[Ovmsdev] MAX942

[William Petefish]

Same problem I had with the BeagleBone. All of the *bone's pins are 3.3v. I
based a couple of level shifters off of a Sparkfun
design<http://www.sparkfun.com/products/8745>.
Basically it is a transistor for the TX line and a resistor for the RX
line. The elegance of this design is it is 4 resistors and one transistor.

William

On Wed, Jul 4, 2012 at 7:27 PM, Mark Webb-Johnson <mark at webb-johnson.net>wrote:

> 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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