1 Introduction:

PIC microcontroller pin diagram:

In the PIC16F87X microcontroller, the 28-pin microcontroller has 3 I/O ports, namely RA, RB and RC; the 40-pin microcontroller has 5 I/O ports, namely RA, RB, RC, RD and RE. Among them, RA has 6 lines, RE has 3 lines, and the rest have 8 lines. The port
lines of the PIC16F87X port can be used as ordinary I/O pins, and can also be used as external pins of certain components or peripheral modules. For example, port pin RC.4 can be used as an ordinary I/O pin, and can also be used as a data input pin for SPI serial communication. Here we only introduce the basic functions and basic usage of the port, and other complex functions of the port will be introduced later.

2. Two registers related to input/output ports

Each I/O port in the PIC microcontroller has two basic dedicated registers: port data register and port direction register, as shown below.      

These registers have a unified address in RAM, that is, the ports of the PIC microcontroller can be accessed as RAM units without the need for special instructions.

3. Basic structure of input/output ports

The 3/5 ports of PIC16F87X not only have structural differences, but the internal structures of the lines of the same port are also slightly different. However, their basic structural model is shown in the figure below.

There are three D flip-flops (also called latches) in the diagram.

Where Data Latch represents the port data register, such as PORTA, PORTB, PORTC;

TRIS Latch port direction register, such as TRISA, TRISB, TRISC; and Input Latch is used as a buffer when the port is set to input.

3.1 Working Principle of Basic Input/Output Ports

The following are the basic operations on the port line:

⒈Write the I/O direction register TRIS Latch.
 When the content in the direction register is 1, the corresponding port line is set to input (Input); when its content is 0, the corresponding pin is set to output (Output).
 ⒉ To output data through the port pin
   , the port line must be set as output in advance, that is, the content of the corresponding TRIS Latch must be 0.
⒊
  To input data from the port pin, the port line must be set as input in advance, that is, the content of the corresponding TRIS Latch must be 1.

3.2 Application examples of basic functions of input/output ports

The following example is a single-touch-triggered 8-bit binary cumulative counter, which is designed based on the port function and the hardware on the ICD online debugger. The hardware circuit used in this example is shown in the figure below.

 In the figure, port RC is connected to 8 branches, which constitute the output circuit of port RC. Among them, 8 resistors play a current limiting role to protect the port pins and the light-emitting diode LED; the LED emits light at a high level. The figure also uses the RB0 line of port RB as an external input pin. Resistor R4 is a current limiting resistor, which protects the RB0 pin; resistor R21 is a pull-up resistor, which pulls the RB0 level up; switch SW1 is used to manually input a low-level pulse signal.

Programming ideas:

When the power is just turned on, all 8 LEDs are off, indicating that the initial value of the counter is 0.
When the switch SW1 is pressed, the counter value increases by 1, D0 lights up, indicating the binary number 00000001B, and then the button is released; when SW1 is pressed again, the counter value increases by 1, D1 lights up, indicating the binary number 00000010B, and then the button is released again; and so on. When the button is pressed 255 times, D7~D0 all light up, and if the button is pressed again, the counter will return to 0. This cycle repeats.

When designing a button input program, one thing you need to pay attention to is that you must handle the jitter phenomenon when the button is pressed or released to avoid misjudgment. The de-jitter of a button is generally eliminated by calling a delay program. That is, in the program design, when the first level change on RB0 is queried, it is immediately delayed by τ (for example, 10ms), and after the state on RB0 is stable, it is queried again to confirm that it is a key action (pressed or released), it is considered valid, otherwise, it is judged as an interference pulse. The program list is as follows:

STATUS     EQU     03H    

PORTB      EQU     06H    

TRISB EQU 86H

PORTC EQU 07H    

TRISC      EQU     87H     

DATA1 EQU 20H; DATA1 is a delay variable

DATA2 EQU 21H; DATA2 is a delay variable

N1 EQU D'13' ;Outer loop delay constant

N2 EQU 0FFH ; Inner loop delay constant

RP0 EQU 5H; Body selection position RP0

ORG        000H       

     BSF STATUS, RP0 ; switch to RAM bank 1

     MOVLW 00H ; Set port C as output

     MOVWF  TRISC       

     MOVLW 0FFH ; Set port B as input

     MOVWF TRISB           

     BCF STATUS, RP0 ; Restore to RAM volume 0

     PORTC EQU 07H    

     TRISC  EQU     87H     

     DATA1 EQU 20H; DATA1 is a delay variable

     DATA2 EQU 21H; DATA2 is a delay variable

     N1 EQU D'13' ;Outer loop delay constant

     N2 EQU 0FFH ; Inner loop delay constant

     RP0 EQU 5H; Body selection position RP0

ORG      000H       

     BSF STATUS, RP0 ; switch to RAM bank 1

     MOVLW 00H ; Set port C as output

     MOVWF  TRISC       

     MOVLW 0FFH ; Set port B as input

     MOVWF TRISB           

     BCF STATUS, RP0 ; Restore to RAM volume 0

Assembly language often requires more hardware knowledge, so for people who focus on software or have good C language programming skills, I prefer C programming.

The following is a program written in C language:

 include

#define N1 13 //External loop

#define N2 0xff //Inner loop

static volatile bit PORTB0 @(unsigned)&PORTB*8+0; //Port variable design

int DATA1,DATA2;

main() {

           TRISC=0xff; //Input ？？？？？？？  TRISC = 0x00;  //Output

           TRISB=0xff; //Input

           PORTC=0; //Clear

check: if PORTB0==0 then goto check; //Monitor whether there is a level signal input Valid level signal 1

            DELAY(); //Delay de-jitter

            if PORTB0==0 then goto check; //debounce confirmation

            PORTC++; //The advantages of C language design are immediately apparent

CHECK1 if  PORTB0==1  goto  CHECK1;  

             DELAY()

             if  PORTB0==1  goto  CHECK1;

             GOTO  check;

             while(1) {  };

}

void DELAY()   

{

        DATA1=N1;      

   LP0: DATA2=N2;            

   LP1: if DATA2-- > 0 goto LP1;                 

        if DATA1-- > 0 goto LP0;

   return;

  }