Best practice using DMA to read from UART and write to SD?

Hello Lars,

there are different way how to do it, most of them depends on project complexity. Specifically, if you are using F722 MCU, I would assume that UART2SDCARD is not the only task you have to handle, thus I would go with operating system for sure. MCU has enough memory to handle and CPU is smart enough to do the job using RTOS. ST provides FreeRTOS port as part of STM32Cube software.

Example, how i would do it is like this

• Create new task (thread), dedicated only for reading UART and saving data to SDCARD
  • FatFS (if you are using it) supports RTOS mode, so you are safe to use it. Please read FatFS documentation what implementation is mandatory for RTOS mode
• Use single message queue to writenotifications from interrupts for DMA HT or TC interrupts
• Use LL drivers to configure UART and DMA for RX mode and enable HT and TC interrupts
• Wait for new event on message queue or timeout value, when you decide to check for new data

Pseudo code example below:

void
uart_to_sdcard_thread(void) {
 last_dma_position = 0;
 queue = create_message_queue(10); //Create message queue for 10 elements
 
 while (1) {
 //Wait for new queue or timeout
 //If there is timeout, you know that neither TC or HT events happened
 //so manually check if there is anything to read
 message_queue_wait_msg(queue, MAX_TIMEOUT_VALUE);
 if (timeout_happened_on_read) {
 //There was no message in queue but it was timeout
 }
 //Remember: NDTR is downcounter and holds number of 'remaining' elements to transfer
 new_position = LL_DMA_GetDataCounter(...)
 //Do some math, and save to card
 }
}
//DMA interrupt handler
void
dma_irq_handler(void) {
 if (tc_irq) {
 message_queue_write(queue, tc);
 }
 if (ht_irq) {
 message_queue_write(queue, ht);
 }
}
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If you need example for message queues on STM32F7, follow this example:STM32Cube_FW_F7_V1.0\Projects\STM32756G_EVAL\Applications\FreeRTOS\FreeRTOS_Queues

Some readings, which may help you to start:

https://community.st.com/thread/42689-efficiently-use-dma-with-uart-rx-on-stm32