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TCA9555: INT Pin possibly preventing our system from Booting properly

Oscar Zuniga
Prodigy 120 points
Part Number: TCA9555
Other Parts Discussed in Thread: TCA9539-Q1

Hello, 

We use the TCA9555 chip to interface with a cable to turn on LEDs and be used as inputs for two buttons. P00 - P07, P10 power LEDs. P11 - P12 are inputs for two buttons. The port pins are driving the LEDs directly, we do not have them set up like in Fig 35 and Fig 36 of your datasheet. Unfortunately, we have the INT pin connected to a boot strap configuration pin that needs to be held high during initial boot. It looks like the INT pin holds the configuration pin low at random at initial boot. After measuring the pin at boot when the cable is connected to the TCA9555 chip and going through a power cycle, the INT pin acts crazy, as shown below: 

When the cable is removed, the INT pin on the TCA9555 acts normal and allows the boot configuration pin to go high as intended: 

What would be causing the INT pin to be acting crazy at boot? Is there a way to delay the interrupt pin from impacting the boot configuration? Are the ports driving the interrupt pin to go low at initial power on? Any insight would be greatly appreciated. 

  • 0
    TI__Guru 50795 points

    Hi Oscar,

    Interrupts for the TCA9555 are triggered when the state of an input pin differs from what is held in memory. Since all pins are set as inputs by default, this noise on the INT/ pin is likely a result of voltages on the cable being sporadic during initialization.

    To stop the device from driving the INT/ line low during boot, I can think of a couple of options.

    1. Keep the TCA9555 unpowered (Vcc < Vpor) until after INT/ line sensitivity.

    2. Configure all I/Os on TCA9555 as outputs before boot. Since interrupts only trigger on input pins, this would prevent any interrupt conditions from being met during the boot period. Once the INT/ sensitive period is over, reconfigure TCA9555 inputs. Note: changing pins from outputs to inputs will likely cause false interrupts. Interrupts triggered during this configuration should be ignored. Care should also be taken that the input devices (push buttons in this case) can withstand the levels that TCA9555 may drive on the pins while configured as outputs.

    Sadly, there is no way to delay the TCA9555 interrupt feature after Vcc > Vpor.

    If possible, you may consider using TCA9539-Q1. This part is similar to the TCA9555 except for the removal of internal I/O pull-ups (power saving) and the replacement of A2 with /RESET. This device could be held in reset during boot to avoid triggering on the INT/ line.

    I hope this helps. Let me know if you have any more questions.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 120 points

    Hi Eric, 

    Thanks for the reply. We delayed powering the TCA9555 by modifying the SS on our regulators. This has allowed our embedded system to boot up properly before the INT pins could have an effect on the overall system. We have taken into account the impact the INT pin might have on the overall system for our next revision. 

    Thanks