[Ovmsdev] MAX942

[Michael Stegen]

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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>

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