Cyclone III I/O standard

Usually, devices from the LVC family.

Generic buffers work well for 5V TTL, for 5V CMOS you really have to go with the level shifters, such as the 74LVC4245A (8 bit): http://www.nxp.com/documents/data_sheet/74lvc4245a.pdf

To be on the safe side, indeed use a level-shifter.

But there is an alternative. Check the chapter "5.0-V Device compatibility" in the CII device handbook. This chapter dissapeared in the CIII device handbook (why?), but the theory is still applicable.

But be carefull, only apply the higher voltage after the device is configured, because the clamping diodes are activated during configuration.

Cheers,

Ton

This suggestion is given without warranty

--- Quote Start ---

This chapter dissapeared in the CIII device handbook (why?)

--- Quote End ---

I guess, the authors finally realized, that it isn't actually safe operation to require the external 5V level to vanish during configuration ... This has been true for previous devices as well, but since Cyclone III, every third handbook page (or so) has a warning keep the Cyclone III voltage limits.

In my opinion, the PCI clamp diodes shouldn't be used for 5V input unless the 5V source is automatically disabled for an unfigured FPGA. Otherwise 74LVC, external schottky diodes or resistive dividers should be used.

Thak you guys.

'cuz I don't wanna take any risks I'll use 74LVC buffer to interface I/O circuit with TTL voltage level :)