Two I2C devices with same address, and only 1 spare pin!

[Hans Wedemeyer]

Sometimes it's difficult to use I2C devices when the address is hard coded in the chip.
I need to use to chips that have no means of setting the address. In a different design I simply used two (soft) I2C ports.

The current project needs the same two chips and to complicate the issue I only have one spare pin.
So making the second I2C port is not possible without some juggling.

Sharing the SDA pin should work, if I remember correctly the SCL wakes up the chips.

Has anyone done this?
Did it work and are there issues to watch out for ?

Thanks..

#use i2c(Master,Fast,sda=PIN_E0,scl=PIN_E1) // for chip #1

code here for access to chip #1

#use i2c(Master,Fast,sda=PIN_E0,scl=PIN_E2) // for chip #2

code here for access to chip #2

[avro698]

You could look at using an I2C multiplexer, try this http://www.standardics.nxp.com/products/pca/pdf/pca9543a.pca9543b.pca9543c.pdf
This would allow both IC's to exist on their own "sub bus", so address conflicts will not occur. The multiplexer will control the data traffic between master device and the selected slave peripheral.
The advantage here is that you do not use any extra pins, the switching is done using I2C commands.
The overhead is that you must provide extended code to actually perform the switch between the two sub devices.


Hope this is of help

ATB 698

[Hans Wedemeyer]

[adrian]

[ckielstra]

Or, just brainstorming, use the extra pin to activate a cheap 74xx-series external multiplexer which switches the CLK and SDA lines. This way you can create two totally separated I2C busses served by a single (hardware) master.

[treitmey]

I think he said that he has no room for more chips.((in the subject line))
what if these are software I2C?
and you revese clock and data on one of the chips.
that way, at one time B1-->clk b2-->sda
then when talking to other chip
B2-->clk and B1-->data.

? ? ? Any chance that would work??

[rnielsen]

I've had to do something similar to this. I had four sensors with the same hard coded address that I wanted to connect to the same I2C bus. I used a SN74CBT3253 mux chip for this. It will mux one bus with four devices. S0 & S1 are the pins used to select which channel you want to use. If you only have one spare pin, on your PIC, you could tie S1 to GND and simply toggle S0 for which device you wanted to talk to. Simply set or clear S0 before you start your I2C communications routine.

Since the inactive channel will be disconnected from the I2C bus you will need to make sure that each Slave has pull-ups on each side of the mux chip, ie. pull-ups on the PIC side and pull-ups on each of the Slave's bus. If you don't do this the Slave will see a clock pulse when that channel is disconnected and reconnected. They come in surface mount and are rather small in size.

Ronald

[Donlaser]

I use a scheme similar to the proposed one by Adrian to control two TDA7449 i2c audio preamp with two data lines and a data line for the shared clock.

[Ttelmah]