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Original question:

TCA9546A: Do we have I3C 4-ch switch like TCA9546A?

TCA9548A: Cascading multiple I2C switches with each downstream channel connecting to multiple slaves for two switches.

Owais Malik
Prodigy 50 points
Part Number: TCA9548A

Hi,

For multiple I2C switches, somebody mentioned that we need to keep an eye on ViL of I2C if we have multiple switches in the path. I was confused with the answer because only one device is able to answer to master which means that only one slave device will be asserted low when it talks to master. Why should we worry about the current limitation on I2C line when its low. Can you please clarify. I want to use three I2C switches in series with two of the switches connected to multiple slave devices on their respective downstream channels as shown below. Do  I need to worry about any thing like overall capacitance and driver strength etc in below circuit diagram. FYi the third switch is just to isolate another master.

Regards

Owais

  • +1
    Guru 317280 points

    Each switch has an on-resistance and thus adds a little voltage drop, and the pull-up resistors on all active bus segments act in parallel. Furthermore, low-voltage (1.8 V) devices often do not have much drive strength.

  • 0
    TI__Mastermind 48158 points

    Hi Owais,

    Clemens is correct. Cascading of multiple I2C switches results in a small RDS_ON voltage drop across each active channel, since each active channel is essentially a passFET. With each channel ON, the pull-up resistors present on all channels also appear in parallel. TCA9548A is not a true buffer, and doesn't separate one side from another in that way.

    For example, if you have all 8 channels ON at once on the right-most TCA9548A in your diagram, you can expect all 8 channels' parasitic bus capacitance to combine which could effect your overall RC time constant (rise-time spec). Also, you will see each pull-up resistor on each channel appear in parallel, which will effect your Vol and could potentially require a stronger I2C driver. 

    Regards,

    Tyler