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PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
PM7322
3.3V SRAM INTERFACE FOR RCMP-800
Issue 1: October 7, 1996
PMC-Sierra, Inc.
105-8555 Baxter Place, Burnaby, BC Canada V5A 4V7 604 415 6000
PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
TABLE OF CONTENTS REFERENCE ................................................................................................. ii INTRODUCTION .......................................................................................... 1 CIRCUIT DESCRIPTION .............................................................................. 1 SUMMARY ................................................................................................... 3 NOTES ......................................................................................................... 4
i
PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
REFERENCE [1] [2] [3] PMC-940904, "PM7322 RCMP-800 Datasheet", Issue 5, PMC-Sierra Inc. App. Note 1, "Zero Delay Bus Switches", Pericom Databook. AN-966, "Using the Bus Switch as a 5V to 3V Translator", National Semiconductor
ii
PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
INTRODUCTION The choice of Synchronous SRAM's to be used with the RCMP-800 (which will be referred to as simply RCMP1) is important from both a technical and cost perspective. Currently, 3.3V Synchronous SRAM's are becoming more common, as mainly driven by demand in the PC market. In general, 3.3V SRAM's dissipate lower power, and have lower costs due to higher levels of integration. However, the majority of these 3.3V SRAM's are not 5V I/O tolerant. In order for the 5V RCMP to take advantage of these 3.3V SRAM's, simple level-translation logic can be used to overcome this 5V tolerance problem. This appnote illustrates how the RCMP can be interfaced to 3.3V Synchronous SRAM's, using very low-power, close to zero-delay bus switches.
CIRCUIT DESCRIPTION Fig.1 shows 5V-to-3.3V interface between the 5V RCMP and 3.3V SRAM's. It illustrates how any signal, uni-directional or bi-directional, can be connected between the 5V circuitry (the RCMP) and the 3.3V circuitry (the SRAM). For the RCMP SRAM interface, the relevant signals are the address, data, control (address strobe, write strobe, output enable) and clock. Fig. 1 5V-to-3.3V Level-Translation Interface
1Note that this appnote applies equally to PM7323, the RCMP-200.
1
PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
The bus switch shown is simply a N-transistor with its gate controlled by the byte-enable (BE, active-low) input. The byte-enable is tied low in this application, which causes the gate of the transistor to be high. The transistor will turn on if either the drain or the source is one threshold voltage (VT) below the gate. The source and drain act as the I/O ports of this level translator. When the transistor is turned on, the switch becomes a very low series resistance, typically about 5 ohms, which in effect connects the input directly to the output. The switch acts as a level translator by clamping the signal level at the I/O's to at most one V T below the VCC. By changing the VCC, the user can therefore control the maximum voltage seen at the I/O's. Since the switch becomes effectively only a resistor, there is close-to-zero delay (max. 250ps) incurred. Also, typical power dissipation is extremely low, about 0.1mA, or 0.2uA for low-power versions. The switch is inherently bidirectional since the transistor is symmetric. As shown in Fig. 1, V CC is limited to about 4.3V by the diode from the 5V supply. A diode that can maintain a constant voltage drop, such as the IN4001, would be suitable. R1 provides the forward biasing current for the diode, in order to maintain a constant voltage drop across the diode. A typical value for R1 is 2.2K ohm. The bypass capacitor, C, provides decoupling for the reduced V CC. A typical value for C is 0.01uF. Also, optionally, R2 can be used to pull-up the signal on the 3.3V side, to ensure that the high voltage level will be close to 3.3V. Similarly, R3 can be used to pull-up the signal on the 5V side to give a 5V high voltage level. Typical value for R2 and R3 is 1K ohm. Table 1 shows a list of vendors offering a variety of these bus switches 2. The most common version is the dual 5-bit bus switch, the 3384, which all four vendors offer. Pericom and Quality Semiconductor have the largest selection of bus switches, tailored for many applications. The parameters that distinguish these bus switches are: power dissipation, configuration, package, and on-resistance. In particular, there are 25-ohm on-resistance versions that provide series resistance to help reduce reflections, which can be applicable to clock signals. Also, the variety of packages allow the user to optimize the board area, or to maintain footprint compatibility with devices such as 74XXX245.
2All of the listed vendors have Internet web pages available.
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PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
Table 1 Bus Switch Vendors
Vendor Cypress Part Number Configuration ICCQ (Typ.) 0.1mA 0.2uA 0.2uA 0.2uA 0.2uA 0.2uA 0.2uA 0.2uA 1.5mA(max) 0.2uA 1.5mA(max) 3mA(max) 3uA Package 24 DIP/SOIC/QSOP 24 DIP/SOIC/QSOP 24 DIP/SOIC/QSOP 24 DIP/SOIC/QSOP 24 DIP/SOIC/QSOP/TSSOP 48 TSSOP 24 SOIC/QSOP 48 SSOP/TSSOP 24 PDIP/SOIC/QSOP/HQSOP 24 PDIP/SOIC/QSOP 24 PDIP/SOIC/QSOP/HQSOP 48 QVSOP 24 SOIC/QSOP
CYBUS3384 Dual 5-bit CYBUS3L384 Dual 5-bit 74LVX3L384 PI5C3384
(25 ohm) PI5C32384
National Semi. Pericom
Dual 5-bit Dual 5-bit Dual 5-bit Dual 10-bit Single 10-bit Dual 10-bit Dual 5-bit Dual 5-bit Dual 5-bit Dual 10-bit Single 10-bit
(Flow-Through Pinout)
PI5C32X384 PI5C3861 PI5C16861
QS3384 QS3L384 (25 ohm) QS32384 QS32X384 (Flow-Through Pinout) QS3861
Quality Semi.
For the RCMP SRAM interface, the total number of signals that needs to be leveltranslated will be: 20 bits address, 45 bits of data, 3 control signals and a clock, which is equal to 69 bits. In this case, 3 of the dual 10-bit switches and 1 of the single 10-bit switches can be used to provide this interface.
SUMMARY This appnote shows how the RCMP can be interfaced to 3.3V SRAM's using very simple bus switches. These switches incur almost zero delay, and dissipate very little power. Also, the board area needed can be minimized with the small packages available.
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PMC-Sierra, Inc. APPLICATION NOTE
ISSUE 1
PM7322 RCMP-800
3.3V SRAM INTERFACE FOR RCMP-800
NOTES Contact us for applications support:: FAX: PHONE: Email: Website: (604) 415-6206 (604) 415-6000 apps@pmc-sierra.bc.ca http://www.pmc-sierra.com
Seller will have no obligation or liability in respect of defects or damage caused by unauthorized use, mis-use, accident, external cause, installation error, or normal wear and tear. There are no warranties, representations or guarantees of any kind, either express or implied by law or custom, regarding the product or its performance, including those regarding quality, merchant ability, fitness for purpose, condition, design, title, infringement of thirdparty rights, or conformance with sample. Seller shall not be responsible for any loss or damage of whatever nature resulting from the use of, or reliance upon, the information contained in this document. In no event will Seller be liable to Buyer or to any other party for loss of profits, loss of savings, or punitive, exemplary, incidental, consequential or special damages, even if Seller has knowledge of the possibility of such potential loss or damage and even if caused by Seller's negligence. (c) 1996 PMC-Sierra, Inc. PMC-961022P1 Printed in Canada Issue date: October 7, 1996
PMC-Sierra, Inc.
105-8555 Baxter Place, Burnaby, BC Canada V5A 4V7 604 415 6000

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