GPIO Registers

The basic and important feature of any controllers is the number of gpio's available for connecting the peripherals. PIC16F877A has 33-gpio's grouped into five ports namely PORTA-PORTE as shown in the below table.

As shown in the above table many I/O pins have 2-3 functions. If a pin is used for other function then it may not be used as a gpio.

Though the gpio pins are grouped into 8-bit ports they can still be configured and accessed individually.
Each Port is associated with 2 registers for direction configuration(Input/Output) and for Read/Write.

note: Here 'x' could be A,B,C,D,E so on depending on the number of ports supported by the controller.

TRISx:TRI-State Register/ Data Direction Register

Before reading or writing the data from the ports, their direction needs to be set. Unless the PORT is configured as output, the data from the registers will not go to controller pins.

This register is used to configure the PORT pins as Input or Output. Writing 1's to TRISx will make the corresponding PORTx pins as Input. Similarly writing 0's to TRISx will make the corresponding PORTx pins as Output.

PORTx:
This register is used to read/write the data from/to port pins. Writing 1's to PORTx will make the corresponding PORTx pins as HIGH. Similarly writing 0's to PORTx will make the corresponding PORTx pins as LOW.
Before reading/writing the data, the port pins should be configured as InputOutput.

Led Blinking Example

After knowing how to configure the GPIO ports, its time to write a simple program to blink the Leds.

1. Configure the PORTS as outputs using TRIS registers.
2. Turn ON all the LED and wait for some time.
3. Turn OFF all the LED and wait for some time.

In this tutorial we will use PICC compiler to write the program and generate the hex file.

#include <16F877A.h>

#device ADC=10

#use delay(crystal=20000000)

#define LED PIN_D6

#define DELAY 1000

void main()

{

   while(TRUE)

   {

      output_low(LED);

      delay_ms(DELAY);

      output_high(LED);

      delay_ms(DELAY);

   }

}

In the program we do not assign any numbers to the TRISx register as the compiler do it automatically.