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[General] Push-Pull and Open Drain

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K Srinivas Pavan Kumar

Member level 3
Jun 20, 2015
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Can anybody explain what is Push-Pull and Open Drain Circuits.
Which one is better and which is multi time switching and one time switching and in which protocols we will use them


First, open a browser and type "push pull wiki". You will have a list of push pull definitions.
You can choose push pull output and you get a better definition with images than we could
do here.
Same method with "open drain wiki". Open collector is the same with bipolar transistors.
Now if we suppose you have read this, here is some more info. Note that I didn't read these
pages, so it might be fully described.

Usually the output of a microprocessor is push-pull. But on ARM processors for instance (at
least STM32), you can setup the output as open drain. Usually you don't bother and you use
it as is. But open drain may help you to generate 5V signal from a 3.3V (or less) processor.
If you want to do that, wire the bit you want to set to 5V to VCC through a resistor. If the output
transistor is conductive, the output will be 0. And if it is not conductive, the output will be 5V.

Now I don't know what you call one time switching or multi time switching. For me there is
switching, period. If you could explain what you mean, you would have better chances to
have relevant replies.


In open drain systems, you will need a pull up resistor to make use of the output or use the open drain where you will connect the load and the supply line. This will sink more current.


Another benefit of open drain is the "wired AND" with an automatic prevention of short circuit.
Like in I2C, IIC, TWI... bus lines.
You may have many devices at the bus, all with open drain outputs. Additionally you have a pullup resistor at the bus lines.
Therefore each device can force the signal lines to be LOW, but no device can force it to be HIGH. Only the pullup resistor can generate a HIGH when no devices output a LOW. ( all must be high, or better say "high impedance")

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