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PCA9555: How to send a low pulse through an action?

Vincent Su
Prodigy 20 points
Part Number: PCA9555
Other Parts Discussed in Thread: SN74LVC1G57, SN74LVC1G123

HI sir,

We now have a CPU connect the reset pin on PCA9555 P16, but when we enable the reset function (set the P16 to Low) the CPU cannot perform actions.

The reset function as I know is

1. set P16 to low

2. wait XX ms

3. set P16 to High

Due to the CPU is under ready to reset, it can not control PCA 9555 anymore. So the CPU reset is keeping at low.

Is there any way to send a low pulse from one command?

thanks!

Vincent

  • 0
    Guru 316780 points

    The PCA9555 (or any other I/O expander) cannot generate timing patterns.

    To generate a pulse, use a monostable multivibrator like the SN74LVC1G123 and an inverter. Alternatively, use an XNOR gate with Schmitt-trigger inputs (e.g., SN74LVC1G57) with an R-C delay at one input.

  • 0
    TI__Mastermind 47448 points

    Hi Vincent,

    What problem are you trying to solve? Are you asking if there is a toggle output on the PCA9555? 

    Are you asking what parameters are needed for a successful reset of the PCA9555? This can be found in the information underneath the power supply recommendations in the datasheet. 

    What do you mean by "Is there any way to send a low pulse from one command?"

    Regards,

    Tyler

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Yes, that what I want to do. 

    Due to the GPIO of the CPU itself is fully used, so its GPI[0:3] and GPO[0:3] are implemented with PCA9555.
    As I say, when the CPU enable the reset function (set the P16 to Low), the whole system will be stuck until P16 becomes "HIGH" (but this is already impossible to control)

    If we add a SN74LVC1G123 for use, and the PCA9555 only send the "LOW" signal. The SN74LVC1G123 will not to output, right?

    Is there any other way to trigger(low trigger IC or something) a pulse when PCA9555 keep "LOW".

    If so, we can set the PCA9555.P16 to HIGH after the system restart.


    thanks!

  • +1 in reply to Vincent Su
    Guru 316780 points

    The '123 can be triggered by a falling edge on /A or a rising edge on B:

    So when you send a high pulse on /A or a low pulse on B (as shown above), then the reset happens only at the end of the pulse. (In any case, the /A and B inputs are edge-triggered, so nothing will happen as long as P16 does not change.)