configurable UART line driver/receiver

[tomk]

Hi,

I've got a design with a 3.3V microprocessor and I need a configurable interface between the UART and the outside world. It needs to be configurable because the micro will need to be able to talk to a few different devices, each with slightly different voltage level requirements on the Tx and Rx lines. The configuration can be set either with digital I/O from the micro (preferable) or with jumpers on the board. In other words, a build time configuration would be fine, but a run time config would be nice.

On the Tx side (from micro to outside world) I need to convert from (0 and 3.3 V) to either (0 and 5 V) or (-5 V and +5 V).

The Rx side (from outside world to micro) I need to convert from either (0 and 5 V) or (-5 V and + 5 V) to (0 and 3.3 V). The Rx lines also need to be able to survive RS232 levels (up to +/- 15 V).

Does anyone know of a part or set of parts that would be a good fit? The only solutions that I've come up with are kind of complicated and cumbersome.

I hope that explanation is clear, but please let me know if not. Thanks in advance for the help!

Tom

 

[betwixt]

On the receive side, a resistor and Zener diode is all you need. The resistor goes in series with the signal and a 3.3V Zener goes across the microprocessor input. It will limit the voltage to 3.3V (you might get away with 2.7V diodes) and clamp any reverse voltage to ground.

For the transmitter side, you could try a MAX3232 without it's charge pump capacitors. It might work for receiving as well, you would have to experiment. Feed either just +5V or +5 and -5V to the pins where the charge reservoir capacitors would connect. I stress I have not tried this but it could be a simple and very inexpensive solution. Be careful, the MAX will also invert the signals but you need that in many cases anyway.

Brian.

 

[FvM]

I hope that explanation is clear.

Although the explanation is almost clear, it doesn't seem to make much sense in the context of serial interface standards. The only configurable interface chips I'm aware of can be switched between RS-422/485 (differential 0/5 V) and RS232 (singled ended +10/-10 V or reduced level). But beyond existing standards, everything is possible of course. Betwixt already mentioned the option to modify operation of standard RS-232 drivers. The commonly used RS-232 receivers have a positive threshold and can work well with +5/0V "poor man's" RS-232.

 

[tomk]

Thanks to betwixt and FvM for your responses. That's plenty to get me off and running.

I know it doesn't make sense in the context of serial interface standards, but it's what I've got to deal with. Thanks again!