Electronic – Open-drain microcontoller ports


I want to understand the concept of open-drain ports like port P0 of the 8051 microcontroller. Why do we need to connect pull up resistors to the port P0?

I have basic knowledge of MOSFETs and other electronic devices.

Best Answer

Ignoring the ins and outs of how mosfets work. The output acts like a switch to ground. It cannot produce a high (+5V) signal by itself. The pull up resistor is used so that when the switch is open the output will be high. When the switch is closed the output will be low (0V)

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