. v cc power +5 volt power supply pins. gnd ground ground pins. pin description 3037 tbl 01
5 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges recommended dc operating conditions symbol parameter min. typ. max. unit v ccm military supply 4.5 5.0 5.5 v voltage v ccc commercial supply 4.5 5.0 5.5 v voltage gnd supply voltage 0 0 0 v v ih input high voltage 2.0 v commercial v ih input high voltage 2.2 v military v il (1) input low voltage 0.8 v commercial & military note: 3037 tbl 03 1. 1.5v undershoots are allowed for 10ns once per cycle. absolute maximum ratings (1) symbol rating commercial military unit v term terminal voltage C0.5 to +7.0 C0.5 to +7.0 v with respect to gnd t a operating 0 to +70 C55 to +125 c temperature t bias temperature under C55 to +125 C65 to +135 c bias t stg storage C55 to +125 C65 to +155 c temperature i out dc output current 50 50 ma note: 3037 tbl 02 1. stresses greater than those listed under absolute maximum rat- ings may cause permanent damage to the device. this is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. exposure to absolute maimum rating conditions for extended periods may affect reliabilty. dt72255l idt72255l idt72265l idt72265l commercial military t clk = 10, 12,15, 20ns t clk = 15, 25ns symbol parameter min. typ. max. min. typ. max. unit i li (1) input leakage current (any input) C1 1 C10 10 m a i lo (2) output leakage current C10 10 C10 10 m a v oh output logic 1 voltage, i oh = C2 ma 2.4 2.4 v v ol output logic 0 voltage, i ol = 8 ma 0.4 0.4 v i cc1 (3) active power supply current 180 250 ma i cc2 (3,4) power down current (all inputs = v cc - 0.2v or 15 25 ma gnd + 0.2v, rclk and wclk are free-running) notes : 1. measurements with 0.4 v in v cc . 2. oe = v ih 3. tested at f = 20 mhz with outputs unloaded. 4. no data written or read for more than 10 cycles dc electrical characteristics (commercial: v cc = 5v 10%, t a = 0 c to +70 c; military: v cc = 5v 10%, t a = C55 c to +125 c) notes: 1. with output deselected, ( oe =high). 2. characterized values, not currently tested. symbol parameter (1) conditions max. unit c in (2) input vin = 0v 10 pf capacitance c out (1,2) output vout = 0v 10 pf capacitance 3037 tbl 05 capacitance (t a = +25 c, f = 1.0mhz) 3037 tbl 04
6 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 ac electrical characteristics (1) (commercial: v cc = 5v 10%, t a = 0 c to +70 c; military: v cc = 5v 10%, t a = C55 c to +125 c) 3037 tbl 06 commercial com'l & mil. commercial military 72255l10 72255l12 72255l15 72255l20 72255l25 72265l10 72265l12 72265l15 72265l20 72265l25 symbol parameter min. max. min. max. min. max. min. max. min. max. unit f s clock cycle frequency 100 83.3 66.7 50 40 mhz t a data access time 2 8 2 9 2 10 2 12 3 15 ns t clk clock cycle time 10 12 15 20 25 ns t clkh clock high time 4.5 5 6 8 10 ns t clkl clock low time 4.5 (2) 5 (2) 6 (2) 8 10ns t ds data set-up time 3.5 3.5 4 5 6 ns t dh data hold time 0 0 1 1 1 ns t ens enable set-up time 3.5 3.5 4 5 6 ns t enh enable hold time 0 0 1 1 1 ns t lds load set-up time 3.5 3.5 4 5 6 ns t ldh load hold time 6.5 8.5 10 10 10 ns t rs reset pulse width (3) 10 12 15 20 25 ns t rss reset set-up time 10 12 15 20 25 ns t rsr reset recovery time 10 12 15 20 25 ns t rsf reset to flag and output time 10 12 15 20 25 ns t fwft mode select time 0 0 0 0 0 ns t rts retransmit set-up time 3.5 3.5 4 5 6 ns t olz output enable to output in low z (4) 000 0 0 ns t oe output enable to output valid 3 7 3 7.5 3 8 3 10 3 13 ns t ohz output enable to output in high z (4) 3 7 3 7.5 3 8 3 10 3 13 ns t wff write clock to ff or ir 89101215ns t ref read clock to ef or or 89101215ns t paf write clock to paf C 89101215ns t pae read clock to pae 89101215ns t hf clock to hf 16 18 20 22 25 ns t skew1 skew time between rclk and wclk 8 10 12 15 20 ns for ff and ir t skew2 skew time between rclk and 15 18 21 25 35 ns wclk for pae and paf notes: 1. all ac timings apply to both standard idt mode and first word fall through mode. 2. for the rclk line: t clkl (min.) = 7 ns only when reading the offsets from the programmable flag registers; otherwise, use the table value. for the wclk line, use the t clkl (min.) value given in the table. 3. pulse widths less than minimum values are not allowed. 4. values guaranteed by design, not currently tested. 3037 tbl 08 input pulse levels input rise/fall times input timing reference levels output reference levels output load gnd to 3.0v see figure 1 ac test conditions figure 1. output load * includes jig and scope capacitances. 3037 drw 04 1.1k 30pf* 680 w 5v d.u.t. 1.5v 1.5v 3ns
7 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges signal descriptions: inputs: data in (d 0 - d 17 ) data inputs for 18-bit wide data. controls: master reset ( mrs mrs ) a master reset is accomplished whenever the master reset ( mrs ) input is taken to a low state. this operation sets the internal read and write pointers to the first location of the ram array. pae will go low, paf will go high, and hf will go high. if fwft is low during master reset then the idt standard mode, along with ef and ff are selected. ef will go low and ff will go high. if fwft is high, then the first word fall through mode (fwft), along with ir and or , are selected. or will go high and ir will go low. if ld is low during master reset, then pae is assigned a threshold 127 words from the empty boundary and paf is assigned a threshold 127 words from the full boundary; 127 words corresponds to an offset value of 07fh. following master reset, parallel loading of the offsets is permitted, but not serial loading. if ld is high during master reset, then pae is assigned a threshold 1023 words from the empty boundary and paf is assigned a threshold 1023 words from the full boundary; 1023 words corresponds to an offset value of 3ffh. following master reset, serial loading of the offsets is permitted, but not parallel loading. regardless of whether serial or parallel offset loading has been selected, parallel reading of the registers is always permitted. (see section describing the ld line for further details). during a master reset, the output register is initialized to all zeroes. a master reset is required after power up, before a write operation can take place. mrs is asynchronous. partial reset ( prs prs ) a partial reset is accomplished whenever the partial reset ( prs ) input is taken to a low state. as in the case of the master reset, the internal read and write pointers are set to the first location of the ram array, pae goes low, paf goes high, and hf goes high. whichever mode is active at the time of partial reset, idt standard mode or first word fall-through, that mode will remain selected. if the idt standard mode is active, then ff will go high and ef will go low. if the first word fall-through mode is active, then or will go high, and ir will go low. following partial reset, all values held in the offset regis- ters remain unchanged. the programming method (parallel or serial) currently active at the time of partial reset is also retained. the output register is initialized to all zeroes. prs is asynchronous. a partial reset is useful for resetting the device during the course of operation, when reprogramming flag settings may not be convenient. retransmit ( rt rt ) the retransmit operation allows data that has already been read to be accessed again. there are two stages: first, a setup procedure that resets the read pointer to the first location of memory, then the actual retransmit, which consists of reading out the memory contents, starting at the beginning of memory. retransmit setup is initiated by holding rt low during a rising rclk edge. ren and wen must be high before bringing rt low. at least one word, but no more than full - 2 words should have been written into the fifo between reset (master or partial) and the time of retransmit setup (full = 8,192 words for the 72255, 16,384 words for the 72265). if idt standard mode is selected, the fifo will mark the beginning of the retransmit setup by setting ef low. the change in level will only be noticeable if ef was high before setup. during this period, the internal read pointer is initialized to the first location of the ram array. when ef goes high, retransmit setup is complete and read operations may begin starting with the first location in memory. since idt standard mode is selected, every word read including the first word following retransmit setup re- quires a low on ren to enable the rising edge of rclk. writing operations can begin after one of two conditions have been met: ef is high or 14 cycles of the faster clock (rclk or wclk) have elapsed since the rclk rising edge enabled by the rt pulse. the deassertion time of ef during retransmit setup is variable. the parameter t rtf1 , which is measured from the rising rclk edge enabled by rt to the rising edge of ef is described by the following equation: t rtf1 max. = 14*t f + 3*t rclk (in ns) where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. regarding ff : note that since no more than full - 2 writes are allowed between a reset and a retransmit setup, ff will remain high throughout the setup procedure. for idt standard mode, updating the pae , hf , and paf flags begins with the "first" ren -enabled rising rclk edge following the end of retransmit setup (the point at which ef goes high). this same rclk rising edge is used to access the "first" memory location. hf is updated on the first rclk rising edge. pae is updated after two more rising rclk edges. paf is updated after the "first" rising rclk edge, followed by the next two rising wclk edges. (if the t skew2 specification is not met, add one more wclk cycle.) if fwft mode is selected, the fifo will mark the beginning of the retransmit setup by setting or high. the change in level will only be noticeable if or was low before setup. during this period, the internal read pointer is set to the first location of the ram array.
8 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 when or goes low, retransmit setup is complete; at the same time, the contents of the first location are automatically displayed on the outputs. since fwft mode is selected, the first word appears on the outputs, no read request necessary. reading all subsequent words requires a low on ren to enable the rising edge of rclk. writing operations can begin after one of two conditions have been met: or is low or 14 cycles of the faster clock (rclk or wclk) have elapsed since the rclk rising edge enabled by the rt pulse. the assertion time of or during retransmit setup is variable. the parameter t rtf2 , which is measured from the rising rclk edge enabled by rt to the falling edge of or is described by the following equation: t rtf2 max. = 14*t f + 4*t rclk (in ns) where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. note that a retransmit setup in fwft mode requires one more rclk cycle than in idt standard mode. regarding ir : note that since no more than full - 2 writes are allowed between a reset and a retransmit setup, ir will remain low throughout the setup procedure. for fwft mode, updating the pae , hf , and paf flags begins with the "last" rising edge of rclk before the end of retransmit setup. this is the same edge that asserts or and automatically accesses the first memory location. note that, in this case, ren is not required to initiate flag updating. hf is updated on the "last" rclk rising edge. pae is updated after two more rising rclk edges. paf is updated after the "last" rising rclk edge, followed by the next two rising wclk edges. (if the t skew2 specification is not met, add one more wclk cycle.) rt is synchronized to rclk. the retransmit operation is useful in the event of a transmission error on a network, since it allows a data packet to be resent. first word fall through/serial in ( fwft/si ) this is a dual purpose pin. during master reset, the state of the fwft/si helps determine whether the device will operate in idt standard mode or first word fall through (fwft) mode. if, at the time of master reset, fwft/si is low, then idt standard mode will be selected. this mode uses the empty flag ( ef ) to indicate whether or not there are any words present in the fifo memory. it also uses the full flag function ( ff ) to indicate whether or not the fifo memory has any free space for writing. in idt standard mode, every word read from the fifo, including the first, must be requested using the read enable ( ren ) line. if, at the time of master reset, fwft/si is high, then fwft mode will be selected. this mode uses output ready ( or ) to indicate whether or not there is valid data at the data outputs (q n) . it also uses input ready ( ir ) to indicate whether or not the fifo memory has any free space for writing. in the fwft mode, the first word written to an empty fifo goes directly to q n , no read request necessary. subsequent words must be accessed using the read enable ( ren ) line. after master reset, fwft/si acts as a serial input for loading pae and paf offsets into the programmable registers. the serial input function can only be used when the serial loading method has been selected during master reset. fwft/si functions the same way in both idt standard and fwft modes. write clock (wclk ) a write cycle is initiated on the rising edge of the write clock (wclk). data set-up and hold times must be met with respect to the low-to-high transition of the wclk. the write and read clocks can either be asynchronous or coincident. write enable ( wen wen ) when write enable ( wen ) is low, data can be loaded into the input register on the rising edge of every wclk cycle. data is stored in the ram array sequentially and indepen- dently of any on-going read operation. when wen is high, the input register holds the previous data and no new data is loaded into the fifo. to prevent data overflow in the idt standard mode, ff will go low , inhibiting further write operations. upon the comple- tion of a valid read cycle, ff will go high allowing a write to occur. wen is ignored when the fifo is full. to prevent data overflow in the fwft mode, ir will go high, inhibiting further write operations. upon the completion of a valid read cycle, ir will go low allowing a write to occur. wen is ignored when the fifo is full. read clock (rclk) data can be read on the outputs, on the rising edge of the read clock (rclk), when output enable ( oe ) is set low. the write and read clocks can be asynchronous or coincident. read enable ( ren ren ) when read enable ( ren ) is low, data is loaded from the ram array into the output register on the rising edge of the rclk. when ren is high, the output register holds the previous data and no new data is loaded into the output register. in the idt standard mode, every word accessed at q n , including the first word written to an empty fifo, must be requested using ren . when all the data has been read from the fifo, the empty flag ( ef ) will go low, inhibiting further read operations. ren is ignored when the fifo is empty. once a write is performed, ef will go high after t fwl1 +t ref and a read is permitted. in the fwft mode, the first word written to an empty fifo automatically goes to the outputs q n , no need for any read request. in order to access all other words, a read must be executed using ren . when all the data has been read from the fifo, output ready ( or ) will go high, inhibiting further read operations. ren is ignored when the fifo is empty. once a write is performed, or will go low after t fwl2 +t ref, when the first word appears at q n ; if a second word is written into the fifo, then ren can be used to read it out.
9 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges serial enable ( sen sen ) serial enable is ( sen ) is an enable used only for serial programming of the offset registers. the serial programming method must be selected during master reset. sen is always used in conjunction with ld . when these lines are both low, data at the si input can be loaded into the input register one bit for each low-to-high transition of wclk. when sen is high, the programmable registers retains the previous settings and no offsets are loaded. sen functions the same way in both idt standard and fwft modes. output enable ( oe oe ) when output enable ( oe ) is enabled (low), the parallel output buffers receive data from the output register. when oe is high, the output data bus (q n ) goes into a high impedance state. load ( ld ld ) this is a dual purpose pin. during master reset, the state of the load line ( ld ) determines one of two default values (127 or 1023) for the pae and paf flags, along with the method by which these flags can be programmed, parallel or serial. after master reset, ld enables write operations to and read operations from the registers. only the offset loading method currently selected can be used to write to the registers. aside from master reset, there is no other way change the loading method. registers can be read only in parallel; this can be accomplished regardless of whether serial or the parallel loading has been selected. associated with each of the programmable flags, pae and paf , are registers which can either be written to or read from. offset values contained in these registers determine how many words need to be in the fifo memory to switch a partial flag. a low on ld during master reset selects a default pae offset value of 07fh ( a threshold 127 words from the empty boundary), a default paf offset value of 07fh (a threshold 127 words from the full boundary), and parallel loading of other offset values. a high on ld during master reset selects a default pae offset value of 3ffh (a threshold 1023 words from the empty boundary), a default paf offset value of 3ffh (a threshold 1023 words form the full boundary), and serial loading of other offset values. the act of writing offsets (in parallel or serial) employs a dedicated write offset register pointer. the act of reading offsets employs a dedicated read offset register pointer. the figure 2. partial flag programming sequence notes: 1. only one of the two offset programming methods, serial or parallel, is available for use at any given time. 2. the programming method can only be selected at master reset. 3. parallel reading of the offset registers is always permitted regardless of which programming method has been selected. 4. the programming sequence applies to both idt standard and fwft modes. selection parallel write to registers: empty offset full offset parallel read from registers: empty offset full offset no operation write memory read memory no operation 3037 tbl 02 0 0 x 1 1 1 0 0 1 1 0 x 1 1 1 0 1 x 0 1 1 serial shift into registers: 26 bits for the 72255 28 bits for the 72265 1 1 1 x x x 0 wclk x x x x rclk x x x x x 1 bit for each rising wclk edge starting with empty offset (lsb) ending with full offset (msb)
10 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 two pointers operate independently; however, a read and a write should not be performed simultaneously to the offset registers. a master reset initializes both pointers to the empty offset (lsb) register. a partial reset has no effect on the position of these pointers. once serial offset loading has been selected, then pro- gramming pae and paf procedes as follows: when ld and sen are set low, data on the si input are written, one bit for each wclk rising edge, starting with the empty offset (13 bits for the 72255, 14 bits for the 72265), ending with the full offset (13 bits for the 72255, 14 bits for the 72265). a total of 26 bits are necessary to program the 72255; a total of 28 bits are necessary to program the 72265. individual registers cannot be loaded serially; rather, both must be programmed in sequence, no padding allowed. pae and paf can show a valid status only after the the full set of bits have been entered. the registers can be re-programmed, as long as both offsets are loaded. when ld is low and sen is high, no serial write to the registers can occur. once parallel offset loading has been selected, then programming pae and paf procedes as follows: when ld and wen are set low, data on the inputs d n are written into the empty offset register on the first low-to-high transition of wclk. upon the second low-to-high transition of wclk, data at the inputs are written into the full register. the third transition of wclk writes, once again, to the empty offset register. to ensure proper programming (serial or parallel) of the offset registers, no read operation is permitted from the time of reset (master or partial) to the time of programming. (during this period, the read pointer must be pointing to the first location of the memory array.) after the programming has been accomplished, read operations may begin. write operations to memory are allowed before and during the parallel programming sequence. in this case, the pro- gramming of all offset registers does not have to occur at one time. one or two offset registers can be written to and then, by bringing ld high, write operations can be redirected to the fifo memory. when ld is set low again, and wen is low, the next offset register in sequence is written to. as an alternative to holding wen low and toggling ld , parallel programming can also be interrupted by setting ld low and toggling wen . write operations to memory are allowed before and during the serial programming sequence. in this case, the program- ming of all offset bits does not have to occur at once. a select number of bits can be written to the si input and then, by bringing ld and sen high, data can be written to fifo memory via d n by toggling wen . when wen is brought high with ld and sen restored to a low, the next offset bit in sequence is written to the registers via si. if a mere interuption of serial programming is desired, it is sufficient either to set ld low and deactivate sen or to set sen low and deactivate ld . once ld and sen are both restored to a low level, serial offset programming continues from where it left off. note that the status of a partial flag ( pae or paf ) output is invalid during the programming process. from the time parallel programming has begun, a partial flag output will not be valid until the appropriate offset word has been written to the register pertaining to that flag. from the time serial programming has begun, neither partial flag will be valid until the full set of bits required to fill all the offset registers has been written. measuring from the rising wclk edge that achieves either of the above criteria; paf will be valid after two more rising wclk edges plus t paf , pae will will be valid after the next two rising rclk edges plus t pae (add one more rclk cycle if t skew2 is not met.) the act of reading the offset registers employs a dedicated read offset register pointer. the contents of the offset registers can be read on the output lines when ld is set low and ren is set low; then, data are read via q n from the empty offset register on the first low-to-high transition of rclk. upon the second low-to-high transition of rclk, data are read from the full offset register. the third transition of rclk, reads, once again, from the empty offset register. it is permissable to interrupt the the offset register access sequence with reads or writes to memory . the interruption is accomplished by deasserting ren , ld , or both together. when ren and ld are restored to a low level, access of the registers continues where it left off. ld functions the same way in both idt standard and fwft modes. frequency select input (fs) an internal state machine manages the movement of data through the supersync fifo. the fs line determines whether rclk or wclk will synchronize the state machine. tie fs to v cc if the rclk line is running at a lower frequency than the wclk line. in this case, the state machine will be synchro- nized to wclk. tie fs to gnd if the rclk line is running at a higher frequency than the wclk line. in this case, the state machine will be synchronized to rclk. note that fs must be set so the clock line running at the higher frequency drives the state machine; this ensures efficient handling of the data within the fifo. if the same clock signal drives both the wclk and the rclk pins, then tie fs to gnd. the frequency of the clock tied to the state machine (referred to as the "selected clock") may be changed at any time, so long as it is always greater than or equal to the frequency of the clock that is not tied to the state machine (referred to as the "non-selected clock"). the frequency of the non-selected clock can also be varied with time, so long as it never exceeds the frequency of the selected clock. to be more specific, the frequencies of both rclk and wclk may be varied during fifo operation, provided that, at any given point in time, the cycle period of the selected clock is equal to or less than the cycle period of the non-selected clock. the selected clock must be continuous. it is, however, permissible to stop the non-selected clock. note, so long as rclk is idle, ef / or and pae will not be updated. likewise, as long as wclk is idle, ff / ir and paf will not be updated. changing the fs setting during fifo operation (i.e. read- ing or writing) is not permitted; however, such a change at the time of master reset or partial reset is all right. fs is an asynchronous input.
11 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges note: 1. any bits of the offset register not being programmed should be set to zero. figure 3. offset register location and default values empty offset register 17 0 07fh if ld is low at master reset, 3ffh if ld is high at master reset full offset register 17 0 default value default value 07fh if ld is low at master reset, 3ffh if ld is high at master reset 12 12 72255 C 8,192 x 18Cbit 3037 drw 05 empty offset register 17 0 07fh if ld is low at master reset, 3ffh if ld is high at master reset full offset register 17 0 default value default value 07fh if ld is low at master reset, 3ffh if ld is high at master reset 13 13 72265 C 16,384 x 18Cbit 3037 drw 06 that updates the flag. t fwl1 includes any delays due to clock skew and can be expressed as follows: t fwl1 max. = 10*t f + 2*t rclk (in ns) where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. since no read can take place until ef goes high, the t fwl1 delay determines how early the first word can be available at q n . this delay has no effect on the reading of subsequent words. in fwft mode, the ouput ready ( or ) function is selected. or goes low at the same time that the first word written to an empty fifo appears valid on the outputs. or goes high one cycle after rclk shifts the last word from the fifo memory to the outputs. then further data reads are inhibited until or goes low again. when writing the first word to an empty fifo, the assertion time of or is variable, and can be represented by the first word latency parameter, t fwl2 , which is measured from the rising wclk edge that writes the first word to the rising rclk edge that updates the flag. t fwl2 includes any delay due to clock skew and can be expressed as follows: t fwl2 max. = 10*t f + 3*t rclk (in ns) where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. note that the first word latency in fwft mode is one rclk cycle longer than in idt standard mode. the t fwl2 delay determines how early the first word can be available at q n . this delay has no effect on the reading of subsequent words. ef / or is sychronized to the rclk. it is double-registered to enhance metastable immunity. outputs: full flag ( ff ff / ir ir ) this is a dual purpose pin. in idt standard mode, the full flag (ff) function is selected. when the fifo is full (i.e. the write pointer catches up to the read pointer), ff will go low, inhibiting further write operation. when ff is high, the fifo is not full. if no reads are performed after a reset (either mrs or prs), ff will go low after 8,192 writes tor the idt72255 and 16,384 writes to the idt72265. in fwft mode, the input ready (ir) function is selected. ir goes low when memory space is available for writing in data. when there is no longer any free space left, ir goes high, inhibiting further write operation. if no reads are performed after a reset (either mrs or prs), ir will go high after 8,193 writes for the idt72255 and 16,385 writes for the idt72265. the ir status not only measures the contents of the fifo memory, but also counts the presence of a word in the output register. thus, in fwft mode, the total number of writes necessary to deassert ir is one greater than needed to assert ff in idt standard mode. ff/ir is synchronized to wclk. it is double-registered to enhance metastable immunity. empty flag ( ef ef / or or ) this is a dual purpose pin. in the idt standard mode, the empty flag ( ef ) function is selected. when the fifo is empty (i.e. the read pointer catches up to the write pointer), ef will go low, inhibiting further read operations. when ef is high, the fifo is not empty. when writing the first word to an empty fifo, the deassertion time of ef is variable, and can be represent by the first word latency parameter, t fwl1 , which is measured from the rising wclk edge that writes the first word to the rising rclk edge
12 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 programmable almost-full flag ( paf paf ) the programmable almost-full flag ( paf ) will go low when the fifo reaches the almost-full condition as specified by the offset m stored in the full offset register. at the time of master reset, depending on the state of ld , one of two possible default offset values are chosen. if ld is low, then m = 07fh and the paf switching threshold is 127 words from the full boundary, if ld is high, then m = 3ffh and the paf switching threshold is 1023 words away from the full boundary. any integral value of m from 0 to the maximum fifo depth minus 1 (8,191 words for the 72255, 16,383 words for the 72265) can be programmed into the full offset register. in idt standard mode, if no reads are performed after reset ( mrs or prs ), paf will go low after (8,192-m) writes to the idt72255, and (16,384-m) writes to the idt72265. in fwft mode, if no reads are performed after reset ( mrs or prs ), paf will go low after (8,193-m) writes to the idt72255, and (16,385-m) writes to the idt72265. in this case, the first word written to an empty fifo does not stay in memory, but goes unrequested to the output register; there- fore, it has no effect on determining the state of paf . note that even though paf is programmed to switch low during the first word latency period (t fwl ), attempts to read data will be ignored until ef goes high indicating that data is available at the output port. this is true for both timing modes. paf is synchronous and updated on the rising edge of wclk. it is double-registered to enhance metastable immu- nity. notes: 1.data in the output register does not count as a 'word in fifo memory". since in fwft mode, the first word written to an empty fifo goes unrequested to the output register (no read operation necessary), it is not included in the fifo memory count. 2. n = empty offset, default values: n = 127 when parallel offset loading is selected or n=1023 when serial offset loading is selected. 3. m = full offset, default values: m = 127 when parallel offset loading is selected or n=1023 when serial offset loading is selected. 4. following a reset (master or partial), the fifo memory is empty and or = high. after writing the first word, the fifo memory remains empty, the data is placed into the output register, and or goes low. in this case, or any time the last word in the fifo memory has been read into the output register; a rising rclk edge, enabled by ren , will set or high. number of words in fifo memory ir ir paf paf hf hf pae pae or or 72255 0l h hl h 1 to n l h hl l (n+1) to 4,096 l h hhl 4,097 to (8192-(m+1)) l h lhl (8,192-m) to 8,191 l l lhl 8,192 h l lhl (2) (3) 72265 0 1 to n (n+1) to 8,192 8,193 to (16,384-(m+1)) (16,384-m) to 16,383 16,384 (2) (3) 3037 tbl 04 (1) (4) table ii status flags for fwft mode number of words in fifo memory ff ff paf paf hf hf pae pae ef ef 72255 0 h hh l l h hh l h (n+1) to 4,096 h hh h h 4,097 to (8192-(m+1)) h hl h h h ll h h 8,192 l ll h h 1 to n (8,192-m) to 8,191 (2) (3) 72265 0 (n+1) to 8,192 8,193 to (16,384-(m+1)) 16,384 1 to n (16,384-m) to 16,383 (2) (3) 3037 tbl 03 (1) notes: 1. data in the output register does not count as a 'word in fifo memory". since in fwft mode, the first word written to an empty fifo goes unrequested to the output register (no read operation necessary), it is not included in the fifo memory count. 2. n = empty offset, default values: n = 127 when parallel offset loading is selected or n=1023 when serial offset loading is selected. 3. m = full offset, default values: m = 127 when parallel offset loading is selected or n=1023 when serial offset loading is selected. table i status flags for idt standard mode
13 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges programmable almost-empty flag ( pae pae ) the programmable almost-empty flag ( pae ) will go low when the fifo reaches the almost-empty condition as speci- fied by the offset n stored in the empty offset register. at the time of master reset, depending on the state of ld , one of two possible default offset values are chosen. if ld is low, then n = 07fh and the pae switching threshold is 127 words from the empty boundary, if ld is high, then n = 3ffh and the pae switching threshold is 1023 words away from the empty boundary. any integral value of n from 0 to the maximum fifo depth minus 1 (8,191 words for the 72255, 16,383 words for the 72265) can be programmed into the empty offset register. in idt standard mode, if no reads are performed after reset ( mrs or prs ), the pae will go high after (n + 1) writes to the idt72255/72265. in fwft mode, if no reads are performed after reset ( mrs or prs ), the pae will go high after (n+2) writes to the idt72255/72265. in this case, the first word written to an empty fifo does not stay in memory, but goes unrequested to the output register; therefore, it has no effect on determin- ing the state of pae . note that even though pae is programmed to switch high during the first word latency period (t fwl ), attempts to read data will be ignored until ef goes high indicating that data is available at the output port. this is true for both timing modes. pae is synchronous and updated on the rising edge of rclk. it is double-registered to enhance metastable immu- nity. half-full flag ( hf hf ) this output indicates a half-full memory. the rising wclk edge that fills the memory beyond half-full sets hf low. the flag remains low until the difference between the write and read pointers becomes less than or equal to half of the total depth of the device; the rising rclk edge that accomplishes this condition also sets hf high. in idt standard mode, if no reads are performed after reset ( mrs or prs ), hf will go low after (d/2 + 1) writes, where d is the maximum fifo depth (8,192 words for the idt72255, 16,384 words for the idt72265). in fwft mode, if no reads are performed after reset ( mrs or prs ), hf will go low after (d/2+2) writes to the idt72255/ 72265. in this case, the first word written to an empty fifo does not stay in memory, but goes unrequested to the output register; therefore, it has no effect on determining the state of hf . because hf uses both rclk and wclk for synchroniza- tion purposes, it is asynchronous. data outputs (q 0 -q 17 ) q 0 -q 17 are data outputs for 18-bit wide data.
14 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 figure 4. master reset timing t rs mrs t rsr ren t rss fwft/si 3037 drw 07 t rsr t fwft t rsr wen t rss ld t rsr t rss (1) t rss rt sen t rss t rsf t rsf oe = high oe = low (1) pae paf , hf q 0 - q 17 t rsf ef / or ff / ir t rsf t rsf if fwft = high, or = high if fwft = low, ef = low if fwft = low, ff = high if fwft = high, ir = low
15 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges figure 5. partial reset timing t rs prs t rsr ren t rss 3037 drw 08 t rss t rsr wen t rss rt sen t rss t rsf t rsf oe = high oe = low (1) pae paf , hf q 0 - q 17 t rsf ef / or ff / ir t rsf t rsf if fwft = high, or = high if fwft = low, ef = low if fwft = low, ff = high if fwft = high, ir = low
16 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 figure 6. write cycle timing (idt standard mode) notes: 1. t skew1 is the minimum time between a rising rclk edge and a rising wclk edge to guarantee that ff will go high (after one wclk cycle plus t wff ). if the time between the rising edge of rclk and the rising edge of wclk is less than t skew1 , then the ff deassertion may be delayed an extra wclk cycle. 2. ld = high wclk d 0 - d 17 wen ff rclk ren t ds t wff t wff data in valid no operation (1) t skew1 3037 drw 09 t ens t dh t enh 1 2 t clkh t clkl t clk
17 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges notes: 1. t fwl1 contributes a variable delay to the overall first word latency (this parameter includes delays due to skew): t fwl1 max. (in ns) = 10*t f + 2* t rclk where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period 2. ld = high figure 7. read cycle timing (idt standard mode) no operation rclk ren ef t clk t clkh t clkl t enh t ref t ref last word t a t olz t oe q 0 - q 17 oe wclk (1) t fwl1 wen 3037 drw 10 d 0 - d 17 t ens t ens t enh t ds t dhs first word t ohz
18 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 wclk d 0 - d 17 wen rclk ef q 0 - q 17 ren t ds t ens d1 d0 d1 first valid write oe 3037 drw 11 t a t ref t oe t olz t fwl1 (1) t a d0 notes: 1. t fwl1 max. (in ns) = 10* t f + 2* t rclk where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period 2. ld = high figure 8. first data word latency (idt standard mode)
19 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges notes: 1. t skew1 is the minimum time between a rising rclk edge and a rising wclk edge to guarantee that ff will go high (after one wclk cycle pus t wff ). if the time between the rising edge of the rclk and the rising edge of the wclk is less than t skew1 , then the ff deassertion may be delayed an extra wclk cycle. 2. ld = high figure 9. full flag timing (idt standard mode) d 0 - d 17 wen rclk ff ren t enh t enh q 0 - q 17 data read next data read data in output register low oe t skew1 data write 3037 drw 12 wclk no write 1 2 1 2 t ds no write t wff t wff t wff t a t ens t ens t skew1 t ds t a wd (1) (1)
20 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 notes: 1. t fwl1 max. (in ns) = 10*t f + 2*t rclk where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the period. 2. ld = high figure 10. empty flag timing (idt standard mode) wclk d 0 - d 17 wen rclk ef q 0 - q 17 oe t ds t ens t a data write 1 word 1 t enh t ds t ens data write 2 t enh ren data in output register low 3037 drw 13 t ref t fwl1 t ref t ref t fwl1 (1) (1)
21 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges figure 12. parallel loading of programmable flag registers (idt standard and fwft modes) figure 11. serial loading of programmable flag registers (idt standard and fwft modes) note: 1. for the 72255, x = 12. for the 72265, x = 13. wclk sen si 3037 drw 14 t enh t ens t lds t ldh ld t ds bit 0 empty offset bit x bit 0 full offset (1) t enh t enh bit x (1) wclk ld wen d 0 - d 17 3037 drw 15 t ld s t clkh t clkl t ens pae offset paf offset t ds t dh t ldh t enh t clk
22 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 notes: 1. pae offset = n 2. data in the output register does not count as a "word in fifo memory". since, in fwft mode, the first word written to an empty fifo goes unrequested to the output register (no read operation necessary), it is not included in the fifo memory count. 3. t skew2 is the minimum time between a rising wclk edge and a rising rclk edge for pae to go high (after one rclk cycle plus t pae ). if the time between the rising edge of wclk and the rising edge of rclk is less than t skew2 , then the pae deassertion may be delayed one extra rclk cycle. wclk t enh t clkh t clkl wen pae rclk ren 3037 drw 17 t ens t enh t ens n words in fifo memory t pae n+1 words in fifo memory t skew2 t pae 1 2 1 2 n words in fifo memory (1,2) (3) figure 13. parallel read of programmable flag registers (idt standard and fwft modes) note: 1. oe =low figure 14. programmable almost empty flag timing (idt standard and fwft modes) rclk ld ren q0 - q17 t ldh t lds t clkh t clkl t ens data in output register pae offset paf offset t enh t enh t enh 3037 drw 16 t clk t a t a
23 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges notes: 1. paf offset = m, d = 8,192 for idt 72255, 16,384 word for idt 72265. 2. data in the output register does not count as a "word in fifo memory". since, in fwft mode, the first word written to an empty fifo goes unrequested to the output register (no read operation necessary), it is not included in the fifo memory count. 3. t skew2 is the minimum time between a rising rclk edge and a rising wclk edge for paf to go high (after one wclk cycle plus t paf ). if the time between the rising edge of rclk and the rising edge of wclk is less than t skew2 , then the paf deassertion time may be delayed an extra wclk cycle. wclk t enh t clkh t clkl wen paf rclk (3) t paf ren 3037 drw 18 t ens t enh t ens d - (m+1) words in fifo memory t paf d - m words in fifo memory (1,2) t skew2 1 2 12 d-(m+1) words in fifo memory wclk wen hf rclk ren t clkh t clkl t ens t enh d/2 + 1 words d/2 words t hf t hf t ens 3037 drw 19 d/2 words figure 15. programmable almost full flag timing (idt standard and fwft modes) note: 1. d = maximum fifo depth = 8,192 for idt 72255, 16,384 word for idt 72265. figure 16. half - full flag timing (idt standard and fwft modes)
24 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 t ref t rts t enh w x t dh t rts w [y+1] t ens t enh t rtf1 3037 drw 20 t a t ens (1,2) t ens w y wclk wen rclk ren d 0 - d 17 rt ef paf hf pae ff q 0 - q 17 t enh w [x + 1] t skew2 1 2 3 2 1 t ens t ds w 1 t paf t hf t pae t a t enh t ens t ds t dh t enh t ref (3) (4) notes: 1. t rtf1 contributes a variable delay to the overall retransmit recovery time: t rftf1 max = 14*t f + 3*t rclk (in ns) where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. 2. retransmit set up is complete after ef returns high, only then can a read operation begin. write operations are permitted after one of two conditions have been met: ef is high or 14 cycles of the faster clock (rclk or wclk) have elapsed since the rclk rising edge enabled by the rt pulse. 3. following retransmit setup, the rising edge of rclk that accesses the first memory location also initiates the updating of hf , pae , and paf . 4. no more than d-2 words (d = 8,192 words for the 72255, 16,384 words for the 72265) should have been written to the fifo between reset (master or partial) and retransmit setup. therefore, ff will be high throughout the restransmit setup procedure. 5. oe =low figure 17. retransmit timing (idt standard mode)
25 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges notes: 1. t fwl2 max. (in ns) = 10*t f + 3*t rclk where tf is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. 2. t skew2 is the minimum time between a rising wclk edge and a rising rclk edge for pae to go high (after one rclk cycle plus t pae ). if the time between the rising edge of wclk and the rising edge of rclk is less than t skew2 , then the pae deassertion may be delayed one extra rclk cycle. 3. ld = high, oe = low 4. pae offset = n, paf offset = m, d = maximum fifo depth = 8,192 words for the idt72255, 16,384 words for the idt72265. w 1 w 2 w n+1 w [n +2] w [d/2+1] w [d/2+2] w [d-m+1] w [d-m)] w d w [d+1] w [n+3] w [n+4] w [d-m+2] w [d-m+3] wclk wen d 0 - d 17 rclk t dh t ds t ens t fwl2 ren q 0 - q 17 paf hf pae ir t ds t ds t ds 1 2 t skew2 12 t a t ref or t pae t hf t paf t wff w [d/2+3] w [d-m+4] w 1 t enh 3037 drw 21 data in output register (1) (2) figure 18. write timing (first word fall through mode)
26 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 notes: 1. t skew1 is the minimum time between a rising rclk edge and a rising wclk edge to guarantee that ir will go low (after one wclk cycle plus t wff ). if the time between the rising ege of rclk and the rising edge of wclk is less than t skew1 , then the ir assertion may be delayed an extra wclk cycle. 2. t skew2 is the minimum time between a rising rclk edge and a rising wclk edge for paf to go high (after one wclk cycle plus t paf ). if the time between the rising edge of rclk and the rising edge of wclk is less than t skew2 , then the paf deassertion may be delayed an extra wclk cycle. 3. ld = high 4. pae offset = n, paf offset = m, d = maximum fifo depth = 8,192 words for the idt72255, 16,384 words for the idt72265. wclk 1 2 12 wen d 0 - d 17 rclk t ens ren q 0 - q 17 paf hf pae ir or w 1 w 1 w 2 w 3 w [d/2] w m+1 w [m+2] t ohz t skew1 t enh t ds t dh t oe t a t a t a t paf t wff t wff t ens oe t skew2 w [d+1] 3037 drw 22 t pae w [d/2+1] w [d-n+1] w [d-n] t a t a t hf t ref w d w [d+1] t a w [d-n+2] w [m+3] 1 2 w [d-n+3] t a (1) (2) figure 19. read timing (first word fall through mode)
27 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges figure 20. retransmit timing (fwft mode) t ref t rts t enh w x t dh t rts w [y+1] t ens t enh t rtf1 3037 drw 23 t a (1,2) t ens w y wclk wen rclk ren d 0 - d 17 rt or paf hf pae ir q 0 - q 17 t enh w [x + 1] t skew2 1 2 3 2 1 t ens t ds w 1 t paf t hf t pae t ens t a t enh t ens t ds t dh t enh t ref (3) (4) t a w 2 notes: 1. t rtf2 contribute a variable delay to the overall retransmit time: t rtf2 max = 14*t f + 4*t rclk (in ns) where t f is either the rclk or the wclk period, whichever is shorter, and t rclk is the rclk period. 2. retransmit set up is complete after or returns low, only then can a read operation begin. write operations are permitted after one of two conditions have been met: or is low or 14 cycles of the faster clock (rclk or wclk) have elapsed since the rclk rising edge enabled by the rt pulse. 3. following retransmit setup, the rising edge of rclk that accesses the first memory location also initiates the updating of hf , pae , and paf . 4. no more than d-2 words (d = 8,192 words for the 72255, 16,384 words for the 72265) should have been written to the fifo between reset (master or partial) and retransmit setup. therefore, ir will be low throughout the retransmit setup procedure. 5. oe =low
28 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 operating configurations single device configuration a single idt72255/72265 may be used when the applica- tion requirements are for 8,192/16,384 words or less. the idt72255/72265 can always be used in single device con- figuration, whether idt standard mode or fwft mode has been selected. no special set up procedure is necessary. figure 21. block diagram of single 8,192x18/16,384x18 synchronous fifo width expansion configuration word width may be increased simply by connecting to- gether the control signals of multiple devices. status flags can be detected from any one device. the exceptions are the ef and ff functions in idt standard mode and the ir and or functions in fwft mode. because of variations in skew between rclk and wclk, it is possible for ef / ff deassertion and ir / or assertion to vary by one cycle between fifos. in idt standard mode, such problems can be avoided by creat- ing composite flags, that is, anding ef of every fifo, and separately anding ff of every fifo. in fwft mode, com- posite flags can be created by oring or of every fifo, and separately oring ir of every fifo. figure 22 demonstrates an 36-word width by using two idt72255/72265s. any word width can be attained by adding additional idt72255/72265s. data out (q 0 - q 17 ) data in (d 0 - d 17 ) master reset ( mrs ) read clock (rclk) read enable ( ren ) output enable ( oe ) empty flag/output ready ( ef / or ) programmable almost empty ( pae ) write clock (wclk) write enable ( wen ) load ( ld ) full flag/input ready ( ff / ir ) programmable almost full ( paf ) idt 72255/ 72265 partial reset ( prs ) first word fall through/serial input (fwft/si) retransmit ( rt ) 3037 drw 24 half full flag ( hf ) frequency select (fs) serial enable( sen )
29 idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 military and commercial temperature ranges note: 1. use an and gate in idt standard mode, an or gate in fwft mode. 2. do not connect any output control signals directly together. depth expansion configuration the idt72255/72265 can easily be adapted to applications requiring more than 8,192/16,384 words of buffering. in fwft mode, the fifos can be arranged in series (the data outputs of one fifo connected to the data inputs of the next)C no external logic necessary. the resulting configuration provides a total depth equivalent to the sum of the depths associated with each single fifo. figure 23 shows a depth expansion using two idt72255/72265s. care should be taken to select fwft mode during master reset for all fifos in the depth expansion configuration. the first word written to an empty configuration will pass from one fifo to the next ("ripple down") until it finally appears at the outputs of the last fifo in the chainCno read operation is necessary. each time the data word appears at the outputs of one fifo, that device's or line goes low, enabling a write to the next fifo in line. the or assertion time is variable and is described with the help of the t fwl2 parameter, which includes including delay caused by clock skew: t fwl2 max.= 10*t f + 3*t rclk figure 22. block diagram of 8,192x36/16,384x36 72255/65 width expansion figure 23. block diagram of 16,384x18/32,768x18 synchronous fifo memory with programmable flags used in depth expansion configuration data in (dn) write clock (wclk) 36 18 18 master reset ( mrs ) read clock (rclk) data out (qn) 18 36 write enable ( wen ) full flag/input ready ( ff / ir ) programmable ( paf ) programmable ( pae ) empty flag/output ready ( ef / or ) #2 output enable ( oe ) read enable ( ren ) 18 load ( ld ) idt 72255/ 72265/ empty flag/output ready ( ef / or ) #1 partial reset ( prs ) idt 72255/ 72265/ 3037 drw 25 full flag/input ready ( ff / ir ) #2 half full flag ( hf ) frequency select (fs) first word fall through/ serial input (fwft/si) retransmit ( rt ) #1 #1 #2 gate (1) gate (1) ? dn input ready write enable write clock wen wclk ir data bus rclk read clock rclk ren oe output enable output ready qn dn ir gnd wen wclk or ren oe qn read enable or data out 72255/ 72265 fs 72255/ 72265 fs transfer clock 3037 drw 26 18 18 18
30 military and commercial temperature ranges idt72255/72265 syncfifo ? 8,192 x 18, 16,384 x 18 where t rclk is the rclk period and t f is either the rclk or the wclk period, whichever is shorter. the maximum amount of time it takes for a word to pass from the inputs of the first fifo to the outputs of the last fifo in the chain is the sum of the delays for each individual fifo: t fwl2(1) + t fwl2(2) + ... + t fwl2(n) + n*t rclk where n is the number of fifos in the expansion. note that the additional rclk term accounts for the time it takes to pass data between fifos. the ripple down delay is only noticeable for the first word written to an empty depth expansion configuration. there will be no delay evident for subsequent words written to the configuration. the first free location created by reading from a full depth expansion configuration will "bubble up" from the last fifo to the previous one until it finally moves into the first fifo of the chain. each time a free location is created in one fifo of the chain, that fifo's ir line goes low, enabling the preceding fifo to write a word to fill it. the amount of time it takes for ir of the first fifo in the chain to assert after a word is read from the last fifo is the sum of the delays for each individual fifo: n*(3*t wclk ) where n is the number of fifos in the expansion and t wclk is the wclk period. note that one of the three wclk cycle accounts for t skew1 delays. in a supersync depth expansion, set fs individually for each fifo in the chain. the transfer clock line should be tied to either wclk or rclk, whichever is faster. both these actions result in moving, as quickly as possible, data to the end of the chain and free locations to the beginning of the chain. ordering information commercial (0 c to +70 c) military (C55 c to +125 c) compliant to mil-std-883, class b pin grid array (pga, g68-1) thin plastic quad flatpack (tqfp, pn64-1) slim thin quad flatpack (stqfp, pp64-1) low power 8,192 x 18 supersync fifo 16,384 x 18 supersync fifo blank b g pf tf 10 commercial only 12 commercial only 15 commercial & military 20 commercial only 25 military only l 72255 72265 idt xxxxx device type x power xx speed x package x clock cycle time (t clk ) speed in nanoseconds process / temperature range 3037 drw 27

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