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SN74CB3T3245: I2C interface with SN74CB3T3245 bus switch

Georgii Palagin
Prodigy 20 points
Part Number: SN74CB3T3245
Other Parts Discussed in Thread: SN74LVC2G66

I have a question about using SN74CB3T3245 as a bus switch for I2C interface. No voltage translation needed, 3.3V levels at both sides of the switch. SDA is bidirectional, SCL is unidirectional, both are open drain. Will it work in the configuration shown below? Are any resistors excessive/missing?

  • 0
    Guru 276980 points

    This switch will work fine. (For only two channels, a smaller device like SN74LVC2G66 would be more suitable.)

    There must be a pullup on I2C2_SCL to prevent this line from floating when the switch is open. (The I2C2 pullups are needed only when the bus is inactive, so you can use larger values if you want to save a little bit of power.)

  • 0
    TI__Mastermind 27818 points

    Georgil,

    I have the same note as Clemens here. You have a 8 channel device and only using 2. Is there a reason for this or can you replace this with a smaller device that may be more suitable such as the SN74LVC2G66. This will also be a general purpose mux rather than a level shifting one, which the 3245 is. Additionally, an extra pullup on the I2C2_SCL to pullup the state when OE\ is high (switch open) would keep the line from floating in that state.

    All TI multiplexers are bidirectional, so you don't need to worry about which signals are bidirectional and which are unidirectional when picking a TI multiplexer.

    The configuration you have will work though. Picking a more suitable mux though would simply optimize the solution. 

    Thanks,
    Rami

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Thanks very much. As for smaller devices - there are other push-pull signals I connect to it, I just didn't mention them in the picture. I only wanted to know how it works with open drain signals. I've also found another TI thread similar to mine (but there was 2.5 <-> 3.3 translation there), and TI answer was that it was ok NOT to place pull-ups on the 2.5V side for I2C, which sounds strange to me. I suppose both sides need pull-ups.

    https://e2e.ti.com/support/logic/f/151/t/246190?CB3T-and-I2C