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Thread: I2C PCF8574, PCF8575 I/O expander I can't drive a pin to Output & High

  1. #1
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    I2C PCF8574, PCF8575 I/O expander I can't drive a pin to Output & High

    I've got a little I2C rig setup and I can communicate with an PCF8575
    I can drive a pin LOW but cannot pull it high.
    eg:
    0x40,0xAA,0xAA turns every other pin low but nothing goes high.
    Anyone else using this type of I/O expander.
    Bill
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  2. #2
    Super Moderator Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent Nigel Goodwin Excellent
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    Aren't they really strange? - pins are IN and OUT at the same time, and I 'think' that outputs are open-collector, so require pull-up resistors to work.
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    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
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    According to the datatsheet, they are active low open drain outputs..

    http://www.nxp.com/acrobat_download/...es/AN469_2.pdf
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    What's the 100uA pullup for? Those pins never show high...
    Bill
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    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
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    Is this what you are referring to?


    4. Question: In the data sheet of PCF8575 it is mentioned that the Port output is HIGH
    after power-on. In the data sheet of PCF8575C it is mentioned that the Port output will
    be in 3-state condition after power-on. We found that there is no difference in the block
    diagram and the simplified schematic diagram of each I/O. What is the condition of the
    outputs after power-on?
    Answer: There is in fact a small difference in the schematic diagrams for PCF8575
    and PCF8575C. The PCF8575C lacks the weak pull-up in the form of the 100 μA
    current source in the output I/Os. This is the reason that PCF8575's I/Os remain HIGH
    after power-on and after being written HIGH. Without this current source, PCF8575C's
    I/Os will be in high-impedance state after the initial strong pull-up transistor is OFF (it
    is ON during the acknowledge phase).
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    Thanks Chippie, I'll see if I can source some with the pull ups.
    Bill
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    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
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    no probs m8ty...Happy hunting...
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    Aye, well I haven't found any other revisions aside from C and D. Now I've got to check for pullups on NXT or TI rev D.

    edit:
    The PCA9555 is pin compatible and appears to have the resistors! Thanks Chippie.
    Last edited by blueroomelectronics; 7th July 2009 at 03:32 PM.
    Bill
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  9. #9
    Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent Chippie Excellent
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    Quote Originally Posted by blueroomelectronics View Post
    Aye, well I haven't found any other revisions aside from C and D. Now I've got to check for pullups on NXT or TI rev D.

    edit:
    The PCA9555 is pin compatible and appears to have the resistors! Thanks Chippie.
    Ahh that's good..

    I do have a use then?......
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    Quote Originally Posted by Chippie View Post
    Ahh that's good..

    I do have a use then?......
    But alas, I'm afraid we'll have to sell you for scientific experiments.
    Bill
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