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TCA9548A: Please confirm that the 'RESET' pin must pull-up to Vcc?

diverger
Expert 1275 points
Part Number: TCA9548A

Hi,

In the datasheet there is one words:

"The RESET input must be connected to VCC through a pull-up resistor."

I think there are two possible explaination:

  1. The pin mustn't be connected to VCC directly.
  2. The pin can't be pulled-down, but must be pulled-up.

So, what's the words are emphasizing? 

  • 0
    TI__Expert 8030 points

    Hello, 

    diverger said:
    The pin mustn't be connected to VCC directly.

    We don't recommend shorting RESET directly to VCC because of the large amounts of inrush current that the device will experience when the pin is connected. Inrush current could damage the device depending on how large it is. Putting a pull-up resistor between the VCC and RESET pin connection helps protect the device by limiting the amount of inrush current experienced by the pin.

    Even though we do not recommend shorting RESET directly to VCC, doing so will probably still be okay since the device does have some built-in internal protection.

    diverger said:
    The pin can't be pulled-down, but must be pulled-up.

    Another reason why we recommend connecting the RESET pin to VCC through a pull-up resistor is to ensure that this pin is biased to VCC when it is not being driven low. If the RESET pin is left floating, its value is uncertain and the device could accidentally be reset if the pin is unintentionally driven low by external noise in the system. Connecting the pin to VCC through a pull-up resistor ensures that the device will only be reset whenever it is intentionally driven low by an external device.

    Best, 

    Kenneth

  • 0 in reply to Kenneth Arnold
    Expert 1275 points

    So, does it mean that I can also pull-down it to ground?

  • 0 in reply to Kenneth Arnold
    Guru 317320 points
    Kenneth Arnold said:
    We don't recommend shorting RESET directly to VCC because of the large amounts of inrush current

    Can there really be inrush current? This is a CMOS input with a capacitance of a few pF, II is specified as 1 µA, and the inputs allow up to 7 V regardless of VCC.

  • 0 in reply to Clemens Ladisch
    TI__Expert 8030 points

    Clemens,

    I agree with you that it will probably still be okay if RESET is shorted directly to VCC. The device has some built-in protections that will probably be enough to protect against inrush current. Inrush current is always a factor whenever there is capacitance on a pin, which is why we can't make any guarantees when it comes to shorting a pin to VCC.

    Best,
    Kenneth

  • +1 in reply to diverger
    TI__Expert 8030 points

    Hello,

    Yes, you can pull-down the RESET pin down to ground without damaging the device, however doing this will keep the device in its RESET state by default. The pin would have to be driven high continuously by another source in order to keep the device in a state other than RESET. 

    Best, 

    Kenneth

  • 0 in reply to Kenneth Arnold
    Expert 1275 points

    Yes, I intend to use a GPIO to control this pin, so that I can reset it any time.  So, no matter I pull it up or down, I need to drive it constantly.