I was finishing a project that does the job of hi-speed polling from a SPI interfaced IMU chip, the frequency is about 5Khz, I know that this could be done by start a DMA transfer by software in the hardware update interrupt. But now I'm searching for a faster solution. Since the interrupt may take a very small period, when the amount of sensor chip or polling frequency increases, the whole cost of context switch may be a problem.
I'm asking that is it possible to use a timer update event to trigger a DMA transfer without using any interrupt (at least, don't trigger the interrupt every time when a poll finishes).
There was a blog that I read says that change the request source could do this job, what he means actually looks like the code below:
hdma_spi1_tx.Instance = DMA1_Channel6;
hdma_spi1_tx.Init.Request = DMA_REQUEST_TIM2_UP;
hdma_spi1_tx.Init.Direction = DMA_MEMORY_TO_PERIPH;
hdma_spi1_tx.Init.PeriphInc = DMA_PINC_DISABLE;
hdma_spi1_tx.Init.MemInc = DMA_MINC_ENABLE;
hdma_spi1_tx.Init.PeriphDataAlignment = DMA_PDATAALIGN_BYTE;
hdma_spi1_tx.Init.MemDataAlignment = DMA_MDATAALIGN_BYTE;
hdma_spi1_tx.Init.Mode = DMA_NORMAL;
hdma_spi1_tx.Init.Priority = DMA_PRIORITY_LOW;
Which was simply changed the CubeMX generated code. I have already been a user of ST MCUs for many years and before I tried that I knew the solution may not work and actually it just didn't work.
But just 2 years before now I find the module named DMAMUX, at that time I was obsessed with it and thought it may be the silver bullet for all efficient transmission but now I still not clear about it.
Finally I found a trick to achieve the goal, but I have never seen anyone mentioned it, which is not possible to be configured in CubeMX. This trick is explained as below:
1.Configure the pwm output （to pin） of a timer;
2.Configure the EXTI line at the same pin （use pin stacking function and make sure the pwm genenration is at top）;
3.Configure the DMA Generator, using synchronization signal of EXTI, and SPI transmission is synchronized by the DMA Generator;
4.Add the code of EXTI configuration since Cube MX won't generate two functions code on one conflict pin.
In summary this solution route the trigger signal as TIM->EXTI->DMA Generator->SPI DMA. Due to the long time that has passed since I firstly tried this, some details are forgotten by me, but I'm certain that it actually works except that this would cost a pin resource inevitably.
So, I'm looking for a better solution that will not cost any pin resource and could achieve the performance as the one I mentioned above or just tell me the trick I tried was just an accident.
