[Ovmsdev] MAX942

Michael Stegen michael at stegen.com
Thu Jul 5 23:03:08 HKT 2012

Yes, William's sparkfun design looks good, and is a lot cheaper then the 
MAX chip.
If you just want a replacement for the MAX942, it's probably the best 


Op 5-7-2012 15:47, Mark Webb-Johnson schreef:
> Michael,
> Seems a bit risky to change at this late stage, and we still need the 
> +5V for the CAN transceiver. I do like the sound of the 18F26K80 
> though - the dual UARTS would be helpful for something I'm working on.
> What do you think of William's suggestion of that sparkfun design? 
> Seems quite elegant, and simpler than the MAX chip now we have access 
> to 3.3v supply.
> Regards, Mark.
> On 5 Jul, 2012, at 9:38 PM, Michael Stegen wrote:
>> Ok i have to correct myself here..
>> The 18*F*2680 will not work on 3.3 volt. You will have to use the 
>> 18*LF*2680 for that.
>> -Michael
>>>> 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
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