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  1. #1
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    open drain mode

    Hello!

    Can you please tell me what is the difference between push-pull and open-drain mode?

    I searched the net and the wiki, but all answers were confusing. Can you please direct me to some site which explains this in detail or maybe you can explain it with some pictures?

    I have a microprocessor which pins can be configured with push-pull or open-drain mode. I know what is pull-up or pull-down resistors and how they are used, but this push, pull, drain thing is new to me.

    Best regards,

    Rostfrei

    •   Alt28th May 2007, 12:48

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  2. #2
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    open drain push pull

    The push-pull output actually uses two transistors. Each will be on to drive the output to the appropriate level: the top transistor will be on when the output has to be driven high and the bottom transistor will turn on when the output has to go low.

    The open-drain output lacks the top transistor. When the output has to go high you simply turn off the bottom transistor, but the line is now pulled high only by the pullup resistor.

    Your micro allows you to select between the two types, which means that by setting some bits in some register you actually enable/ disable the top transistor and enable/disable the pullup (if internal, otherwise you just disable the top transistor and have to use an external pullup)

    The advantage of the push-pull output is the higher speed, because the line is driven both ways. With the pullup the line can only rise as fast as the RC time constant allows. The R is the pullup, the C is the parasitic capacitance, including the pin capacitance and the board capacitance.
    The push-pull can typically source more current. With the open-drain the current is limited by the R and R cannot be made very small, because the lower transistor has to sink that current when the output is low; that means higher power consumption.

    However, the open-drain allows you to cshort several outputs together, with a common pullup. This is called an wired-OR connection. Now you can drive the output low with any of the IO pins. To drive it high all ouputs have to be high. This is advantageous in some situations, because it eliminates the external gates that would otherwise be required.


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    •   Alt28th May 2007, 17:20

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    push pull open drain

    I couldn't wish for any more clearer answer than this. Thank you very much!



    •   Alt28th May 2007, 20:40

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    push pull vs open drain

    In Push-Pull register, is that used High and Low byte. I would like to know about High and Low byte. Answer me pls.



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    Re: question: push-pull or open-drain mode

    Very good and clear explanation, I could not expect more clear answers. Thanks a lot...! Keep doing good work..!



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    Re: push pull open drain

    Quote Originally Posted by Rostfrei
    I couldn't wish for any more clearer answer than this. Thank you very much!
    +1. Precise and clear.


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