[Ovmsdev] MAX942

[Michael Stegen]

Op 5-7-2012 2:27, Mark Webb-Johnson schreef:
> For the hardware guys - Michael Stegen, Brian, William - anyone :-)
>
> 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.

Actually i used the internal pullup of the UART in the GSM modem (in my 
original design) as a logic "1", the PIC will pull the line low when it 
wants to send a logic "0". The diode also blocks any 5V from going into 
the modem.

> 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.
>
You can also power the PIC from 3.3v, i don't think that wil cause any 
issues on the CAN transceiver.
The Can transceiver sees a logic "1" at 2 volts, so a level converter 
between these two chips is not needed.
As the X-tal operates at 20Mhz, it should work. According to the 
datasheet the max clock @ 3.3v is 25Mhz

Another way to make it a bit cheaper is to use a 18F26K80, an improved 
version of the 18F2680. But that will likely require some changes to the 
code, as it's not a direct replacement.
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en550199
It also has two UARTS, and comperators, which you can use instead of the 
MAX942.

-Michael