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PCF8574: Inquiry about the combination key issue of PCF8574

jie ma1
Prodigy 30 points

Part Number: PCF8574
Other Parts Discussed in Thread: TCA8418E

When using PCF8574 as the input port, currently, the correct value can be read through I2C for each individual key press. However, when two or more keys are pressed simultaneously, only the value of one key is returned. Could you please explain what is going on?

  • 0
    TI__Guru 65368 points

    Hi Jie,

    This is probably because the interrupt is shared amongst all input changes. I am not sure how your software is reading the io expander, but it sounds like it reads while an input is changing state. 

    If you need a device with a FIFO buffer, TCA8418E is the better device to catch multiple key presses in a system. 

    Regards,

    Tyler

  • 0
    Prodigy 30 points

    The problem has been identified. All that is needed is to reduce the I2C speed from 1M to within 400k, and then the values of multiple key presses can be read.