PCA9545A and usage with different voltages levels on I2C busses

The VCC operating voltage is from 2.3V to 5.5V.

However the switches themselves can handle 1.8V to 5V translation.  The level shifting is only done with the assistance of external pull ups.

Hi Roland,

Thanks for the answer. This was my understanding as well until I hit figure 3 in the datasheet[:)]. This more or less implies, that you need to run the chip at 2.3V - 2.5V VCC in case you want to use it with 1.8V logic, since it's only in this case the chip ensures not to make the 1.8V I2C bus higher than the 1.8V from the pull-ups. - Right? In case running the chip at 3.3V minimum garanteed I2C voltage is ~2.7V - Correct?

Since I don't have any 2.3V (nor 2.5V) supply in my system, I plan to use a level shifter (txs0102 - http://focus.ti.com/docs/prod/folders/print/txs0102.html) for converting my 1V8 I2C bus to a 3V3 I2C bus. Do you see any problems with this approach?

And a second question: Do you see any problems in cascading two PCA9545A chips (one PCA9545A chip connected to one of the 4 outputs of the first PCA9545A chip - Of cause the chips should be configures for different I2C addresses[:)]). For some of the other chips (like PCA9515A) I know it's problematic to connect the chips in serie due to the directions sensing method. (See i.e. the thread at: http://e2e.ti.com/forums/t/8919.aspx). I can't find any warnings like this for the txs0102 and the PCA9545A chips, but would anyway like to ask if you foresee any problems using any combinations of these two chips? I.e.:
1) Two txs0102 in series
2) two PCA9545A in series
3) a txs0102 after/before a PCA9545A

Best regards and thanks again - It's highly appreciated
  Søren

Well, usually for these I2C open drain translation we recommend using the PCA9306.

1) Two txs0102 in series, do see a problem.  But not quite sure why you would want to do this.

2) Should be okay.  But remember. These are switches, so you need to consider the RON of the device, typically 9-16 ohms depending on the VCC.

3) Should be okay as well, but would like to understand why.

Hi Roland,

Thanks for your answers and sorry for not being able to get back to you prior to now, but the last week has been pretty tough.[:)]

1 & 3) In order to follow up on the thread I'll try to describe my system as I had it while posting the question. It consisted of an OMAP3 (running I2C at 1V8), a lot of I2C peripherals (30+ devices on the 4 paths after the I2C mux)  running at 3V3, except for one single I2C device which was running at 5V0.

In order to interface the OMAP3 to the mux I would need to put a level converter between the OMAP3 and the mux (or run the mux at 2V5 in order to ensure 1V8 operation of the mux). In order to interface the mux to the 5V0 I2C device I would need another level converter, since I needed the 5V0 device to be on an I2C path from the mux together with some 3V3 devices. This was my main reason for asking about having two txs0102 in series with a pca9545a in between.

In the mean time I however managed to find a 3V3 compatible part for my single 5V0 device => I don't any longer have the problem.

2) With respec to the RON for the switches in the  PCA9545A I think I have accounted for them, but thanks for reminding me in case I hadn't...

Best regards and thanks for your fast and helpful support
  Søren

I wanted to mention that this applies to all the series of switches PCA9543A, PCA9544A, PCA9545A, and PCA9548A series of I2C switches.  I wanted to get these part numbers in this thread in case somebody searches for it.