To start off, I am using an ATMEGA328P Xplained Mini board. I am writing an "echo" program on the AVR in C and in MATLAB. The MATLAB code sends a value (3) to the USART of the AVR asynchronously (via USB), and the AVR code receives the data and sends it back via USART to the MATLAB program, which stores the received value and prints it to the command window in MATLAB.
My problem is that unless I add a time-delay ( pause(1) on line 8 of the MATLAB program ) after calling the fopen() function, the AVR will not receive the data sent by MATLAB using the fwrite() function, and the MATLAB code will stay forever in the "while Bytes.Available == 0" loop. To be clear, the time delay solves the problem. However, I want to know why I need to add the delay. I have read through the datasheet of the 328P and several MATLAB documents, and I suspect that there must be some sort of time for the connection between MATLAB and the USART to stabilize. I hope that by understanding why the time delay solves the problem, that I can properly handle the issue and reduce the amount of time per transmission cycle.
Is there some pin or property that I can poll that will tell me that the AVR is ready to be sent to (similar to a CTS pin)? Or is this even the problem? I could be completely wrong with my thinking that the AVR is suspect; maybe it is MATLAB that is the problem. I appreciate any insight!
Here is the MATLAB Code:
PORT = 'COM4';
BAUD = 9600;
BITS = 8;
AVR = serial(PORT, 'BaudRate', BAUD, 'DataBits', BITS);
fopen(AVR);
pause(1);
fwrite(AVR, 3, 'async');
while AVR.BytesAvailable == 0
end
response = fread(AVR, AVR.BytesAvailable, 'uint8');
disp('AVR:');
disp(response);
fclose(AVR);
delete(AVR);
clear AVR;
Here is the AVR C Code:
#define F_CPU 16000000UL
#define BAUD 9600
#define MYUBRR F_CPU/16/BAUD-1
#include <avr/io.h>
#include <util/delay.h>
void USART_Init( unsigned int );
void USART_Transmit( unsigned char );
unsigned char USART_Receive( void );
int main(void)
{
unsigned char data0;
USART_Init(MYUBRR);
while(1)
{
data0 = USART_Receive();
USART_Transmit(data0);
}
}
void USART_Init( unsigned int ubrr )
{
/* Set baud rate */
UBRR0H = (unsigned char)(ubrr>>8);
UBRR0L = (unsigned char)ubrr;
/* Enable receiver and transmitter */
UCSR0B = (1<<RXEN0)|(1<<TXEN0);
/* Set frame format: 8data, 1stop bit */
UCSR0C = (1<<UCSZ01)|(1<<UCSZ00);
}
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSR0A & (1<<UDRE0)) )
{
}
/* Put data into buffer, sends the data */
UDR0 = data;
}
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSR0A & (1<<RXC0)) )
{
}
/* Get and return received data from buffer */
return UDR0;
}
