[Ovmsdev] MAX942

Mark Webb-Johnson mark at webb-johnson.net
Fri Jul 6 21:04:40 HKT 2012


Brian,

Interesting. That was originally used because of the old DX modem, but the thresholds on the SIM908 / SIM900 do seem more generous.

The SIM908 is powered at 3.3V, and the PIC18F2685 at 5.0V.

From the SIM908 manual:



From the PIC18F2685 manual:





That would seem to say that the SIM908 will use 2.7V as it's minimum output voltage, and the PIC18F2685 will accept anything above 2.0V. So, SIM908 TX -> PIC RX should be ok.

For the other way, the PIC18F2685 will transmit at VDD-0.7 - 4.3V, and the SIM908 will accept anything above 2.4V.

From that description, the entire MAX942 circuit is not required. The RX and TX pins of the SIM908 and PIC18F2685 can be directly connected to each other.

What do you think? Perhaps it is time to find my old breadboard and try it...

Regards, Mark.

On 6 Jul, 2012, at 5:28 PM, HONDA S-2000 wrote:

> 
> 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 V PIC TX.
> 
> Brian
> 
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