On Feb 24, 12:41 pm, "Mike Noone" <nleah...@gmail.com> wrote:
> Hi - I have a system that consists of one Atmel SAM7X256 connected via
> CAN to 6 MicrochipMCP2515chips. The MCP2515s have 20MHz clocks, and
> the SAM7X256 has a 55MHz clock. I have the CAN bus running at 1MHz
> right now, but the chips aren't perfectly matched. Specifically, the
> SAM7X256 uses 11TQ, while theMCP2515uses 10TQ. I had to do this as
> otherwise I'd have to have fractional baudrate divisors, which
> obviously is not possible. Communication is perfect without any CAN
> errors whatsoever - but I'm just wondering - is this allowed within
> spec? Or am I just getting lucky? I have the propagation segment on
> the SAM7X256 1TQ longer than on theMCP2515. Is this the best part to
> lengthen? These are my settings:
>
> MCP2515:
> syncseg = 1TQ
> PS1 = 3TQ
> PS2 = 3TQ
> propseg = 3TQ
>
> AT91SAM7X256:
> syncseg = 1TQ
> PS1 = 3TQ
> PS2 = 3TQ
> propseg = 4TQ
>
> Thanks!
>
> -Mike

If the bit times are exactly the same, I think you should have no
problems caused by clock mis-match.  I've often used nodes with
different clocks and different CANbus "time quanta" but I've almost
always been able to match bit times precisely.

(Bus length and speed are still as constrained as always.)

My belief is that the only testing that counts for much is testing
that forces a lot of bus contention (arbitration).  The fact that
nodes can hear each other when the packets don't collide never
convinces me I have a functional bus.  I create a test case with a
heavily loaded bus, and make sure there are many messages whose IDs
are the same for the first many bits.  If THAT runs without retries, I
trust my cabling, transceivers, PropSeg, SynchSeg, sample point, bit
times  etc.

Rick Corey
(Am I paranoid _enough_?)

On 24 Feb 2007 12:41:31 -0800, "Mike Noone" <nleahcim@gmail.com> wrote
in comp.arch.embedded:

> Hi - I have a system that consists of one Atmel SAM7X256 connected via
> CAN to 6 Microchip MCP2515 chips. The MCP2515s have 20MHz clocks, and
> the SAM7X256 has a 55MHz clock. I have the CAN bus running at 1MHz
> right now, but the chips aren't perfectly matched. Specifically, the
> SAM7X256 uses 11TQ, while the MCP2515 uses 10TQ. I had to do this as
> otherwise I'd have to have fractional baudrate divisors, which
> obviously is not possible. Communication is perfect without any CAN
> errors whatsoever - but I'm just wondering - is this allowed within
> spec? Or am I just getting lucky? I have the propagation segment on
> the SAM7X256 1TQ longer than on the MCP2515. Is this the best part to
> lengthen? These are my settings:
> 
> MCP2515:
> syncseg = 1TQ
> PS1 = 3TQ
> PS2 = 3TQ
> propseg = 3TQ
> 
> AT91SAM7X256:
> syncseg = 1TQ
> PS1 = 3TQ
> PS2 = 3TQ
> propseg = 4TQ
> 
> Thanks!
> 
> -Mike

We've been doing this for quite some time, without any problems, using
a master with external CAN controllers with 8 MHz clock (8 TQ at 500
KHz), and the on-chip CAN controllers on slave DSPs that wind up using
20 TQ.  Never had a problem.

If you're not having any problems, I wouldn't change anything.

-- 
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

Hi - I have a system that consists of one Atmel SAM7X256 connected via
CAN to 6 Microchip MCP2515 chips. The MCP2515s have 20MHz clocks, and
the SAM7X256 has a 55MHz clock. I have the CAN bus running at 1MHz
right now, but the chips aren't perfectly matched. Specifically, the
SAM7X256 uses 11TQ, while the MCP2515 uses 10TQ. I had to do this as
otherwise I'd have to have fractional baudrate divisors, which
obviously is not possible. Communication is perfect without any CAN
errors whatsoever - but I'm just wondering - is this allowed within
spec? Or am I just getting lucky? I have the propagation segment on
the SAM7X256 1TQ longer than on the MCP2515. Is this the best part to
lengthen? These are my settings:

MCP2515:
syncseg = 1TQ
PS1 = 3TQ
PS2 = 3TQ
propseg = 3TQ

AT91SAM7X256:
syncseg = 1TQ
PS1 = 3TQ
PS2 = 3TQ
propseg = 4TQ

Thanks!

-Mike