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The difference in open-drain and push-pull

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elarson

New Member
I see these terms used in trying to program a microcontroller and haven't found a clear definition of what they are and how they differ. thanks.
 

Roff

Well-Known Member
Open drain can only sink current. You have to provide a pullup resistor if you want to source current. Open drain is useful if you have a bus with several devices on it, any of which can pull down the output to logic "0". Logic "1" is provided by the pullup resistor.

Push-pull outputs can source and sink current.
 

Dean Huster

Well-Known Member
And in digital circuits, push-pull outputs tend to have a higher frequency capability (faster switching speed) than open drain outputs.

Dean
 

elarson

New Member
reply to open-drain and push-pull

So, an open-drain would work the same as a push-pull if it has a pull-up resistor?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: reply to open-drain and push-pull

elarson said:
So, an open-drain would work the same as a push-pull if it has a pull-up resistor?

With some limitations! - you need to be aware that it's open-drain and design accordingly. It's effectively just an on/off switch - when switched on the pin is taken to ground, when switched off the pin is O/C (just as if you'd disconnected the wire from the pin). Using a pull-up resistor is limited by the amount of current the resistor can provide - this current has to be absorbed by the open-drain pin when it's taken low, so it can't provide more current than that will sink.
 
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