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PCF8575C: IO's are high at power-up

Art Mecina
Genius 16805 points

Part Number: PCF8575C
Other Parts Discussed in Thread: TCA9535, TCA9555

Hello Guys,

Good day.

Our customer is using PCF8575C on their design. However  all IO's of expander IC are high. How can they initialize all IO's of expander IC to low at power up?

In their application, they are not driving I2c bus, not using INT pin and used only first 8 IOs. Their INT pin is connected to VCC with a  pull up resistor and and remaining 8 IO's are unconnected and configured as outputs.

Thanks in advance!

Art

  • 0
    TI__Guru* 81031 points

    Hi Art,

    "Our customer is using PCF8575C on their design. However  all IO's of expander IC are high."

    This seems a bit unlikely since our device doesn't actively pull up the outputs (it's an open drain output device). The only time the device drives high is during the ACK after the two bytes are sent from a write 'HIGH' transaction, temporarily turning on the rise time accelerator on the outputs.

    The HIGH your seeing may be due to external pull up resistors in the system unless there is leakage current present.

    "How can they initialize all IO's of expander IC to low at power up?"

    None of our devices (IO expanders) power up in an all LOW OUTPUT setting and there isn't any kind of NVM inside of our device to allow for something like that to be programed into it. You would need to program the device through I2C immediately after power up.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 16805 points

    Hello Bobby,

    Just a follow-up inquiry from the customer.

    As mentioned in datasheet that all IO's having Push-pull design structure, at initial power on condition, if all IO's of expander IC are high, then why they are not capable to drive his output?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    TI__Guru* 81031 points

    Hi Art,

    PCF8575C doesn't have a push pull design in the sense that it holds high:

    The High side driver you see on the high level diagram is only active during the write 'high' ACK and used as a temporary rise time accelerator. The non-C version of this device has the push pull drivers.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 16805 points

    Hello Bobby,

    Thanks for the support on this. Customer decided to useTCA9535 or TCA9555 to meet his requirement, as per datasheet at power on it will be in high Z state. Is it OK ? Customer is using only P01 to P07 port pins. He will not be using remaining 8 port pins (P10 to P17) & Interrupt pin. A1,A2,A0 connected to ground. How should he terminate the unused pins?

    If you could direct me to the right person who could answer this, It is highly appreciated.

    Best regards,

    Art

     

  • +1 in reply to Art Mecina
    TI__Guru* 81031 points

    Hi Art,

    unused P-Pins for the 9555 can be left hanging since internally, the pins are biased via an internal pull up resistor. (This device does not power up in a HI-Z state since it has internal pull ups)

    Unused P-Pins on 9535 should be referenced to either GND or Vcc with a resistor.

    INT is an open drain driver, it can be left floating is unused.

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 16805 points

    Thanks Bobby!

  • 0 in reply to Art Mecina
    TI__Genius 16805 points

    Hi Bobby,

    Just a follow-up inquiry to confirm, for TCA9535, at power on condition for first 8 IOs , customer don't want high signal on these pins (that's why he have selected without internal pull up ICs). For these 8 IOs for output mode only, can he use pull down resistors for each IO, so that his circuit will work fine?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    TI__Guru* 81031 points

    Yes,

    This is how I would expect for a low level to be ensured at power up.

    -Bobby