In many microcontroller projects, you need to read and write data. It can be reading data from the peripheral unit like ADC and writing values to RAM. In another case, maybe you need to send chunks of data using SPI. Again you need to read it from RAM and continuously write to the SPI data register. When you do this using processor – you lose a significant amount of processing time. To avoid occupying the CPU, most advanced microcontrollers have a Direct Memory Access (DMA) controller. As its name says – DMA does data transfers between memory locations without the need for a CPU.

Low and medium density ST32 microcontrollers have a single 7 channel DMA unit, while high-density devices have two DMA controllers with 12 independent channels. In STM32VLDiscovery, their ST32F100RB microcontroller with a single DMA unit having 7 channels.

DMA controller can do automated memory to memory data transfers, also do peripheral to memory and peripheral to peripheral. DMA channels can be assigned one of four priority levels: very high, high, medium, and low. And if two same priority channels are requested simultaneously – the lowest number of the channel gets priority. DMA channel can be configured to transfer data into the circular buffer. So DMA is an ideal solution for any peripheral data stream.

Speaking of physical DMA bus access, it is important to note that DMA only accesses bus for actual data transfer. Because of the DMA request phase, address computation and Ack pulse are performed during other DMA channel bus transfers. So when one DMA channel finishes bus transfer, another channel is already ready to do transfer immediately. This ensures minimal bus occupation and fast transfers. Another exciting feature of DMA bus access is that it doesn’t occupy 100% of bus time. DMA takes 5 AHB bus cycles for single word transfer between memory – three of them are still left for CPU access. This means that DMA only takes a maximum of 40% of bus time. So even if DMA is doing intense data transfer, the CPU can access any memory area, peripheral. If you look at the block diagram, you will see that the CPU has a separate Ibus for Flash access. So program fetch isn’t affected by DMA.

Stm32 Array Declaration Form

Programming DMA controller

Simply speaking, programming DMA is relatively easy. Each channel can be controlled using four registers: Memory address, peripheral address, number of data, and configuration. And all channels have two dedicated registers: DMA interrupts the status register and interrupts the clear flag register. Once set, DMA takes care of memory address increment without disturbing the CPU. DMA channels can generate three interrupts: transfer finished, half-finished, and transfer error.

As an example, let’s write a simple program that transfers data between two arrays. To make it more exciting, let’s do the same task using DMA and without it. Then we can compare the time taken in both cases.

Don't forget those <pre> and </pre> tags! All your files need to be able to 'see' the structure type definition; only one file must actually define the variables (as it is the definition which allocates storage). Previously we have tried to do a single conversion of one ADC channel. We were waiting for the ADC result in a loop, which isn’t an effective way of using processor resources. It is better to trigger a conversion and wait for the conversion to complete the interrupt. This way, a processor can do other tasks rather than wait for ADC conversion to complete. This time we will go through another. One way to fix this problem is to declare message as a volatile array. The C standard is unambiguous that volatile side effects must not move past sequence points, so this will work. On the other hand, adding more volatile qualifiers may suppress interesting optimizations elsewhere in the program. Material declaration: ST has adopted the IPC-1752 standard for reporting declarable substances and material groups. The ST list of banned, exempted and declarable substances is regularly reviewed to include regulations and customer requirements and ensure our suppliers respect our rules regarding the use of chemicals and hazardous substances.

Here is a code of DMA memory to memory transfer:

First of all, we create two arrays: source and destination. Size of length is determined by ARRAYSIZE, which in our example is equal to 800

We use the LED library from the previous tutorial – they indicate a start and stop-transfer for both modes – DMA and CPU. As we see in the code, we must turn on the DMA1 clock to make it functional. Then we start loading settings into DMA_InitStructure. For this example, we selected DMA1 Channel1, so first of all, we call DMA_DeInit(DMA1_Channel1) function, which makes sure DMA is reset to its default values. Then turn on memory to memory mode, then we select normal DMA mode (also, we could select circular buffer mode). As priority mode, we assign Medium. Then we choose the data size to be transferred (32-bit word). This needs to be done for both – peripheral and memory addresses.

NOTE! If one of the memory sizes would be different, say source 32-bit and destination 8- bit – then DMA would cycle four times in 8-bit chunks.

Then we load destination, source start addresses, and amount of data to be sent. Afterload these values using DMA_Init(DMA_Channel1, &DMA_InitStructure). After this operation, DMA is prepared to do transfers. Any time DMA can be fired using DMA_Cmd(DMA_Channel1, ENABLE) command.

To catch the end of DMA transfer, we initialized DMA transfer Complete on channel1 interrupt.

Where we could toggle the LED and change the status flag giving a signal to start the CPU transfer test.

CPU based memory copy routine is simple:

Measuring DMA and CPU based transfer speeds

Since LEDG is connected to GPIOC pin 9 and LEDB is connected to GPIOC pin 8 we could track start and end pulses using scope:

Stm32 Initialize Array

So using 800 32-bit word transfer using DMA took 214μs:

While using the CPU memory copy algorithm, it took 544μs:

Stm32 Array Declaration Code

This shows a significant increase in data transfer speed (more than two times). And with DMA most considerable benefit is that the CPU is unoccupied during transfer and may do other intense tasks or go into sleep mode.

I hope this example gives an idea of DMA’s importance. With DMA we can do loads of work only on the hardware level. We will get back to it when we get to other STM32 features like ADC.

Stm32 Array Declaration Codes

Download Sourcery G++ Lite Eclipse project files here: STM32DiscoveryDMA.zip