[Ovmsdev] MAX942

Mark Webb-Johnson mark at webb-johnson.net
Thu Jul 5 08:27:00 HKT 2012

For the hardware guys - Michael Stegen, Brian, William - anyone :-)

On the v1 board, we have:

The MAX942 chip is used for the SIMCOM TX -> PIC RX line, to convert the 3.3v SIMCOM async communication to 5v TTL for the PIC.

My understanding of this is that the SIMCOM chip, while expecting 3.3v can accept 5v (TX from PIC, RX to SIMCOM) - and a diode is used on the circuit for this. But on the line from the SIMCOM to the PIC (TX from SIMCOM, RX to PIC), the SIMCOM only gives out 3.3v, but the PIC requires 5v TTL levels. One half of the MAX942 is used for that, and the other half is unused.

I've seen some reference designs for such conversion, but all required both 5v and 3.3v power to handle it. In the original design (two separate boards), we had no 3.3v power on the PIC board, so could only do it the MAX942 way.

Now, in the new v2 design, we have both 5v and 3.3v power lines on the same board, so perhaps we can do it a simpler way. The MAX942 chip is (relatively) expensive.

The China guys are asking what we want to do with this, and suggesting using resistors to handle the 3.3v->5v conversion now. I guess some sort of voltage divider.

Has anyone done anything similar?

Regards, Mark.

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