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
F2680 will not work on 3.3 volt. You will
have to use the 18
LF2680 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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