I have been working thru the tutorials on Usart with and without interrupts.
I have the following code:
#include#include #include #define USART_BAUDRATE 4800 #define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1) int uart_initiate(void); int uart_putc(char c, FILE *stream); unsigned char uart_rec; FILE uart_str = FDEV_SETUP_STREAM(uart_putc, NULL, _FDEV_SETUP_WRITE); int main (void) { //Initiate Port A for output and set to high (on stk500 led's are "active low") DDRA = 0xff; PORTA = 0x00; uart_initiate(); stdout = &uart_str; UCSRB |= (1 << RXCIE); sei(); uart_rec = '2'; for (;;) // Loop forever { if(uart_rec == '1') { printf("%:c \n", uart_rec); printf("Hello world!\n"); uart_rec = '2'; } } } //Initiate uart int uart_initiate(void) { UCSRB |= (1 << RXEN) | (1 << TXEN); // Turn on the transmission and reception circuitry UCSRC |= (1 << URSEL) | (1 << UCSZ0) | (1 << UCSZ1); // Use 8-bit character sizes UBRRH = (BAUD_PRESCALE >> 8); // Load upper 8-bits of the baud rate value into the high byte of the UBRR register UBRRL = BAUD_PRESCALE; // Load lower 8-bits of the baud rate value into the low byte of the UBRR register return 0; } /* *************************** */ //Put function int uart_putc(char c, FILE *stream) { if(c == '\n') { uart_putc('\r', stream); } //end if loop_until_bit_is_set(UCSRA, UDRE); UDR = c; return 0; } //Receive interrupt ISR(USART_RXC_vect) { char ByteRec; uart_rec = UDR; // Fetch the received byte value into the variable "uart_rec" ByteRec = uart_rec; UDR = ByteRec; //echo the caracter received PORTA ^= 0xff; // Toggle all leds on Port A }
When I connect my stk500 to a serial port and send any character (including a "1" (without quotes)), the LEDs toggles on and off and the character echos (as would be expected based on the code in the interrupt routine). But if I enter "1" (again, without quotes), neither of the "printf" statements in the if block execute.
However if I modify the code to add an "else" block after the "if" block as follows:
for (;;) // Loop forever
{
if(uart_rec == '1')
{
printf("%:c \n", uart_rec);
printf("Hello world!\n");
uart_rec = '2';
}
else
{
printf("%:c \n", uart_rec);
}
}
Then the terminal on my laptop will print a continuous stream of "2"s until I enter another character, in which case it prints that character. If I enter a "1", both "printf" statements are executed, after which the continuous stream of "2"s returns.
Why does the "else" block cause the code in the "if" block to execute? What am I missing?
