Is it safe to just down shift output pins on 5v SRAM?

Rolf van Dam

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If I understand correctly the input pins on the Papilio One are not 5v safe and I should convert any input into the Papilio to 3v3. I only have SRAM modules that output 5v and use a 5v power supply but can read logic at 3v3 (which the Papilio outputs). Is it safe to just downshift the output pins on the SRAM from 5v to 3v3 and connect the address and control pins directly to the Papilio?

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Practically speaking it is usually safe to only provide level translation on only the pins that are output pins on a 5V part. The input pins on a 5V part will work correctly with 3.3V inputs. The thing that could get you into trouble is what happens when the device is being programmed or being held in reset. Some devices might output 5V on all of their pins during programming which would cause a problem in this scenario, so just be sure to consider what happens in all states. With an SRAM module it probably won't behave that way. 


With the Xilinx Spartan 3E and 6 FPGA chips you can provide 5V tolerance on your remaining pins by just putting a 390 ohm resistor in series with the pin you are connecting to. That will limit the current that can flow through the pin and provide 5V tolerance. We use this method with most of our Wings and MegaWings that use 5V chips and it works just fine...



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