STM32F1 USART DMA TX does not complete the second transfer - c

Question

Hopefully someone will throw some light on my incompetence.
I have setup the USART for DMA transfers using DMA interrupts.
void DMA1_Channel7_IRQHandler(void)
{
// USART2 TX handler
/* Test on DMA Stream Transfer Complete interrupt */
if (DMA_GetITStatus(DMA1_IT_TC7))
{
/* Clear DMA Stream Transfer Complete interrupt pending bit */
DMA_ClearITPendingBit(DMA1_IT_GL7 | DMA1_IT_TC7);
}
DMA_Cmd(DMA1_Channel7, DISABLE);
}
I am trying to send data to the TX buffer with a command such as:
SCICommsStringIntoTxQ("\n\rTEST_CODE V0.01: \r\n");
where the command is
void SCICommsStringIntoTxQ(int8_t * str_out_string)
{
memset(TxBuffer, '\0', sizeof(TxBuffer));
strcpy(TxBuffer, str_out_string);
USART_DMACmd(USART2, USART_DMAReq_Tx, ENABLE);
DMA_Cmd ( DMA1_Channel7, ENABLE );
}
My DMA Init is:-
/* Configures the DMA1 Channel7 for UART2_CHAN Transmission. */
DMA_DeInit(DMA_Channel_USART2_TX);
DMA7_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)&(USART2->DR);
DMA7_InitStructure.DMA_DIR = DMA_DIR_PeripheralDST;
DMA7_InitStructure.DMA_BufferSize = (uint16_t)sizeof(TxBuffer) -1;
DMA7_InitStructure.DMA_MemoryBaseAddr = (uint32_t)TxBuffer;
DMA7_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
DMA7_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
DMA7_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
DMA7_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
DMA7_InitStructure.DMA_Mode = DMA_Mode_Normal;
DMA7_InitStructure.DMA_Priority = DMA_Priority_High;
DMA7_InitStructure.DMA_M2M = DMA_M2M_Disable;
DMA_Init(DMA_Channel_USART2_TX, &DMA7_InitStructure);
USART_DMACmd(USART2, USART_DMAReq_Tx, ENABLE); // Enable USART2 DMA TX
DMA_ITConfig(DMA_Channel_USART2_TX, DMA_IT_TC, ENABLE);
The first send works fine, however as soon as I try and send the next string it corrupts out my first. Can anyone advise what I have done wrong?

Answer 1

Thanks for taking time to respond. Im not checking or waiting for the USART to finish. I assumed the DMA took care of it. Should I be looking at the UART XE (transmit data empty flag ) before sending the next string to the DMA?

Answer 2

I am not sure from your code if you are waiting the USART finishes sending your first message before sending the second string.
If not, and if TxBuffer points to the same memory location for both strings, the second time you call the memset you are changing the memory portion that the DMA is serving to the USART while the latter is still sending out the previous string.
EDIT:
Answering to your question, Wikipedia reports:
With DMA, the CPU first initiates the transfer, then it does other operations while the transfer is in progress, and it finally receives an interrupt from the DMA controller when the operation is done.
This means that when you start a transmission over the USART bus, the DMA controller gets data from RAM and serves this data to the USART controller while the CPU can do other operations. This means that your CPU can potentially change on the fly the portion of RAM the DMA controller is accessing to while serving the USART controller.
I do not know the specific details of your implementation and hardware configuration but I think you should check when the USART or the DMA controller ends the transmission before starting a second transfer. How to do this depends on your current configuration. Checking XE might work or maybe you can rely on an interrupt that raises when the DMA ends its transmission.
For instance maybe you can set a your internal flag able_to_transmit inside this block:
/* Test on DMA Stream Transfer Complete interrupt */
if (DMA_GetITStatus(DMA1_IT_TC7)) {
able_to_transmit = 1;
}
And do something like this before transmitting:
while(!able_to_transmit)
; //wait or do something else
SCICommsStringIntoTxQ("\n\rTEST_CODE V0.01: \r\n");
able_to_transmit = 0;
I hope this helps.