Generic Software Debug UART implementation for Microcontrollers.

         In this post I am giving code snippet of software debug uart implementation which can be used for any microcontroller. I did not implemented receive functionality as for debug uart it is rarely required. I will post it soon. Now understand the code details in short. What i did is you need to have start bit, a stop bit and data bits. Start bit is High to low transition for one bit period (1/baud rate) and stop bit is low to high transition for 1 bit period. and you need to shift data bits (LSB first) of a character byte in these two bits over a GPIO. For Tx configure port as Output. You can use almost any port pin for this.I have written and tested below code on atmega8 with maximum 9600 baud rate. You can use this uart for debugging purpose if your code is complicated and lengthy.


softuart.h
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/*
 * File :softuart.h
 * Created on: 21-Jun-2013
 * Author: Shrenik S. Shikhare
 * Copyright(c): 2013
 */


#define START_BIT 1
#define DATA_BITS 8
#define NUMBER_OF_STOP_BIT 1

#define F_CPU 1000000
#define TRANSMIT_PIN PD1
#define RECEIVE_PIN PD0
#define DEBUG_BAUD_RATE 9600 //work only upto 9600
#define DELAY_VALUE (F_CPU /DEBUG_BAUD_RATE)

void InitDebugUart();
void putDebugChar(char data);
void DEBUG(char *data);

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softuart.c

------------------------------------------------------------------------------------------------------------

 * File :softuart.c

 * Created on: 21-Jun-2013

 * Author: Shrenik S. Shikhare

 * Copyright(c): 2013

 */

#include <avr/io.h>

#include <avr/interrupt.h>

#include <avr/delay.h>

#include "softuart.h"

/****************************************************************************************

 * Function name : InitDebugUart()

 *Discription : This configures uart related pins like TxPIN as output and RxPIN as input

****************************************************************************************/

void InitDebugUart()

{

DDRD &= ~(1<<RECEIVE_PIN) ; //PD0 is RXD

DDRD |= (1<<TRANSMIT_PIN)|(1<<PD2); //PD1 is TXD

PORTD |= (1<<TRANSMIT_PIN); //Make Trasmit pin default high at start

}

/****************************************************************************************

 * Function name : putDebugChar()

 *Discription : This function toggles Tx GPIO as per data input with delay between two bit = bit period

   for the required baud rate bit period=1/BAUD_RATE

****************************************************************************************/

void putDebugChar(char data)

{

uint8_t Iterator;

PORTD &= (0<<TRANSMIT_PIN); //Make trasmit pin high to low to send start bit

//_delay_us(104);

_delay_us(DELAY_VALUE);

for (Iterator=0; Iterator<8;Iterator++)

{

if (data & 0x01)

PORTD |= (1<<TRANSMIT_PIN);

else

PORTD &= (0<<TRANSMIT_PIN);

_delay_us(DELAY_VALUE-4);//this is done to compensate for m/c cycles that are used in further instruction execution

data = data >> 1; //approx delay for 9600Baud

//data = data >> 1;

}

PORTD |= (1<<TRANSMIT_PIN); //Send logic 1 stop bit for a bit period

_delay_us(104);

}

void DEBUG(char *data)

{

while(*data)

putDebugChar(*data++);

}

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You can change this code as per the MCU you are going to use. You wont required to change much apart from delay routine to get required bit period, controller specific header files.

I will update receive uart code soon. Hope this will be useful for you.

Thanks 

Shrenik.

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