Objective

In a previous post, I explained how to make AVR communicate with my PC using UART. Since I will be using UART in other projects, I wanted to write a simple library for my own use.

Link to github repo

What functions should this library have? To begin with:

  1. Initialize UART
  2. Transit a byte to a receiver
  3. Receive a byte from a transmitter
  4. Print a string
  5. Print a byte as a number

I will primarily use this library to print to a console for debugging purposes, so these functions should suffice. I will add more when the need arises.

Code

My code is broken down into myUART.c and myUART.h

  1. // myUART.c
void initUART(int baud) {
// Calculate UBBR
uint16_t baud_setting = (F_CPU / 16 / baud) - 1;

// Set BAUD
UBRR0H = baud_setting >> 8;
UBRR0L = baud_setting;

// Set Transmitter/Receiver enable
UCSR0B |= (1 << TXEN0) | (1 << RXEN0);
	
// Set 8 data bits and 1 stop bit
UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
}

    First, we set Baudrate using F_CPU and baud using the formula from the datasheet. Macro F_CPU holds the clock frequency and needs to be defined for this to work. Then we enable Transmitter/Receiver, and set data format.

  2. void transmitByte(uint8_t ch) {
while (!(UCSR0A & (1 << UDRE0)));

UDR0 = ch;
}

    Run a blocking loop until Data Registery Empty UDRE0 flag bit is set. Then, write the byte to Data Register UDR0.

  3. uint8_t receiveByte(void) {
while (!(UCSR0A & (1 << RXC0)));

return UDR0;
}

    Run a blocking loop until Receive complete RXC0 bit is set. Then, return the byte from Data Register UDR0.

  4. void printString(const char myString[]) {
for (int i = 0; myString[i]; i++)
{
    transmitByte(myString[i]);
}
}

    Loop over the string and use transmitByte() to send one byte at a time.

  5. void printByte(uint8_t ch) {
transmitByte('0' + (ch / 100));
transmitByte('0' + ((ch / 10) % 10));
transmitByte('0' + (ch % 10));
}

Transmit ASCII code of each decimal digit of the byte.

Complete Code

#include <avr/io.h>
#include "myUART.h"


// Initialization
void initUART(int baud) {
	// Calculate UBBR
	uint16_t baud_setting = (F_CPU / 16 / baud) - 1;

	// Set BAUD
	UBRR0H = baud_setting >> 8;
	UBRR0L = baud_setting;

	// Set Transmitter/Receiver enable
	UCSR0B |= (1 << TXEN0) | (1 << RXEN0);
	
	// Set 8 data bits and 1 stop bit
	UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
}


// Transmitting functions
void transmitByte(uint8_t ch) {
    while (!(UCSR0A & (1 << UDRE0)));

    UDR0 = ch;
}


// Receiving functions
uint8_t receiveByte(void) {
	while (!(UCSR0A & (1 << RXC0)));

	return UDR0;
}


// Printing functions
void printString(const char myString[]) {
    for (int i = 0; myString[i]; i++)
    {
        transmitByte(myString[i]);
    }
}


void printByte(uint8_t ch) {
	transmitByte('0' + (ch / 100));
	transmitByte('0' + ((ch / 10) % 10));
	transmitByte('0' + (ch % 10));
}