How slow is too slow and timing settings

I put a scope on the CAN bus on the bench today and discovered that, due to
a programming error in setting the clock, my CAN bus is running at about 10
kbps rather than 125 kbps.  Can this cause problems?, i.e.: ‘Too’ slow?

I have been reading AN754 (“Understanding Microchip’s CAN Module Bit
Timing”) and checking the timing settings I currently have in the code which
I had copied from some example code I found on the Internet some time ago
when I first started with the MCP2515.

The settings are:
	BLTMODE = 1
	Sample = 1x
	SJW = 1 (0 coded in CFG1)
	PSEG = 3 (2 coded in CFG2)
	SEG1 = 8 (7 coded in CFG2)
	SEG2 = 8 (7 coded in CFG3)


In light of comments here and my readings.  I’m thinking that I need to
change things to:

Given:
       Desired bus rate = 61.442kbps
       Max bus length = 2,000 feet = ~610m
       One-way propagation delay (only cable is significant in my case) =
5ns/m * 610 = 3us
       MCP Clock = 1.8432 Mhz 

	Note: Due to the current PCB design I am restricted to:
	
		MCP Clock = (14.7456Mhz CPU clock) / (2 * (1 + OCR2))
	
	where OCR2 is a control value for Timer-Counter #2 of ATMega1284p
processor)

Using AN754, a spreadsheet and this timing calculator:
http://www.kvaser.com/support/calculators/bit-timing-calculator/

I come up with:

NBT = 16.2760 us
TQ = 1.0851 us
TQs per bit time = 15

PSEG = 5 (actually allows bus up to 1,085 meters)

10 TQ left over for SEG1 & SEG2 allocated as:

PSEG1 = 6
PSEG2 = 4

This results in a sample point of 77.5%

My understanding is that I can use an SJW of 4 which should allow for
maximum clock tolerance.

Misc:
	BLTMODE = 1
	Sample = 1x

Should I consider 3-point sampling?
 
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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