Receiving number as string (uart) - string

Question

I'm trying to receive a number through uart which is packed as a string. I'm sending number 1000, so I get 4 bytes + null character. But when I convert the array to number with atoi() and compare the integer with 1000 I don't always get a correct number. This is my interrupt handler function for receiving the number. What could be wrong?
void USART1_IRQHandler(void)
{
if( USART_GetITStatus(USART1, USART_IT_RXNE) )
{
char t = USART1->RDR;
if( (t != '\n' && t!='\0') && (cnt < 4) )
{
received_string[cnt] = t;
cnt++;
}
else
{
cnt = 0;
}
t = 0;
received_string[4] = 0;
}
if(cnt==4)
{
data = atoi(received_string);
}
}

Answer 1

Try this code instead. Here I check the max number of received bytes to avoid buffer overflow (and possible hardware fault). I created a specific function to clear the reception buffer. You can find also a definition for the string length because the code is more flexible. I suggest also to check the reception errors (after read the incoming byte) because in case of errors the reception is blocked.
//Define the max lenght for the string
#define MAX_LEN 5
//Received byte counter
unsigned char cnt=0;
//Clear reception buffer (you can use also memset)
void clearRXBuffer(void);
//Define the string with the max lenght
char received_string[MAX_LEN];
void USART1_IRQHandler(void)
{
if( USART_GetITStatus(USART1, USART_IT_RXNE) )
{
//Read incoming data. This clear USART_IT_RXNE
char t = USART1->RDR;
//Normally here you should check serial error!
//[...]
//Put the received char in the buffer
received_string[cnt++] = t;
//Check for buffer overflow : this is important to avoid
//possible hardware fault!!!
if(cnt > MAX_LEN)
{
//Clear received buffer and counter
clearRXBuffer();
return;
}
//Check for string length (4 char + '\0')
if(cnt == MAX_LEN)
{
//Check if the received string has the terminator in the correct position
if(received_string[4]== '\0'){
//Do something with your buffer
int data = atoi(received_string);
}
//Clear received buffer and counter
clearRXBuffer();
}
}
}
//Clear reception buffer (you can use also memset)
void clearRXBuffer(void){
int i;
for(i=0;i<MAX_LEN;i++) received_string[i]=0;
cnt=0;
}