In many microcontroller projects, you need to read and write data. It can read data from the peripheral unit like ADC and write 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. Most advanced microcontrollers have a Direct Memory Access (DMA) controller to avoid occupying the CPU. 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.
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 example to set up more than one channel and read ADC values using interrupt service routine. How does multichannel ADC conversion works? If we need to convert several channels continuously, we need to set up Sequence registers (ADC_SQRx). There are three sequence registers: ADC_SQR1, ADC_SQR2, and ADC_SQR3 where we can set up a maximum of 16 channels in any order. Conversion sequence starts with SQ1[4:0] settings in ADC_SQR3 register. Bits [4:0] hold the number of ADC channels.