RE: How slow is too slow and timing settings

> From: John Dammeyer
> 
> ....
> http://www.intrepidcs.com/support/mbtime.htm

Thanks for that.

> Unless you expect your oscillators to vary widely an SJW of 1 is all you
need.  I
> ran the MCP2515 at 20kbps but with a 16Mhz crystal.  The problem you may
> run into is the addition of other hardware that isn't based on the ATMEL
> clock.  

All of the devices on my bus are of my own design (same board in fact) and
are all ATMega1284p based so I only need to be compatible with myself.  I
suppose there is a very remote possibility that something could come up that
might want to co-exist but I doubt it.  But, I do agree that I should have
used standard CAN bus speeds for the sake of industry compatibility.

> It's one of the reasons to use a standard clock frequency for the CAN
> part and fudge the UART.  Remember the 1.8432MHz crystals are designed
> for best UART frequencies to generate the odd numbers like 57600 or
> 9600 baud.  ....

At the time I designed the board I had used RS-485 but never CAN so I knew
about RS-485 bit rates but was naive on CAN requirements.  In fact, my board
has solder jumpers that would have allowed me to fall back to RS-485 if the
CAN experiment had not worked out ... now I wish I had gone with an Atmel
device with built-in CAN as interfacing would have been easier and I would
have saved board real estate.  Just another step in the learning process :-)
 
Cheers,

Chuck Hackett
"Good judgment comes from experience, experience comes from bad judgment"
7.5" gauge Union Pacific Northern (4-8-4) 844
http://www.whitetrout.net/Chuck


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