USART
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This document contains code for receiving and transmitting data using interrupts on an AVR microcontroller. It initializes the USART for 9600 baud communication and sets up interrupt service routines. The receive ISR stores incoming characters in a string and checks for an accuracy condition. The transmit ISR increments a counter and re-calls the transmit function to continuously send a test string character-by-character until complete.
![Receive.c
/* Since we never know when we are going to get data, we write a
Interrupt Routine that will execute on receiving data.
You can write different conditions depending on your convenience to
test whether data reception is complete and also
whether it is accurate
*/
#include <avr/io.h>
#include <avr/interrupt.h>
#define USART_BAUDRATE 9600
#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)
void initusart(void); // Set the baud
rate ( communication speed in bits per second)
int i=0; // Global variable acts as a counter
unsigned char str[8]; // String to be sent, declared global if to be
used in interrupts
int main (void)
{
DDRA=0xff; // Initialise all ports
DDRC=0xff;
DDRB=0xff;
DDRD=0X00; // Set PORTD for input, since it contains RX and
TX pins
PORTB=0x00; // Set all ports
PORTC=0xff;
PORTA=0x01;
PORTD=0XFF; // Pull up PORTD
initusart(); // Initialise USART
while(1); // Infinite loop to keep the program running
}
ISR(USART_RXC_vect)
{
PORTC -= 0x01; // Decrement PORTC by one
str[i]=UDR; // Load the character in the receive buffer
to string variable
if (str[6]=='g') { PORTC=0x00;} // Condition to test accuracy of
last variable
i++; // Increment counter to read next character
}
void initusart(void)](https://image.slidesharecdn.com/usart-090814004611-phpapp01/85/usart-1-320.jpg)
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USART
- 1. Receive.c /* Since we never know when we are going to get data, we write a Interrupt Routine that will execute on receiving data. You can write different conditions depending on your convenience to test whether data reception is complete and also whether it is accurate */ #include <avr/io.h> #include <avr/interrupt.h> #define USART_BAUDRATE 9600 #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) void initusart(void); // Set the baud rate ( communication speed in bits per second) int i=0; // Global variable acts as a counter unsigned char str[8]; // String to be sent, declared global if to be used in interrupts int main (void) { DDRA=0xff; // Initialise all ports DDRC=0xff; DDRB=0xff; DDRD=0X00; // Set PORTD for input, since it contains RX and TX pins PORTB=0x00; // Set all ports PORTC=0xff; PORTA=0x01; PORTD=0XFF; // Pull up PORTD initusart(); // Initialise USART while(1); // Infinite loop to keep the program running } ISR(USART_RXC_vect) { PORTC -= 0x01; // Decrement PORTC by one str[i]=UDR; // Load the character in the receive buffer to string variable if (str[6]=='g') { PORTC=0x00;} // Condition to test accuracy of last variable i++; // Increment counter to read next character } void initusart(void)
- 2. { UCSRB |= (1 << RXEN); // Turn on the transmission and reception circuitry UCSRC |= (1 << URSEL) | (1 << UCSZ0) | (1 << UCSZ1); // Use 8-bit character sizes UBRRL = BAUD_PRESCALE; // Load lower 8-bits of the baud rate value into the low byte of the UBRR register UBRRH = (BAUD_PRESCALE >> 8); // Load upper 8-bits of the baud rate value into the high byte of the UBRR register UCSRB |= (1 << RXCIE); // Enable the USART Recieve Complete interrupt (USART_RXC) sei(); // Enable global interrupts, necessary if you want to use ANY interrupt }
- 3. Transmit.c /* The execution of the code below is such: After initialising USART, I call a send function to transmit the character. An interrupt is triggered on completion, the counter goes to the next character and calls the send function. So, the exectution will be in the form of continuous loops and will enter the main function only on completion of transmission of all characters */ #include <avr/io.h> #include <avr/interrupt.h> #define USART_BAUDRATE 9600 // Set the baud rate ( communication speed in bits per second) #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) // Prescale factor,calculated automatically void initusart(void); // Function for USART initialisation void send(void); // Function to send characters int i=0; // Global variable acts as a counter unsigned char str[8]="Testing"; // String to be sent, declared global if to be used in interrupts int main (void) { initusart(); // Initialise USART send(); // Call send function while(1); // Infinite loop to keep the program running } void send() { if (i<7) // Send the character only if i is less than string size { UDR=str[i]; // Loads the character into the transmit buffer } } ISR(USART_TXC_vect) // Interrupt Service Routine, fires on Transmit Complete (Tx C) { i++; // Increments i to send next character
- 4. UCSRA &= (0 << TXC); // Clears the Transmit complete flag, Do this to avoid errors send(); // Call the send function again } void initusart(void) { UCSRB |= (1 << TXEN); // Turn on the transmission and reception circuitry UCSRC |= (1 << URSEL) | (1 << UCSZ0) | (1 << UCSZ1); // Use 8-bit character sizes UBRRL = BAUD_PRESCALE; // Load lower 8-bits of the baud rate value into the low byte of the UBRR register UBRRH = (BAUD_PRESCALE >> 8); // Load upper 8-bits of the baud rate value into the high byte of the UBRR register UCSRB |= (1 << RXCIE); // Enable the USART Transmit Complete interrupt (USART_TXC) sei(); // Enable global interrupts, necessary if you want to use ANY interrupt }