Common SCL for several I2C busses

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Colleagues,

Since many of you have been using I2C bus for a long time, I just wanted to run this idea by you. Is it possible to have two I2C busses that share a common SCL line? Like this:

newguy · Joined: 24 Jun 2004 Posts: 1911

I don't see an issue with it. When bus 1 is active, the devices on bus 2 shouldn't repond at all since they won't see any valid start or stop conditions.

Now that I think of it, I'm not sure how the slave devices will respond to a clock toggling but the data line staying high (I assume). You know for a fact that the slaves will ignore data that's not meant for them. If the data line on the inactive bus idles high, that equates to an address of 0xff - and you'll only have problems if one of your slaves has this address (which I doubt).

To begin, try coding things so that i2c_start() and i2c_stop() put valid start and stop conditions on both busses simultaneously. Then your i2c reads and writes can be directed to one or the other. The inactive bus will then see:

valid start - address 0xff - data 0xff - valid stop

This should pretty much guarantee that the slaves on the inactive bus won't interfere/respond/get screwed up.

If this works, then try sending valid start & stop conditions to only one bus or the other and see if things keep working properly. My first impression is that they will, but I'm not 100% sure. In this case the inactive bus will see the clock toggling but the data line will remain high all the time. I have a sneaking suspicion that the slaves may inadvertently get sent part of a start condition or part of a stop and that may cause some flaky operation when you try to address them.

Hope this makes sense.

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

May I ask why you want to do this?

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

I would share the data line and keep the clock separate.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

jds-pic · Joined: 17 Sep 2003 Posts: 205

if you can't spare the pins on the PIC to synthesize an additional SW-driven i2c port, use an external i2c switch/mulitplexer, e.g. Philips PCA9546A
http://www.standardics.philips.com/products/pca/pdf/pca9546a.pdf

this device also provides a level-translating function.

lots of variants of the above are listed here:
http://www.standardics.philips.com/products/pca/all/

jds-pic