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PCA9548A: About asserting RESET_B

Toshiyuki
Mastermind 7075 points
Part Number: PCA9548A

Hi,

The datasheet of PCA9548A describes that POR reset initializes the I2C/SMBus state machine, and asserting RESET_B causes the same. Is it correct? If customer has a state machine error of PCA9548A, do they only have to assert RESET_B? In addition, could they assert RESET_B during accessing I2C?

Best Regards,

Toshiyuki

  • 0
    TI__Genius 17245 points

    Hello Toshiyuki-san,

    I am not sure what you mean by "RESET_B" ??? 

    The reset pin will reset the part when the pin is held LOW.  The part will also be in reset as long as Vcc is less than Vpor.  Both ways initiate the reset state.  The reset can be done at any time.  If you do it during I2C communications then the part should ignore it assuming it doesn't see it's address, but I recommend waiting for an idle state before initiating a reset just as a precaution.

    -Francis Houde

  • 0 in reply to fhoude
    Mastermind 7075 points

    Hi Francis - san,

    I'm sorry for confusing you. It means RESET_B is reset pin. The situations that the reset pin couldn't initiate the reset state has ever occurred for our customer such as state machine error. Please let me know when POR has to be use for reset the part.

    Best Regards,

    Toshiyuki

  • 0 in reply to Toshiyuki
    TI__Guru 62910 points
    Hello Toshiyuki-san,

    Resetting the device through the RESET pin performs the same function as resetting it via POR, so there isn't a reason why a particular method would need to be used over another one. Either method would be effective in resolving errors related to the state machine. I may be misunderstanding your questions, though - please let me know if so.

    Best regards,
    Max
  • 0 in reply to Toshiyuki
    TI__Genius 17245 points

    Hello Toshiyuki-san,

    There are several reasons having a RESET pin is useful, especially in a switch.

    Lets look at the above diagram and assume there is a short between SD0 and GND.  If you were to send a command to connect to the PCA9548A on the SDA and SCL lines to connect to but 0 (SD0/SC0) then all of a sudden the both SDA and SD0 would be held to ground and no devices on the SDA/SCL bus would be able to communicate.  You couldn't even send the command to the PCA9548A to disconnect the SD0 and SC0 lines.  The only way you could regain communication to the PCA9548A or any other devices on the SDA/SCL bus would be to pull the nRESET low to reset the device which by default disconnects everything from SDA and SCL.  The same thing could happen if a slave device got into a stuck bus condition where it pulls the SD0 line low.  The master/micro controller would then be able to determine if that bus is damaged or invalid and thus not allow to connect to it allowing the master to still communicate to channels 2 to 7. 

    I am not sure if you are just asking why the RESET pin is useful, but if that is the questions, then this is one of the answers. 

    -Francis Houde