Using DMA for timer or PWM in stm32duino or leaflab maple

While stm32duino has a generic API for using DMA for any peripheral, it is not well known. I had to search various examples to get an idea. There too older examples use deprecated API, while newer tube API is intuitive and easy.

Setting up of DMA and peripherals

On the look of it, DMA API looks sufficient to set up and execute transfers. But stm32 hardware requires you to set up both DMA and peripheral for DMA. Which means calling API for both DMA and peripheral configuration. Many a times these calls look like redundant since same information has already been configured, but nevertheless, to set-up registers, you must call both APIs.

Setting up DMA

1. Create an instance of dma_tube_config
2. Put address of array as source/destination
3. src/dest size is 8/16/32 bit, which is minimum unit of transfer.
4. Find the peripheral data register for DMA transfer. In most cases there is single data register named DR used for transfer. But in case of timers it is more complicated as explained below.
5. Set up flags for interrupt, circular transfers etc.
6. Set up DMA channel associated with peripheral.
7. Initialize DMA
8. Call dma_tube_cfg with this configuration.

Setting up peripheral

This is more of a peripheral dependant task. Usually you have to enable DMA requests by peripheral. Sometimes addresses need to be set.

Finalize and start DMA

1. Attach interrupt handler routine if interrupts are enabled
2. Start DMA for given channel
3. Start peripheral

Although this much information can be deduced from examples found on various blogs, setting up timer device (in PWM mode) for DMA was a tricky task due to added complexities and no handy reference. I had to go though many pages of reference manual to find some info. It must be miracle that it worked for me in 2nd or 3rd attempt given that I had no idea what I was doing.

PWM

Pulse Width Modulation, generally used for dimming effects or generating analog signal, works using hardware timers.

• Timer Counter counts number of clock pulses elapsed
• Timer CCR (compare) register sets number of pulses after which output inverts (usually high to low)
• Timer ARR (reload) sets total width of pulse.

After initialization in PWM mode, ARR is set to full 2¹⁶ value. Prescaler is also set to 1, which divides system clock frequency by half. So effective frequency of normal PWM is just 36MHz/2¹⁶ = 549 Hz.

So to change this frequency, we must change reload value. But this reload value limits vertical resolution (voltage levels).

Anyway, back to PWM using DMA:

Setting up DMA

Most of the steps mentioned above. For PB7 pin tube_req_src was DMA_REQ_SRC_TIM4_CH2. DMA update register is DMAR instead of CCR directly.

Setting up Timer

This was interesting. But thankfully timer API header has enough documentation to ease it. From ref. manual, I got that there are many timer registers that can be updated using DMA. So, what registers reflect values in DMAR is decided by base_addr and burst_len. To simplify:

• base_addr is address of first Timer register to be updated (starting from CR1)
• burst_len provides number of continuous registers to be updated from base_addr
• burst_len values are fetched one after other from DMAR and registers starting from base_addr are updated.

So, my timer set up function had calls to

1. timer_dma_set_base_addr
2. timer_dma_set_burst_len
3. Finally timer_dma_enable_req to enable DMA requests.

After that I enabled DMA and started timer and voilà!! DMA was working. I enabled completion interrupt just to check whether it was working or not.

References:

• STM32F103 reference manual
• timer.h from stm32duino->system/libmaple/include/libmaple
• dma.h from aforementioned folder
• http://polpla.cat/tutorials/SPI+DMA.html