Buffering in STM32F4 uC for UART

[JCorl.1]

I recently got my STM32F407VG uC working with UART4. I can receive a byte and send a byte. Though I have been reading on how to acquire more than one byte at a time. It seems as though people like to use DMA. I am perfectly okay with polling for my experiments and projects.

I have been looking through the UART section of the reference manual. I see a lot of mention of multibuffering with DMA but I cannot seem to find how big the UART buffer is. I am trying to do something similar to what serial LCDs do where they have a command prefix and then an actual command with optional parameters.

In my mind, there would be a buffer of fixed size that would hold these byte command requests from another device transmitting to the uC. I could read each byte command in this buffer one by one and take the correct action for each one. For example, the Arduino I used had a 64 byte ring buffer and grabbing one byte at a time was possible.

Note: I am not a professional firmware programmer or engineer. Just trying to learn something here.

Any hints are appreciated! Thanks for your time!

[Tesla DeLorean]

The DMA buffers will be as large as you make them.

The USART on the F4 has a 1-byte (9-bit) holding register, and needs to be serviced to avoid over/under run conditions.

Some of the newer families have a small FIFO, perhaps 8 or 16 deep separately for Rx and Tx paths.

For Rx I'd likely use the DMA to create a small FIFO/RING Buffer, and that would sweep/feed a larger FIFO buffer.

For Tx, large blocks of established length can typically be DMA'd directly, perhaps using scatter-gather / chaining methods.

More generally, and where the scope of the source buffer is dubious/transient, I'd likely use a larger FIFO buffer, and service that via DMA IRQ, dispatching available data blocks via DMA, and perhaps POLL/IRQ for byte only.

[TDK]

Since it's a software buffer, you decide how large it is. Technically, the buffer is limited to 65535 words or less due to DMA restrictions. Using UART with DMA in circular mode using a circular buffer is the best option, IMO.

[JCorl.1]

Thank you both for your answers. Looks like there is quite a bit to configure! When I get somewhere, I will let everyone know here and post code for future readers. I really only need to have a DMA for the RX line. I am okay with a single byte for TX with no DMA.

[S.Ma]

You might choose the buffer size depending on the write speed=baudrate, the largest message expected, the time it takes to read and process the message. If the baudrate is many times lower than core speed, byte interrupt method maybe enough.

[Piranha · Accepted solution]

https://github.com/MaJerle/stm32-usart-uart-dma-rx-tx

[JCorl.1]

Thank you everyone! I decided to move onto using the HAL now as it seems it might be easier to use for DMA and many examples show DMA with it. Though I am stuck on it so please consider this thread closed. I have another thread in using HAL and UART here: https://community.st.com/s/question/0D53W00001BRSpgSAH/usart1-for-stm32f411ce-using-hal

Thanks again everyone for their time in responding!