Do not expect the MAX232 to charge pump correctly to the normally generated voltages, to achieve the drive capability and the speed of operation if you do not run the part at 5V. The newer parts are designed with different types of internal circuits.

If you used a 3.3V processor like a Cygnal part then the I/Os of the processor are 5 volt tolerant. In these cases the outputs from a MAX232 that swings 0 to 5 volts could be safely into the processor. For the processor TxD output that would be swinging from 0 to 3.3V you would have to check the Vin characteristerics of the MAX232 part. I looked at the data sheet and see that the logic 1 Vin threshold is 2.0 volts. This means that the MAX232 @5V can work with a Cygnal. Of course you now have to make the circuit board with two power supplies. And do use care to evaluate power sequencing to make sure you know what happens with other components on board that may not be 5v tolerant that are connected either directly or indirectly to the two supplies.

Michael Karas