Driving 3.3V I/Os on a 5V device

Question: - Can I drive 3.3V I/Os into a 5V part? - What are the minimum input levels for 5V devices? - Will my 3.3V processor be strong enough to connect to a 5V device? - Will a -1V undershoot on the inputs cause problems?

 

Answer:

A 5V, non-"V" FIFO can be connected to another device that uses 3.3V I/Os under a few conditions.

First, the FIFO must be powered by a +5V power supply. This may mean a different power supply is needed for the FIFO and the 3.3V device.

Second, as defined by the datasheets, all 5V FIFOs have the following input and output electrical characteristics.

 

                                                                                                                                   
    Parameter    Description    Values
              Minimum    Maximum
    Voh    Output HIGH Voltage    2.4 V    See Datasheet
    Vol    Output LOW Voltage    See Datasheet    0.4 V
    Vih    Input HIGH Voltage    2.0 V    See Datasheet
    Vil    Input LOW Voltage    See Datasheet    0.8 V



Based on the table above, all 5V FIFOs can accept 3.3V inputs as long as the device driving it has outputs of greater than 2.0V to meet the Vih minimum, and do not swing below the minimum specified Vil which differs from FIFO to FIFO. Cypress can only guarantee operation of the FIFOs if they are used within specification.

It is also important to note that the 3.3V device must be able to tolerate the 5V outputs coming from the FIFO for this system to work.