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Different types of Ports in 8051

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The differences vary depending on the specific 8051 variant to which you are referring.

Depending of the specific device, the pins could be attached to several different types of peripheral modules, ADC, DAC, PWM, UART, SPI, TWI, etc.

BigDog
 

Jeffrey Peter said:
I'm using 89s52... I found that the port 0 is a open drain. What is the function of open drain and advantage of having an open drain ?
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If you use open drain as output then you have to use pull up resistors from open drain outputs to Vcc.
 

Open drain output pins does not have pullup resistor inside, just a mosfet at ground side. If you don't have special requirement of open drain pins, just connect external pull up resistor. You can use 9pin SIL resistor pack for total port.

In case of open drain output: for low output, the mosfet conducts whereas for high output it doesn't. But you will not get +5V at output if you don't use pullup resistor.

The benefit is that you can connect two or more such outputs together. If any output is low, you get low level, otherwise high (sort of AND gate).
 
One of the important characteristics to remember of open drain/collector outputs is they can only sink current, never source current.

They essential only offer two states 0V (Ground) or High Impedance.

You might find the information contained in the following of interest concerning open drain/collector outputs:

Open Collector Outputs



Open Collector

Open Collector Outputs

Basics: Open Collector Outputs

Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputs




Those should keep you busy for a while.

BigDog
 
in 8051, port 1,2,3 is having internal pull up resistor, but port 0 doesn't have a pull up resistor.
 

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