Page 74 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Output pin
A logic zero (0) is applied to a bit of the P register. The output FE transistor is turned on, thus connecting the
appropriate pin to ground.

Input pin
A logic one (1) is applied to a bit of the P register. The output FE transistor is turned off and the appropriate pin
remains connected to the power supply voltage over a pull-up resistor of high resistance.

In short, the logic state (voltage) of any pin can be changed or read at any moment. A logic zero (0) and logic one
(1) are not equal. A logic one (0) represents a short circuit to ground. Such a pin acts as an output. A logic one
(1) is “loosely” connected to the power supply voltage over a resistor of high resistance. Since this voltage can be
easily “reduced” by an external signal, such a pin acts as an input.

Port 0
The P0 port is characterized by two functions. If external memory is used, then the lower address byte (addresses
A0-A7) is applied on it. Otherwise, all bits of this port are configured as inputs/outputs. The other function is
expressed when it is configured as an output. Unlike other ports consisting of pins with built-in pull-up resistor
connected by its end to 5 V power supply, pins of this port have this resistor left out. This apparently small
difference has its consequences: Input Configuration If any pin of this port is configured as an input, then it acts
as if its “floats”. Such an input has unlimited input resistance and indetermined potential.

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CITS : E & H - Electronics Mechanic - Lesson 34 - 42