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SN74CBT3253: Unidirectional switch? Can it support I2C

Hirotaka Matsumoto
Guru 27525 points
Part Number: SN74CBT3253
Other Parts Discussed in Thread: TCA9546A, TCA9545A, TCA9544A, SN74CBTLV3253, SN74CB3Q3253

 Hi all

Would you mind if we ask SN74CBT3253?

We assume that SN74CBT3253 is unidirectional switch.
It is not possible to use for I2C transmission, right?(I2C requires bidirectional switch.)

So, function table indicates unidirection, right? 
1A to 1B1 and 2A to 2B1
1A to 1B2 and 2A to 2B2
1A to 1B3 and 2A to 2B3
1A to 1B4 and 2A to 2B4

Because, we have to explain the customer why SN74CBT3253 is not used for I2C transmission.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Mastermind 45340 points

    Hirotaka,

    All the passive FET switches like the SN74CBT3253 devices are bi-directional.  All the passive FET switches like the SN74CBT3253 devices can support any protocol signal like I2C as long as the bandwidth and voltage of the protocol is within the recommended operating conditions in the datasheet.  

    You should make sure that you place pull up resistors on both the master and slave sides of the switch as seen in this post below.  

    Why do you need a 4:1 switch in an I2C application? 

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Guru 27525 points

    Adam san

    Thank you so much for your reply!


    Why do you need a 4:1 switch in an I2C application? 
    ->The customer would like to use 1(master):4(salve) multiplexer like as TCA9546A, TCA9544A and TCA9545A.
        However, they would like to switch the port using DI signal. 
       That's why they need DUAL(SDA and SCL) 1-OF-4 FET MULTIPLEXER.

    We would like to confirm two points.

    This device's Vcc is 5V, that why we should use pull-up with 5V, right?
    (In case of Vcc=3.3V, should we use SN74CBTLV3253?)
    And then, are inside FETs back to back construction?
    (If they are not back to back construction, we are concerned about body diode.)


    Kind regards,

    Hirotaka Matsumoto 

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 45340 points

    Hirotaka-san,

    The pull-up resistors on the I2C bus should be at the level of your master and slave.  If the SN74CBTLV3253 devices is used with VCC = 3.3 V and 5V signals are applied to it, it will violate the abs max rating in the datasheet. 

    If you would like to operate the switch with Vcc = 3.3 V and 5V signals on the I2C bus, you will need to use something like the SN74CB3Q3253. 

    What is you concern about the signal switch construction? Switch design information would be considered proprietary.  The datasheet limits will help explain the capability of the device.  Maybe you can explain what your system is trying to do that there might be an issue.

    Thank you,

    Adam

      

  • 0 in reply to Adam Torma
    Guru 27525 points

    Adam san

    Thank you for your reply!

    OK, we got it.

    Kind regards,

    Hirotaka Matsumoto