Using UART of PIC Microcontroller – MPLAB XC8

Using UART of PIC Microcontroller – MPLAB XC8

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

TXSTA – Transmit Status and Control Register

  • Bit 0 TX9D : This is the 9th bit of data in the 9 bit transmission mode which is commonly used as a Parity Bit.
  • Bit 1 TRMT : This is the Transmit Shift Register (TSR) status bit. This can be used to check whether the data written to transmit register is transmitted or not. When the TRS is empty this bit is set and when the TSR is full this bit will be 0.
  • Bit 2 BRGH : This is the High Baud Rate Select bit for Asynchronous mode operation and is unused in Synchronous mode. Setting this bit selects High Speed and clearing this bit selects Low Speed baud rates. You will can see the baud rate calculation later in this article.
  • Bit 3 Unimplemented : This bit is unimplemented and will read as 0.
  • Bit 4 SYNC : This is the USART Mode select bit. Setting this bit selects Synchronous mode while clearing this bit selects Asynchronous mode.
  • Bit 5 TXEN : Setting this bit enables the transmission. In the synchronous mode operation CREN and SREN bits of RCSTA register overrides this bit.
  • Bit 6 TX9 : When this bit is set it enables the 9 bit transmission otherwise 8 bit transmission is used. 9th bit in the 9 bit transmission mode is commonly used as parity bit.
  • Bit 7 CSRC : Clock Source Select Bit, this bit has no application in the Asynchronous mode operation of USART module. It is used to select master or slave mode in Synchronous mode operation.

RCSTA – Receive Status and Control Register

RCSTA - Receive Status and Control Register

RCSTA – Receive Status and Control Register

  • Bit 0 RX9D : This is the 9th bit of Received Data and is commonly used as Parity Bit.
  • Bit 1 OERR : Overrun Error bit. A high at this bit indicates that Overrun error has occured.
  • Bit 2 FERR : Framing Error bit. 1 at this bit stands for Framing Error while 0 stands for No Framing Error.
  • Bit 3 ADDEN : Address Detect Enable bit. This bit is applicable only in Asynchronous 9 bit mode. Setting this bit enables Address Detect.
  • Bit 4 CREN : Continuous Receive Enable bit. Setting this bit will enable Continuous Receive. In the Synchronous Mode CREN overrides SREN.
  • Bit 5 SREN : Single Receive Enable bit. This bit has no effect on Asynchronous mode and Synchronous Slave mode. Setting this bit will enables Single Receive. This bit will cleared after the reception is complete.
  • Bit 6 RX9 : Setting this bit enables 9 bit reception otherwise it will be in 8 bit reception mode.
  • Bit 7 SPEN : Serial Port Enable bit. Setting this bit enables serial port and configures RC7, RC6 as serial port pins.

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

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

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

Adding a Header File – MPLAB X IDE

Download

You can download entire project files here…

 

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41 Comments on "Using UART of PIC Microcontroller – MPLAB XC8"

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Guest
Guest

can I read text its length is unknown ?

Ligo George
Guest

Yes, you can continuously read using UART_Read() function in a loop…. until a limiting character is reached.

sanj
Guest

Hi ligo ,
can i get pls knw how to send a string from controller to a PC using virtual serial port VSPE.
thanks in advance

sanj
Guest

Hi Ligo george ,

I am trying to send string of data from controller to PC via VSPE , virtual serial port . i am not able to receive any data on the Hyper terminal .
i am using the baud rate .
Please help me how do i do this .

sanj
Guest

Hi Ligo george ,

I am trying to send string of data from controller to PC via VSPE , virtual serial port . i am not able to receive any data on the Hyper terminal .
i am using the baud rate 9600 .
could send me the working code for this .
Please help me how do i do this .

Ligo George
Guest

I hope that you can use above functions without any change..

Ligo George
Guest

1. Set baud-rate correctly..
2. Use No parity..
3. 8 bit data
4. Single Stop bit..

Ligo George
Guest

Above code is 100% working and it is tested in hardware..

Arnau Perez Talens
Guest

i need to make a “if” when is receiving something if not the program shall continue.
how i could do that?
this exemple will work?
if (!TRMT){// the data is ready
//do something
}
//if not ready continue with the program

Sorry for my english
thanks for your help

Ligo George
Guest

Use like this :
if(UART_Data_Ready())
{
}

Aamil Zayan
Guest

Can you guide me to interface PIC16F877a with GSM (i have TTL outputs) using MPLABX anc xc8??

Ligo George
Guest

You can simply use above functions to send AT commands .. and to read responses for GSM modem.

Leonardo
Guest

I tried this code to send a matrix between 2 PIC18, and worked. Thanks so much.
Leo Módulos – From Brazil

shez
Guest

Hi, Can you please give me an example of how to use the functions to send a string, I am trying at the moment and the slave does not seem to be receiving anything, however it works with your example with port B. Thanks in advance

Ligo George
Guest

You can use simply, UART_Write_Text(“HELLO”);

ANTONIO
Guest
Hi, I’m trying to send a value got from de adc of the PIC, is it possible to send a float over the uart, if it so can you tell me how to do it? I’ve tried doing this: #define _XTAL_FREQ 5000000 #define RS RD2 #define EN RD3 #define D4 RD4 #define D5 RD5 #define D6 RD6 #define D7 RD7 #include ; #include “lcd.h”; #include; #include “uart.h” // BEGIN CONFIG #pragma config FOSC = XT // Oscillator Selection bits (HS oscillator) #pragma config WDTE = OFF // Watchdog Timer Enable bit (WDT disabled) #pragma config PWRTE = OFF // Power-up… Read more »
Ligo George
Guest

Size of float is 4byte.. You can send 1 byte through uart at a time. So you can send 4 bytes sequentially. You may also add a start byte and stop byte to indicate start and stop.

yugendar
Guest

hi Can I use this code to interface stx882 and srx882
Please help with my question.
I have to build a wireless transmission using pic18lf14k22

Ligo George
Guest

I hope that you can use it. Use the lowest baud rate as far as possible.
The above tutorial is for PIC 16F877A, you might need to make some changes to work with PIC 18F14K22.
Please read the device specific datasheet for that.

OCybress
Guest
Quick question, using a PIC18F14K50 I just want to echo what I type into the terminal. eg Terminal “inforn” pic echo “info” so far i can send information through UART but I cannot seem to get the right structure to receive and act upon the received information. I’m new to pics and am transitioning from Atmels to PIC’s I do know C and C++ but its a bit rusty any help would be great. so far this is what i have now in main. I have tried a few other structures but cant seem to get the pic to echo… Read more »
Jared
Guest

Would it be possible to see your full assembly code? (opposed to just the main loop). Thanks in advance 🙂

Ligo George
Guest

Sorry, I don’t have assembly codes.

Dnyaneeshwar
Guest

How to use serial interrupt in pic15=6f877a…plz can u post related to that.?

Dnyaneshwar
Guest

sorry pic16f877a

Ligo George
Guest

It is very simple, you need to just enable the interrupt by writing to corresponding registers. Then the interrupt function will be automatically called.

jabez
Guest

hi, thanks for your code.

i would like to ask how could i receive text from the serial data and manipulate it?

example:

If (received text = “hello”)
{
do something
}

Ligo George
Guest

You cannot compare string like that. You have 2 options.
1. Compare each character in the array. OR
2. Use strcmp(), don’t forget to include string.h

Angelo
Guest

Hi, Ligo George,
Thanks a lot for the useful examples!
Can you tell me how to modify this code to make it work with PIC18F2550? The only difference that I found is the Send Break bit (bit 3 from the TXSTA register).
Thanks in advance! 🙂

Vinod Rajbhar
Guest

sir please UART use PIC16f877a but receive another chip

Ligo George
Guest

First you should compare the difference of modules of both pic microcontrollers by checking the datasheet. Then you might need to make some minor changes to make it function.

Ligo George
Guest

You can use the same code in any pic microcontrollers… but you might need to make minor changes depending on the microcontrollers. Please check the device specific datasheet for more information.

Vinod Rajbhar
Guest

I have PIC16F877A microcontroller, i am sending data from UART to PIC18F67K22 , i want to send with the help of two different chip. I failed to do this. Kindly send me program.

Ligo George
Guest

Sorry, we cannot provide programs as per the requirements of each and everyone visiting our website. You should try to develop it yourself based on the above tutorial.
You can also hire one of our engineers if you need support like that.

Yolcos
Guest

Hi, I have a problem.
How I can read a text string, for example read a ” Hello”?

char data;
unsigned int num=4;
unsigned char text[3];

text =UART_Read_Text (data,num); // wrong

I do not know how to write the code

Gabriel Sa Pinto
Guest

Hi there I have an issue. My code never leaves the while(!TRMT); condition in the UART_Write(char data) function, or better it runs once but then it waits there forever. What can be wrong? Thanks in advance.

Ligo George
Guest

Just write a loop and use the function UART_Read().

Ligo George
Guest

Are you using 16F877A and above codes ??

Make sure that your UART is initialized.

Sangsu Yun
Guest

Thank you so much!
It works very well in my PIC16F883+HC-06 bluetooth module with android app.

Christophe Nabil Nicolas El-Kh
Guest
Christophe Nabil Nicolas El-Kh

Great tutorial! thank you.

Ligo George
Guest

Thanks for the feedback.
You are welcome to share your project here. Mail us at [email protected]

Andreas Gradinger
Guest

Great!