[time-nuts] Form factor

Chris Albertson albertson.chris at gmail.com
Wed Dec 22 22:11:02 UTC 2010


What is the minimum size micro controller that can support CAN?
Every module, not matter how simple will need a uP that can suport
whatever bus we choose.  It would be great if we could get away with
an 8-pin AVR "tiny"  these sell for abut a buck each.  I hate to use
up a minimum 1 square inch of PCB space just to handle the system bus.
  But mmaybe CAN is easy and the software fits in 1K of on-chip flash?
  I don't know.

I really don't think we need anything so complex as CAN.  A HYGE Open
Counter system might have a dozen modules and I'd figure a typical
number might be 2 or 4.
With I2C we'd make up our own message dictioary and the uPs on each
card would run at the "bit level" not using any kind of protocol
stack.    If noise on the wire is an issue then either every cards
needs a balanced driver and we use twisted pair as out physical layer
or we put the single ended i2c inside coax jumpers between cards.

Again I don't care much what is used but I think we need
1) Reasonable speed
2) ALL cards should be peers with no "master"
3) in-band addressing
4) Small minimum size for the uP and low Flash memory footprint

Can "CAN" do this?  I'm worried about #2 and #4.  I don't have time to
read up on this right now and the Wiki article was not detailed enough


On Wed, Dec 22, 2010 at 12:10 PM, Robert LaJeunesse
<rlajeunesse at sbcglobal.net> wrote:
> The I2C bus is moderately well defined for the hardware interface, but hardly at
> all at the software level. This could be problematic for a system that supports
> multiple hardware configurations. It also uses single ended signalling which
> creates RF noise if not treated extremely carefully. Might I suggest the use of
> a more robust, extensible interface with strong software support and
> differential signalling? The best that comes to minde is 2-wire CAN based on the
> CANopen standard. See http://en.wikipedia.org/wiki/CANopen and
> http://www.canopen.us/
>
> Bob L.
>
> ________________________________
> From: Chris Albertson <albertson.chris at gmail.com>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Wed, December 22, 2010 2:58:10 PM
> Subject: Re: [time-nuts] Form factor
>
> The use of i at c that I proposed was ONLY for module-to-module
> communication.  I wrote that none of the internal chips in a module
> were to be exposed to the i2c bus.  A module may very well have it's
> own internal bus but that design detai needs to be hidden from the
> rest of the system.
>
> In an open system built with modules yu realy, have to be careful to
> never expose the inner working on a module.  Else other modules come
> to depend on the internal design of other modules and then you can
> never upgrade of replace any of them because you have a circular
> dependency loop or likely even multiple loops.  So we have to be
> carfull to never expose the kind of chip or it's internal bus address
> between modules.
>
> ...
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-- 
=====
Chris Albertson
Redondo Beach, California



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