Connectiong two boards operating at different voltages

Hopefully someone can help.

Board #1: Cypress EZ-USB FX2-LP (CY7C68013A) microcontroller on pcb operating at 3.3V.

Board #2: Analog Devices (AD669 DACPORT) 16-bit monolithic D/A converter plus other analog IC's operating at 5V.

The Cypress board has 3 of its I/O ports connected to the other board through a 26-pin ribbon cable. We use the microcontroller ports to set bits on the 16-bit DAC which controls a voltage (-10 to +10V) for a piezo crystal. We also connect ground between the boards.

The problem is, a couple of days ago the DAC board died (I don't know why I didn't build that board and that person is not forthcoming with info). Now 2 bits on the Cypress board no longer work. Could a voltage spike come through to my board and kill these 2 bits? If so, is there a way to prevent it? We only use the ports as output pins (see Cypress note below). I also attached my schematic if it is at all useful.

Thank you,
Roberto

Notes from Cypress datasheet
DC Input Voltage to Any Input Pin.. 5.25V[15]
15. It is recommended to not power I/O with chip power is off.

U have connected a 3.3V digital logic compatible device directly to a 5V digital logic which should be always avoided for safety . Check the maximum current delivering capacity of microcontroller board (at the output ports). It should be a minimum of 10µA(max i/p current of DAC) for better derating.

Otherwise U use a driver with 5V Vcc or isolate the two boards with an optocouler with Vcc = 5V (In this case also check whtether current delivering capacity of the microcontroller board with that of the driver or optocoupler).

Dont take risk in connecting a 3.3V logic device directly to a 5V logic device...

Check whether there is any possibilty of increasing the pull up to 5V in the microcontroller port with out any harm...

Narendrakumar,

Thank you for the advice. I will look into isolating the two boards.

Roberto

use a schmitt trigger hex inverter ic like 40106...74hc/hct14....connect the two in series...hence the input is isolated form the output which is 5V....since it changes state
at 3V .....but it is different for motorola ....it changes if the voltage is higher than 1V i think.....check the manufacturers datasheet