Microcontrollers 8051 Input Output Ports

 8051 microcontrollers have 4 I/O ports each of 8-bit, which can be configured as input or output. Hence, total 32 input/output pins allow the 

• Pin configuration refers to the arrangement of a pin such that, depending on the logic state, it can be set to 1 for input and 0 for output.

•  The microcontroller's input/output (I/O) pins are connected to all of its circuits, with the exception of the P0 port, which is not equipped with pull-up resistors.

•  The input pin receives logic 1 applied to a portion of the P register, turning off the output FE transistor while keeping the other pin connected to the power supply voltage through a high-resisting pull-up resistor.

• Port 0: There are two functions that define the P0 (zero) port.

• The lower address byte (addresses A0A7) is applied to the external memory when it is used; otherwise, all bits of this port are set up as input/output.

• Other ports with built-in pull-up resistors connected by their ends to a 5V power source are connected by pins to the P0 port when it is configured as an output; this port's pins do not contain this resistor.

When a pin of this port is configured as an input, it acts as if it "floats," meaning that it has infinite input resistance and undetermined potential. 

When a pin is configured as an output, it acts as a "open drain." When logic 0 is applied to a port bit, the corresponding pin is connected to ground (0V), and when logic 1 is applied, the external output continues to "float." To apply log

Port 1

P1 is a true I/O port as it doesn’t have any alternative functions as in P0, but this port can be configured as general I/O only. It has a built-in pull-up resistor and is completely compatible with TTL circuits.

Port 2

P2 is similar to P0 when the external memory is used. Pins of this port occupy addresses intended for the external memory chip. This port can be used for higher address byte with addresses A8-A15. When no memory is added then this port can be used as a general input/output port similar to Port 1.

Port 3

In this port, functions are similar to other ports except that the logic 1 must be applied to appropriate bit of the P3 register.

Pins Current Limitations

• When pins are configured as an output (i.e. logic 0), then the single port pins can receive a current of 10mA.

• When these pins are configured as inputs (i.e. logic 1), then built-in pull-up resistors provide very weak current, but can activate up to 4 TTL inputs of LS series.

• If all 8 bits of a port are active, then the total current must be limited to 15mA (port P0: 26mA).

• If all ports (32 bits) are active, then the total maximum current must be limited to 71mA.