Well Sanjeev Kumar....Since you need specific implementation experience to go on.....I have used a 3.3V Cygnal implementation to drive the 5v logic inputs to:

- SRAM chips
- LCD Modules
- RS232 drivers
- Lattice CPLDs

In all cases I configured the Cygnal outputs to push-pull and then made sure from the data sheets that the logic '1' requirement of the inputs to the 5 volt logic were such that an input in the 2.7 -> 3.3v range was high enough to ensure that the input was indeed high. In cases where I am interfacing to something bi-directional like the data lines to an LCD module I have logic in the read subroutine which turns the pins back to bi-d mode before asserting the -RD line. After the -RD is takes away I set the lines back into push-pull output mode.

I can think of two cases where I used the Cygnal parts in situations where the 3v push-pull outputs were not swinging high enough for the 5v logic. In these cases I put pullup resistors on the signal lines to +5v and programmed the Cygnal I/O pins to the normal bi-d mode (ie open drain bi-directional mode). In one case it was a slow logic system and so the pullups were 10K ohms and the low->high transitions were several microseconds. For one faster setup I was bit banging a 5v Optrex graphics LCD module. With that I used 1K pullups to acheive a pullup transition time at less than 40 nsec.

Thus I would not hesitate for one second to connect a 3v Cygnal part's TxD line to the input of a MAX232 chip where that chip was powered from 5v, provided that you arrange for the port pin where the TxD comes from is programmed for push-pull output. Likewise I would not hesitate to connect the RxD input of a 3v Cygnal part directly to the receiver output of the MAX232 chip. Whilest the output of the MAX232 receiver will swing up to near 5v this is of no care to the Cygnal part because its inputs are just happy as peach jam with a 5v input level. Note that the pin to which the RxD line is connected will (on most Cygnal Parts) be automatically forced to an input it is still a good idea to make sure NOT to program that particular port pin onto push-pull output mode.

You should have no need to spend time, money, and energy to design in an extra translator/buffer circuit.


Michael Karas