Bi-directional DMA UART with STM32F4 Discovery

This example firmware demonstrates using DMA for both rx and tx for minimal CPU load during transfers, and is 100% compatible with the Electric UI Quickstart Tutorial.

For a loopback example without Electric UI integration refer to 796ce88.

Why DMA?

DMA (Direct Memory Access) lets the microcontroller's peripherals transfer data to/from memory specified by the firmware developer, rather than making the developer read data out of registers manually.

DMA lets our USART/UART operate much efficiently than the more typical polling or per-byte interrupt based methods, as it seamlessly handles inbound and output transfers in the background with occasional interrupts when:

• the inbound buffer is half full,
• the inbound buffer is full,
• or the serial line has been idle for >1 frame!

This means the microcontroller isn't doing any work while data transfers, other than occasionally ensuring the buffer's don't overflow!

As a result, DMA transfers give one of the best UART experiences with Electric UI, with measured roundtrip heartbeat latency/jitter of 0.6ms ±0.2ms at 115200 baud. For reference, an Arduino Mega is about 6x slower, 4.5ms +-1.1ms, for the same baudrate.

The actual time between the rx Idle Line interrupt firing at the end of the inbound packet, and the response packet's DMA transfer starting was measured around 0.65us +-0.21us.

Hardware Setup

• Hardware used was the ol' faithful green STM32F4 Discovery Board, STM32F407VGT6.
• Connect an external USB-UART adaptor to PD8/PD9 (and ground).
• The board is configured to run at 115200 baud by default.
• The orange led, LD3 on PD13, is used as a blinker for the standard hello-electric example.

Changing the microcontroller family

This example is setup against the STM32F407VGT6, but because it uses the ST HAL, changing to different F4 parts isn't too hard.

• If you're using cmake, change the -DSTM32F407xx to match your part i.e -DSTM32F405xx.
• Ensure your linker file STM32F407BVGTx_FLASH.ld file matches your micro, as your part might have more/less RAM and flash.
• Adjust the USART peripheral in use, GPIO pins, and LED pin as required for your development board.

Firmware Setup

This project was created with CubeMX (.ioc file included), but the usart.h/.c are heavily modified. The project was exported for SW4STM32, but imported and edited with CLion.

We've used the ST LL hal libraries.

1. The electricui-embedded library isn't included in this repo.
   • Either download the zip from GitHub and put the contents in Drivers/electricui-embedded.
   • Or git clone https://github.com/electricui/electricui-embedded into /Drivers.
2. Import the project into your IDE, or build using CMake.
3. Configure the build target as required for your IDE.
4. Build and flash the project to the devboard.

If you are not using CMake, you may need to delete or 'IDE exclude' the electricui-embedded sub-directories test and examples from the build.

Pre-built binaries for the STM32F407VGT6 discovery board are in /binaries.

Acknowledgements

Tilen Majerle's USART DMA examples are the best available resource for DMA USART on the STM32, this project basically combined several examples with some cleanup.