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PCF8575 I/O expander question

Discussion in 'General Electronics Chat' started by ADWSystems, Aug 3, 2017.

  1. ADWSystems

    ADWSystems Member

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    The TI datasheet for the PCF8575 specifically denotes the ability to drive LEDs. But I see little (to nothing) about inputs. How do I tell if I need a pullup resistor on the pins when using them as an input with a dry contact (relay or switch contact; not the output of another chip)?
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi ADW,
    Are you referring to the SDA and SCL input pins, its required to have pull ups, I use 2k2.

    If you are setting the address pins using dry switches, I would use pull ups, say 10k, assuming your switches pull to 0v, when closed.

    E
     

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  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Hi Eric,

    I imagine he's asking about using the I/O pins as inputs - as there's no method of selecting then as IN or OUT it's very confusing.
     
  4. dave

    Dave New Member

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  5. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi Nigel,
    I see what you are saying, I would agree on re-reading the post. :banghead:
    E
     
  6. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    They are designed the same as the Intel port pins When the pins are set high then an input can pull it low..
    This device is basically based on the 8255 parallel port chip..

    They call it Quasi directional!!
     
  7. JonSea

    JonSea Well-Known Member

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    NXP also has this same chip. Their data sheet clears up the mystery. It's "quasi-bidirectional". When you read the register, you are reading tbe state of the pins. When you write to the register, you are setting the port pins.

    From the NXP datasheet:

    SmartSelectImage_2017-08-04-01-40-25-1024x461.png
     
  8. ADWSystems

    ADWSystems Member

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    If you cross out all of the 'output' related sentences above there are only two left, and neither indicate if a pull up or pull down resistor is required for a reliable signal.
     
  9. JonSea

    JonSea Well-Known Member

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    The chip is only going to see a high or low signal. If you afe monitoring switches, you'll need pullups.
     
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