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TCA9555: How to have a default 0V at I/O port before I2C is ready

Antony Lin
Genius 13480 points
Part Number: TCA9555

Hi,

My customer is using TCA9555 with Vcc @ 3.3V now.  For I/O port of P00, they are using it as an output, but expect it can be 0V before I2C is ready to configure it.

Currently they implement an external pull-down 100kohm on P00, and expect it can drive P00 to 0V by default.  But the result is as below,

  • By default (I2C is not ready yet) P00 is 1.65V.
  • When P00 is configured as output-high, P00 is 1.65V.
  • When P00 is configured as output-low, P00 is 0V.

I think the 1.65V comes from the combination of TCA9555 internal PU 100kohm and customer external PD 100kohm.  So, it's an expected result, right?

If so, what can we propose for them to achieve their requirement as below?

  • By default (I2C is not ready yet) P00 is 0V.
  • When P00 is configured as output-high, P00 is 3.3V.
  • When P00 is configured as output-low, P00 is 0V.

Thanks!

Antony

  • 0
    TI__Mastermind 23116 points

    Hello Antony,

    I can't think of any way to make it completely 0V.  You could add a much lower resistance pull-down such that the signal is interpreted as a low.  What you are doing right now is essentially a resistor divider until the internal fets are turned on for output-high or output-low.  This will dissipate more current for both set to HIGH or LOW but it will be much worse for when the port is set to HIGH because of the lower resistance pull down you will have to use to insure low voltage at node during startup.

    -Francis Houde