mark at webb-johnson.net
Thu Jul 5 21:47:49 HKT 2012
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.
On 5 Jul, 2012, at 9:38 PM, Michael Stegen wrote:
> Ok i have to correct myself here..
> The 18F2680 will not work on 3.3 volt. You will have to use the 18LF2680 for that.
>>> 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.
>> It also has two UARTS, and comperators, which you can use instead of the MAX942.
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>> OvmsDev at lists.teslaclub.hk
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