If you have programmed anything with C you should be familiar with common data types like char, unsigned char, int, unsigned int, long int, long long int, etc. It is really hard to tell by looks of type how many bytes this variable takes on memory and how it looks in different system. For instance in 8-bit AVR-GCC compiler int is 16-bit type, while in ARM-GCC int is 32-bit. So int size is dependent on platform, compiler and runtime libraries. And switching between systems may trick you if you are not careful enough.
You can always check the size of variable by using sizeof() function. What to do if you need your code to be portable between different systems. Some great libraries could work on any system including 8-bit, 16-bit, 32-bit or 64-bit. In order avoid this annoying problem the ISO C99 standard introduced portable data types that are defined in stdint.h header file. If you open this header file of your default compiler tool-set you will find how things are organized. First of all it checks the length of int types and then defines portable types by using special naming format [u]intN_t. If number is for unsigned number you add u in front of type name and if do not add anything is unsigned. N indicates number of bits the variable will take. So this is simple table of common portable integer types: Continue reading
After we had a quick overview of STM32 ADC peripheral we can start digging deeper in to specifics. In order to understand simple things lets go with simplest case – single conversion mode. In this mode ADC does one conversion and then stops. After ADC conversion result is stored in to 16-bit ADC_DR data register (remember that conversion result is 12-bit), then End of Conversion (EOC) flag is set and interrupt is generated if EOCIE flag is set. Same situation is if injected channel is converted. The difference is that result is stored in to corresponding ADC_DRJx register, JEOC flag is set and interrupt generated if JEOCIE flag is set.
In our example we are going to measure the internal temperature sensor value and send it using USART. Temperature sensor is internally connected to ADC1_IN16 channel. Algorithm will start single conversion and wait for conversion complete flag EOC. Then we are going to read ADC value from ADC_DR register, which later will be used to calculate in temperature value in Celsius and sent via USART. So we should see value in terminal screen. Continue reading
STM32 ADC is pretty complex peripheral. It is designed to be flexible enough to accomplish complex tasks. We are going to dedicate few posts where we will try to cover main features and give working examples of code.
The block schematic may look scary at first time but if you look closer it can be split in to several pieces that are responsible for different functions. Will will go through them step by step to make it look more clear.
In many microcontroller projects you need to read and write data. It can be reading data from peripheral unit like ADC and writing values to RAM. In other case maybe you need send chunks of data using SPI. Again you need to read it from RAM and constantly write to SPI data register and so on. When you do this using processor – you loose a significant amount of processing time. In order to avoid occupying CPU most advanced microcontrollers have Direct memory Access (DMA) unit. As its name says – DMA does data transfers between memory locations without need of CPU.
Low and medium density ST32 microcontrollers have single 7 channel DMA unit while high density devices have two DMA controllers with 12 independent channels. In STM32VLDiscovery there ST32F100RB microcontroller with single DMA unit having 7 channels. Continue reading