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Muxing I2C question

Started by Mook Johnson October 26, 2014
I have an application where I have 8 identical I2C devices that I cannot 
change the address on.    I need to talk to all 8 in sequence and 
considering a 1:8 MUX.  I'm pondering if I need to MUX both the SCL and 
SDA lines.

If I mux only the SCL line so only the one I'm talking to sees the 
clock, but all of them see the activity on SDA would the device care.

I wouldn't think so because technically it should "see" the activity on 
the SDA without a SCL line toggling.

Just wanted to confirm.

thanks
Den mandag den 27. oktober 2014 01.39.36 UTC+1 skrev Mook Johnson:
> I have an application where I have 8 identical I2C devices that I cannot > change the address on. I need to talk to all 8 in sequence and > considering a 1:8 MUX. I'm pondering if I need to MUX both the SCL and > SDA lines. > > If I mux only the SCL line so only the one I'm talking to sees the > clock, but all of them see the activity on SDA would the device care. > > I wouldn't think so because technically it should "see" the activity on > the SDA without a SCL line toggling. > > Just wanted to confirm. > > thanks
just get one of these: http://www.nxp.com/documents/data_sheet/PCA9547.pdf -Lasse
On 27.10.14 02:40, Mook Johnson wrote:
> > I have an application where I have 8 identical I2C devices that I cannot > change the address on. I need to talk to all 8 in sequence and > considering a 1:8 MUX. I'm pondering if I need to MUX both the SCL and > SDA lines. > > If I mux only the SCL line so only the one I'm talking to sees the > clock, but all of them see the activity on SDA would the device care. > > I wouldn't think so because technically it should "see" the activity on > the SDA without a SCL line toggling. > > Just wanted to confirm. > > thanks
The idle chips may feel sick: If you keep SCL at idle state (up) and toggle SDA, the idle chips will see a stream of start and stop conditions. You may be better off by multiplexing SDA lines and keeping the idle ones up, so the idle chips see plenty of data changes without the start and stop conditions. -- Tauno Voipio