UART write buffer with PDC - c

Question

I'm having a problem with writing to a USARt using const char buffer and char arrray.
Here is my UART write function:
unsigned int USART_Send( unsigned char *p_pucData,
unsigned int p_unLen)
{
AT91C_BASE_US2->US_TPR = (unsigned int)p_pucData;
AT91C_BASE_US2->US_TCR = p_unLen;
AT91C_BASE_US2->US_PTCR = AT91C_PDC_TXTEN;
while((AT91C_BASE_US2->US_CSR & ((0x1 << 11) | (0x1 << 4) ) ) == 0);
AT91C_BASE_US2->US_PTCR = AT91C_PDC_TXTDIS;
return p_unLen;
}
Below function working with const char* like:
USART_Send("IsitDone?",9); //Working
If I use a array buffer like below it is showing garbage characters, wonder why ?
unsigned char arr[10];
memcpy(arr, "HelloWorld", 10);
USART_Send(arr, sizeof(arr)); //Not working properly displaying Garbage chars

Answer 1

Ricardo Crudo is correct. You run into the following problem:
arr is created on the stack
arr is filled
call USART_Send
fill transmit pointer, counter, enable tx requests
/* peripheral state is TXBUFE = '0' and ENDTX = '0' because: */
/* TXBUFE = '0' when PERIPH_TCR != 0 and */
/* ENDTX = '0' when PERIPH_TCR != 0 */
/* but we just wrote to PERIPH_TCR, so it's != 0 */
/* both conditions are satisfied, b/c the transfer hasn't started yet! */
wait until (TXBUFE = '0' and ENDTX = '0')
/* your code thinks PDC is done here */
/* but in reality, PDC is getting started */
disable tx requests
return from sub-function
overwrite stack (and arr) with unrelated data here
/* PDC might push out last word(s) here due to pipelining/ */
/* instruction cache/side effects/you-name-it */
/* even though its matrix requests were disabled a few cycles ago */
Solutions:
copy to a global buffer or
wait some cycles between enabling tx requests and checking if the PDC is done (possibly a whole baud tick) or
read back PERIPH_TCR and check if it's zero instead of checking the flags
Ideally, you would allocate some dynamic memory for strings and deallocate it after the PDC is done asynchronously to your actual code. You might want to check if you can get some kind of interrupt after the PDC/peripheral is done, then deallocate the memory it read from.
If you don't have dynamic memory allocation, then use the a global ring buffer and abstract your string/char send function to use this buffer instead.