[Ovmsdev] MAX942

HONDA S-2000 s2000 at sounds.wa.com
Fri Jul 6 17:28:50 HKT 2012

On Jul 4, 2012, at 17:27, Mark Webb-Johnson wrote:
> 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.
> Has anyone done anything similar?

Yes, this is very common. Many newer chips are 3.3 V while many older  
chips are 5 V, and sometimes it is difficult to design with all chips  
the same.

The thing to do is start with the data sheets.

A) What is the minimum output voltage of the SIMCOM TX pin?
B) What is the minimum input voltage for the PIC RX pin to detect a  
logic one?

If A is greater than B, then you don't have to do anything at all. I  
have many designs where this is the case.

Looking at the data sheet for the PIC18F2580, on page 433, it shows  
that anything over 2.0 V is considered a one, so long as the PIC is  
getting at least 4.5 V of power. If you're running the board at 5.0 V  
then you're set. Just connect the SIMCOM TX directly to the PIC RX,  
with no resistor, diode, or anything that costs money.

The only exception would be if the SIMCOM dips below 2.0 V output  
under certain conditions.

I'm sure you still need the diode to protect the SIMCOM RX from the 5  


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