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Using UART of PIC Microcontroller – MPLAB XC8

Ligo George

UART Communication

UART stands for Universal Asynchronous Receiver / Transmitter. It is a very popular serial communication interface which provides Full Duplex communication between two devices. UART uses two data lines for sending (TX) and receiving (RX) data. Ground/Reference of both devices should be made common. As the name indicates it is an asynchronous communication interface, which means that it doesn’t need to send CLOCK along with data as in synchronous communications. UART is the communication interface used by our old computer’s RS-232 port.

Some of the Microchip’s PIC Microcontroller has built in USART Module. USART stands for Universal Synchronous Asynchronous Receiver Transmitter and it can be configured in following modes.

  1. UART – Asynchronous (Full Duplex)
  2. USRT Master – Synchronous (Half Duplex)
  3. USRT Slave – Synchronous (Half Duplex)

In this tutorial we will learn how to use UART Mode of USART Module using MPLAB XC8 Compiler. For demonstration we are using PIC 16F877A microcontroller.

PIC 16F877A USART Module

USART Registers

TXSTA – Transmit Status and Control Register

TXSTA – Transmit Status and Control Register

RCSTA – Receive Status and Control Register

RCSTA – Receive Status and Control Register

USART Baud Rate Generator (BRG)

Baud Rate Generator provides the required clock for the data transmission and reception. USART module has a dedicated 8 bit baud rate generator which supports both Synchronous and Asynchronous modes. The 8-bit SPBRG register controls the time period of this free running timer. In Asynchronous mode BRGH, 2nd bit of TXSTA register also controls the generated baud rate but in Synchronous mode it is ignored. Baud Rate can be calculated from the following equations, where FOSC is the clock frequency of the microcontroller.

PIC USART – Baud Rate Formula

MPLAB XC8 Programming

Initializing UART

char UART_Init(const long int baudrate)
{
  unsigned int x;
  x = (_XTAL_FREQ - baudrate*64)/(baudrate*64);   //SPBRG for Low Baud Rate
  if(x>255)                                       //If High Baud Rage Required
  {
    x = (_XTAL_FREQ - baudrate*16)/(baudrate*16); //SPBRG for High Baud Rate
    BRGH = 1;                                     //Setting High Baud Rate
  }
  if(x<256)
  {
    SPBRG = x;                                    //Writing SPBRG Register
    SYNC = 0;                                     //Setting Asynchronous Mode, ie UART
    SPEN = 1;                                     //Enables Serial Port
    TRISC7 = 1;                                   //As Prescribed in Datasheet
    TRISC6 = 1;                                   //As Prescribed in Datasheet
    CREN = 1;                                     //Enables Continuous Reception
    TXEN = 1;                                     //Enables Transmission
    return 1;                                     //Returns 1 to indicate Successful Completion
  }
  return 0;                                       //Returns 0 to indicate UART initialization failed
}

Note : 6th and 7th bit of TRISC registers are set as prescribed in the datasheet.

Sending Data Through UART

Writing a Character

void UART_Write(char data)
{
  while(!TRMT);
  TXREG = data;
}

Checking Transmit Register

char UART_TX_Empty()
{
  return TRMT;
}

Writing Text

void UART_Write_Text(char *text)
{
  int i;
  for(i=0;text[i]!='\0';i++)
    UART_Write(text[i]);
}

The following function can be used to write a string or array of characters to UART. It is accomplished by continuous use of character writing function UART_Write().

Receiving Data Through UART

Data Received or Not

The following function can be used to check whether the data is ready to read from the Receive Register. It uses the flag bit RCIF which will be set when the data reception is completed.

char UART_Data_Ready()
{
  return RCIF;
}

Reading a Character

The following function wait till the reception is complete and reads 8 bit data from the Receive Register.

char UART_Read()
{
  while(!RCIF);
  return RCREG;
}

Reading Text

The following function can be used to read a desired length of text or sequence of characters continuously.

void UART_Read_Text(char *Output, unsigned int length)
{
  unsigned int i;
  for(int i=0;i<length;i++)
  Output[i] = UART_Read();
}

For simplifying the program readability we put all the above functions to a header file ‘uart.h’. Thus you just need to include this header file and use required functions. For demonstrating the working of these functions we are using the following example.

PIC to PIC Communication using UART

In this example we are controlling LEDs connected to a PIC using Switches connected to another PIC Microcontroller. For the sake of explanation call these microcontrollers Slave and Master respectively. In the circuit diagram given below a DIP 8 Switch is connected to PORTB of the Master Microcontroller which is configured as Input Port. When a Switch is turned ON, the corresponding pin will be Grounded (LOW). Data Read from the PORTB of Master Microcontroller is send to Slave Microcontroller using UART interface. The Slave Microcontroller writes the received data to its PORTB which is configured as Output. Thus LED’s connected to Slave Microcontroller will Glow depending upon the status of the DIP Switch connected to the Master Microcontroller.

Circuit Diagram

PIC to PIC Communication using UART – Circuit Diagram

Note 1 : TX of Master Microcontroller is connected to RX of Slave Microcontroller and RX of Master Microcontroller is connected to the TX of Slave Microcontroller.

Note 2 : Instead of using PULL UP or PULL DOWN resistors along with switch, we are using INTERNAL PULL UP of PORTB which is enabled in the program.

MPLAB XC8 Codes

Master Code

#define _XTAL_FREQ 8000000
#include <xc.h>
#include <pic16f877a.h>
#include "uart.h"

// BEGIN CONFIG
#pragma config FOSC = HS 
#pragma config WDTE = OFF 
#pragma config PWRTE = OFF
#pragma config BOREN = ON 
#pragma config LVP = OFF 
#pragma config CPD = OFF 
#pragma config WRT = OFF 
#pragma config CP = OFF 
//END CONFIG

void main()
{
  TRISB = 0xFF; //PORTB as Input
  nRBPU = 0;    //Enables PORTB Internal Pull Up Resistors
  UART_Init(9600);

  do
  {
    UART_Write(PORTB);
    __delay_ms(100);
  }while(1);
}

Slave Code

#define _XTAL_FREQ 8000000
#include <xc.h>
#include <pic16f877a.h>
#include "uart.h"
// BEGIN CONFIG
#pragma config FOSC = HS
#pragma config WDTE = OFF 
#pragma config PWRTE = OFF
#pragma config BOREN = ON
#pragma config LVP = OFF
#pragma config CPD = OFF
#pragma config WRT = OFF 
#pragma config CP = OFF 
//END CONFIG

void main()
{
  TRISB = 0x00; //PORTB as Output
  UART_Init(9600);

  do
  {
    if(UART_Data_Ready())
       PORTB = UART_Read();
    __delay_ms(100);
  }while(1);
}

Want to See the Output ?


Note : Don’t forget to copy the header file ‘uart.h’ to your project folder and add it to project tree when you create a new project.

Adding a Header File – MPLAB X IDE

Download

You can download entire project files here…