 |
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
| Datasheet File OCR Text: |
| Microcontrollers ApNote AP164601 APX164601.EXE available o additional file Port Initialization and Direction Control This Application Note discusses various aspects of the initialization of port pins as outputs: effects of single Bit and Bit Field (BFLDL/H) instructions, ports with bit protection, initialization of alternate output functions, effects of interrupts during port initialization, and bitfield function in C compilers Author: Christoph Meinhold / HL DC AT Semiconductor Group 1998-02, Rel. 1.0 Port Initialization and Direction Control 1 Port Structure - Overview..............................................................................................3 2 Initialization of Single Port Pins as Outputs................................................................3 2.1 Configuration as General Purpose Outputs .................................................................3 2.2 Configuration of Alternate Output Function ..................................................................5 2.3 Configuration of CAPCOM Pins....................................................................................5 3 Initialization of several Port Pins on the same Port ...................................................5 AP164601 ApNote - Revision History Actual Revision : Rel. 1.0 Previous Revison: Rel. 0.0 Page of Page of Subjects changes since last release) Actual Rel. prev. Rel. 7 7 Example for bit field function in C added Semiconductor Group 2 of 7 AP164601 1998-02 Port Initialization and Direction Control 1 Port Structure - Overview Each of the general purpose I/O ports (except Port 5) consists of a Port data register Px and a Port Direction register DPx. Each port pin is individually bit-addressable and configurable. The value in the direction register determines whether the corresponding port pin is configured as output (output driver(s) enabled) or input (pin is in high impedance state). Some of the ports have an additional Open Drain Control register ODPx (not implemented in 8xC166 devices). When in open drain output mode, only a logic '0' is actively driven, while a logic '1' must be provided by an external pullup. When reading a pin which is configured as input, the logic state of the pin is read, while reading a pin configured as output returns the value of the corresponding bit in the port data register. When writing to a port, always the port data register is written to. After reset, all I/O port pins are configured as inputs, and the port data registers Px contain the default value 0000h. Port pins which are used as address and/or data bus from reset on, including the BHE#/WRH# signal (P3.12) for 16-bit data bus modes, are configured automatically by hardware. All other pins have to be configured by software when they are to be used as outputs. 2 2.1 Initialization of Single Port Pins as Outputs Configuration as General Purpose Outputs When a port pin is to be used as general purpose output, first the initial output value should be written to the port data register, and then the direction of this pin should be set to output to achieve a defined transition from the floating state to the actively driven state. This could be done, e.g. for a Port 1 pin, via the following instruction sequence: BSET P1.7 BSET DP1.7 NOP BSET P1L.7 EXTR #1 BSET DP1L.7 NOP ; set initial ouput value (syntax for 8xC166 microcontrollers) ; set direction to output ; or other instruction which does not access P1 ; set initial ouput value (syntax for other C166 microcontrollers) ; set direction to output ; or other instruction which does not access P1 Note 1: When the direction of a port pin has been changed, the next instruction should not yet access this pin, since the modification is not effective until after the instruction following the modifying instruction. Otherwise, the next instruction may still read the pin status instead of the port data register value, or vice versa, depending whether the direction has been switched from input to output, or vice versa (pipeline effect, see also following examples, and section 'Particular Pipeline Effects' in respective User's Manual, e.g. C167 Derivatives User's Manual, V2.0, p. 4-7). Semiconductor Group 3 of 7 AP164601 1998-02 Port Initialization and Direction Control Note 2: When a port is in the initial input configuration after reset, and no external source is driving the pins to defined logic levels, it is not guaranteed that the reset value '0' for a specific bit will still be in the port data register after a read-modify-write operation has been performed on any bit of this port. This applies to explicit AND/OR masking as well as to Bit and Bit Field instructions, which are implicit read-modify-write instructions and always perform a complete word access on a port. The reason for this effect is that any read access on port pins configured as inputs returns the logic levels of the individual pins. Therefore, when the value in the port data register is different from the logic level at the pin, the value in the port data register may unintentionally be overwritten by a read-modify-write operation. This may also result in sporadic problems when the level at the input pin is floating to random logic levels. For ports which are associated with the CAPCOM/CAPCOM2 or PWM unit (P2, P7, P8), a specific bit protection mechanism has been implemented which prevents single bits which are modified by the on-chip periperal units from unintentionally being overwritten by a bit read-modify-write sequence via Bit and Bit Field instructions (see section 'Protected Bits' and 'Bit-Handling and Bit-Protection' in respective User's Manual, e.g. C167 Derivatives User's Manual, V2.0, p. 2-17 and p. 4-9). It is therefore advised to always set the port data register and the port direction register (at least for all ports without bit protection) as one unit in two consecutive instructions. In case interrupts could occur after the instruction which writes to the port data register, and in the interrupt service routine read-modify-write instructions are executed which also access this port, it is additionally recommended to temporarily disable interrupts, e.g. BFLDH PSW, #0F0h, #0F0h NOP BSET P1.7 BSET DP1.7 BFLDH PSW, #0F0h, #00h ; (syntax for 8xC166 microcontrollers) ; set PSW.ILVL to highest priority level ; Note: NMI# may still interrupt this sequence ; set initial ouput value ; set direction to output ; restore previous priority ; (syntax for other C166 microcontrollers) ; disable interrupts and class A traps for 2 instruction cycles ; set initial ouput value ; set direction to output ; or other instruction which does not access P1 ATOMIC #2 BSET P1L.7 EXTR#1 BSET DP1L.7 NOP This procedure is also recommended when a port pin is used in a bidirectional manner, i.e. when its direction is dynamically changed from input to output. Semiconductor Group 4 of 7 AP164601 1998-02 Port Initialization and Direction Control 2.2 Configuration of Alternate Output Function In order to use an alternate output function of a Port 3 pin, its direction must be set to output AND the corresponding port data register bit must contain a `1'. Otherwise, the alternate output function is not enabled and the pin will be stuck at `0' (see figure of Port 3 pins in chapter 'Parallel Ports' in respective User's Manual, e.g. C167 Derivatives User's Manual, V2.0, p. 6-18). Configuration of the WR# signal which is an alternate output function of P3.13 on 8xC166 microcontrollers or SSC shift clock output on most other C166 microcontrollers could be performed as follows: BSET P3.13 BSET DP3.13 NOP ; enable WR#/SCLK as alternate output function of P3.13, ; initial level is inactive high ; set direction to output ; or other instruction which does not access P3 Note that when a port data register still contains its reset value 0000h, and the direction of a pin is set to output before the corresponding bit in the port data register is set to `1', a low level will appear at the pin. For P3.13/WR# of 8xC166 microcontrollers in particular, this low level will be regarded as write strobe by external memory devices, and unexpected results may occur. 2.3 Configuration of CAPCOM Pins When a Port 2 pin is to be used as compare signal output (alternate function) for the CAPCOM unit of 8xC166 microcontrollers (respectively Port 7 and Port 8 for CAPCOM2 unit on other C166 microcontrollers), it is not required to write a `1' into the corresponding bit of the port data register. It is only required to set the direction to output. 3 Initialization of several Port Pins on the same Port When several port pins are to be configured as outputs on the same port, the port data register should be programmed before the direction of the selected port pin is set to output. This is preferably done using MOV or Bit Field instructions. When for example the alternate function of P3.13 shall be used (WR# function on 8xC166 microcontrollers, SSC shift clock output SCLK on most other C166 microcontrollers), and P3.10 shall be used as transmit data output TxD0, this could be done with the following instruction sequence: BFLDH P3,#24h,#24h BFLDH DP3,#24h,#24h NOP ; set P3.13/WR#/SCLK and P3.10/TxD0 to '1' ; switch direction to output ; or other instruction which does not access P3 Semiconductor Group 5 of 7 AP164601 1998-02 Port Initialization and Direction Control However, the following sequence is not recommended: BSET P3.13 ; set P3.13/WR#/SCLK to '1' BSET DP3.13 ; set direction of P3.13 to output BSET P3.10 ; set P3.10/TxD0 to '1' ; CAUTION: P3.13 is not yet switched to output when this instruction reads P3 ; (pipeline effect) ; ==> P3.13 value read from pin, maybe undefined ; ==> P3.13 SFR value may be overwritten with pin value by this instruction ; ==> no WR# strobe or shift clock may be output BSET DP3.10 ; set direction of P3.10 to output ... Therefore, when several pins are to be initialized with single bit instructions on the same port (and this port does not have bit protection), it is important that an instruction which does not access this port is placed between two BSET Px / BSET DPx instruction pairs, and in general after each instruction which has modified the port direction, as follows: BSET P3.13 BSET DP3.13 NOP BSET P3.10 BSET DP3.10 NOP Or, on C level: _pubit (P3,13); _pubit (DP3,13); _nop(); _pubit (P3,10); _pubit (DP3,10); _nop(); When several pins in different bytes of a port (without bit protection) are to be initialized with Bit Field instructions, also care must be taken that subsequent instructions will read the correct port status, e.g. BFLDH P3,#24h,#24h BFLDH DP3,#24h,#24h NOP BFLDL P3,#0Ah,#0Ah BFLDL DP3,#0Ah,#0Ah NOP ; set P3.13/WR#/SCLK and P3.10/TxD0 to '1' ; switch P3.13 and P3.10 to output ; or other instruction which does not access P3 ; set P3.1/T6OUT and P3.3/T3OUT to '1' ; switch P3.1 and P3.3 to output ; or other instruction which does not access P3 ; set P3.13/WR#/SCLK to '1' ; set direction of P3.13 to output ; or other instruction which does not access P3 ; set P3.10/TxD0 to '1' ; set direction of P3.10 to output ; or other instruction which does not access P3 Semiconductor Group 6 of 7 AP164601 1998-02 Port Initialization and Direction Control As discussed before, if interrupts can occur after the instruction which modifies the port data register, and in the interrupt service routines read-modify-write instructions are executed which also access this port, it is additionally recommended to temporarily disable interrupts. The built-in bitfield function of some C compilers allows to modify up to 16 bits of a bitaddressable word. When this function is used, care mut be taken when several pins in different bytes of a port are initialized, since the compiler might generate consecutive BFLDL/H instructions which access the same port, e.g.: _bfld (P3, 0x240A, 0x240A); _bfld (DP3, 0x240A, 0x240A); Keil-Syntax: _bfld_ (... This instruction sequence may result in the following assembler instruction sequence: BFLDH P3,#24h,#24h ; set P3.13/WR#/SCLK and P3.10/TxD0 to '1' BFLDL P3,#0Ah,#0Ah ; set P3.1/T6OUT and P3.3/T3OUT to '1' ; CAUTION: P3.13 and P3.10 are still inputs when this instruction reads P3 ; ==> P3.13 and P3.10 values read from pin, maybe undefined ; ==> P3.13 and P3.10 SFR values may be overwritten with pin values ; ==> no WR# strobe or shift clock may be output BFLDH DP3,#24h,#24h ; switch P3.13 and P3.10 to output BFLDL DP3,#0Ah,#0Ah ; switch P3.1 and P3.3 to output Therefore, this bitfield function should not be used in particular when Port 3 pins are initialized on 8xC166 microcontrollers for Port 1 and Port 0 pins, since these ports are implemented as 16-bit wide ports, while on other C166 microcontrollers, these ports are implemented as two 8-bit wide ports each, like most other I/O ports For ports with bit protection (Port 2, 7, 8), these types of problems will not occur. Semiconductor Group 7 of 7 AP164601 1998-02 |
Price & Availability of AP164601
 |
|
|
|
|
All Rights Reserved © IC-ON-LINE 2003 - 2022 |
| [Add Bookmark] [Contact Us] [Link exchange] [Privacy policy] |
|
Mirror Sites : [www.datasheet.hk]
[www.maxim4u.com] [www.ic-on-line.cn]
[www.ic-on-line.com] [www.ic-on-line.net]
[www.alldatasheet.com.cn]
[www.gdcy.com]
[www.gdcy.net] |