dimtass / stm32f103-dual-gang-rotary-pot

STM32F103 Dual gang non-stop rotary pot

STM32F103 Dual gang non-stop rotary pot

This is a small project only for testing a dual gang free-spinning rotary pot I got my hands on. The interesting thing with this pot is that the two wipers (one for each internal resistor) have a 90 degrees phase between each other. Therefore, the code logic is based this difference.

The beautiful and awesome ascii graphics below show the Vadc on the two wipers of the pot while the pot is turning. So, if you have a 12-bit ADC then the max value will be in the range of [0-4095) ((1<12)-1=4095), which means that min=0 and max=4095. Each wiper is sampled with using a different ADC channel ADC1 and ADC2.

The logic of the code is just to detect the direction where the pot is turning and increment its value when it's turned right and decrement when it's turned left. To do this we split the below graph in to 4 main quarters, each for 1/4 of the half period (aka 1 full turn of the pot). With this trick by comparing two sequential ADC values of ADC1 and ADC2 we can detect the exact location of the pot and also in which direction is turning.

To be more specific, in Q1 we know that ADC1 is in the range [min-1/2] as also ADC2. But by comparing two different ADC samples, then we can detect if the pot is turned right or left. Therefore, in Q1 if ADC1 is increment (+) and ADC2 is decrement (-), then we know that the pot is turned left. And if ADC2=(+) and ADC1=(-) then is turned right.

           |1|2|3|4|1|2|3|4|
      max  _________________
               /\      /\
ADC2  1/2  ___/__\___ /__\___
             /    \  /    \
           _/______\/______\_
      min

      max  ____________________
                 /\      /\
ADC1  1/2  _____/__\___ /__\___
            \  /    \  /    \
           __\/______\/______\_
      min

The code also supports dead-zone for each individual (ADC/wiper). The dead zone is used for two reasons. The one is the to minimize ADC noise and the other is to increase or decrease the pot sensitivity. Dead-zone means that the next sampled value must be larger or smaller than the current by the dead-zone value. Therefore, if the current ADC value is 1000 and the dead zone value is set to 30, then to actually run the calculation algorithm the next sample but be >=130 or <=70. That also means that this 60-bit dead zone will affect the pot sensitivity, which means that you need to do more turns to get from the relative min to relative max value.

The algorithm is based on relative values and supports both signed and unsigned integers and also floats if the MCU is capable. Also the init code can set the initial value. Also, each pot can have individual settings. Therefore, one pot and only have integer values in the range of [0-111] with initial value of 72. Another have range [-100,100] with initial value 0 and another one [-120.25,70.50] with initial value of 2.25. Also, each different pot supports an individual step, which also can be integer of float.

Therefore, the pot API provides individual range, min, max, step and dead-zone values and you can use any number of pots as long you initialize the proper size in the init function.

How to compile and flash

You need cmake to build this project either on Windows or Linux. To setup the cmake properly follow the instructions from here. Then edit the cmake/TOOLCHAIN_arm_none_eabi_cortex_m3.cmake file and point TOOLCHAIN_DIR to the correct GCC path.

e.g. on Windows

set(TOOLCHAIN_DIR C:/opt/gcc-arm-none-eabi-4_9-2015q3-20150921-win32)

or on Linux

set(TOOLCHAIN_DIR /opt/gcc-arm-none-eabi-4_9-2015q3)

Then on Windows run build.cmd or on Linux run ./build.bash and the .bin and .hex files should be created in the build-stm32/src folder. Also, a .cproject and .project files are created if you want to edit the source code.

To flash the HEX file in windows use st-link utility like this: "C:\Program Files (x86)\STMicroelectronics\STM32 ST-LINK Utility\ST-LINK Utility\ST-LINK_CLI.exe" -c SWD -p build-stm32\src\stm32f103-dual-gang-pot.hex -Rst

To flash the bin in Linux: st-flash --reset write build-stm32/src/stm32f103-dual-gang-pot.bin 0x8000000

FW details

• CMSIS version: 5.3.0
• StdPeriph Library version: 3.6.1