Re: [avr-gcc-list] Anyone interface an AVR with a CAN bus?

[Bruce D. Lightner]

Reza Naima wrote:

> I'm looking at interfacing with the OBD2 CAN inteface that is starting
> to appear on cars these days.  However, the document that describes how
> they encapsulate OBD2 over CAN is not yet available, although it was
> drawn up over 2 years ago.  So I'm not even sure what speed the bus is
> going to operate at either -- but I've got a good logic analyzer that
> should help determine the clocking speed..

Both 500 Khz and 1 Mhz are used in U.S. vehicles, starting with model year 
2003.  That's fast enough that you likely will need a dedicated "CAN 
controller" chip.

In fact there are both SAE *and* ISO standards which apply to the OBD-II 
automotive CAN-bus.  They are in fact "available"...for a fee.  However, 
getting everthing you need is pretty expensive.  (One of the ISO standards 
is less than 5 pages long, and, as I recall, each ISO standard costs 
~$100...what a rip-off!)

> I'm looking at going with the MCP2515 controllre IC, though I'm not sure
> how fast of an oscillator it needs.

For 1 MHz OBD-II CAN, you will need 16 MHz clock with that chip.

> I found one schematic that shows it
> hooked up to a 16mhz resonator.  The data sheet doesn't provide a value
> either, just a description of how to hook it up.  I'm going to draw up a
> schematic, order the parts, and try to get it work.  Wish me luck ;)

You shouldn't have any problems if you follow the Microchip app note. 
I've made it work with a couple of different AVR chips.

> I'm curious, if I want both the MCP2515 and the ATmega16 to operate at
> 16mhz, any issues with having one of them clock the other one to save on
> xtal costs?

You probably would have to use the AVR part as the source of a "common" 
16MHz clock.  As I recall, the MCP2515 oscillator doesn't run until it is 
"turned on", which would require a running AVR part.  Using a 16 MHz 
oscillator chip will surely work, but that may be a power/cost issue.

Sharing the AVR's 16 MHz clock with the CAN controller may be asking for 
trouble because of the extra load.  It *may* work, but given the lack of 
decent detailed electrical specs for both chip's oscillator circuits, I'd 
be afraid to go into "production" with such a solution.

So, are you making your own OBD-II "scan tool"?

Best regards,

Bruce

--
 Bruce D. Lightner
 Lightner Engineering
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