During building my pedals, I decided to make my own pedal controller using cheep and affordable components. Already having some experience with Arduino boards, I know their capabilities and decided to route a STM32 way this time (mostly to learn something new though).
Here is the comparision of Arduino Leonardo (which is a most likely candidate for our goal) and STM32F103C8T6 (which I used)
|Architecture||8-bit ATMEL AVR||32-bit ARM CORTEX-M3|
|Frequency||16MHz||up to 72MHz|
|Timers||1x 16-bit, 2x 8-bit||3x 16-bit, 1x pwm|
|Peripheral||10-bit ADC (6 channels)
|2 x 12-bit, 1 μs A/D converters (7 channels)
7-channel DMA controller
As you can see, specs for stm32f103c8 are not so bad. And it’s cheaper than Leonardo!
One big drawback is that you need additional external programmer. But its not expensive also, total price for mini board + programmer is still comparable with Arduino clones.
Schematics for the dev board:
Pinout in pdf — The-Generic-STM32F103-Pinout-Diagram
So, after some time, I programmed simracing-oriented firmware for the device.
Device capabilities are:
- 6x 12-bit (4096 steps) axises
- rotary encoders support
- rotary switches support
- matrix buttons support
- single buttons support
- axises calibration
- 1kHz exchange rate with PC, which means 1ms maximum latency for axises
It can be used for DIY buttons boxes, button rim plates, pedals, hand brakes, gear shifters, etc, etc — everywhere you need joystick/gamepad interface with PC.
Here is pinout you get after flashing the firmware first time:
STM32F103C8T6 ------------------- - |3VB +3.3V| - - |C13 GND| - COL1 - |C14 +5V| - COL2 - |C15 B9| - ROW6 ADC0 - |A0 B8| - ROW5 ADC1 - |A1 B7| - ROW4 ADC2 - |A2 B6| - ROW3 ADC3 - |A3 B5| - ROW2 ADC4 - |A4 B4| - ROW1 ADC5 - |A5 B3| - COL4 ADC6 - |A6 A15| - ROT11 COL5 - |A7 A12| - ROT1 - |B0 A11| - ROT2 - |B1 A10| - ROT8 ROTA - |B10 A9| - ROT7 ROTB - |B11 A8| - ROT6 - |R B15| - ROT5 - |+3.3V B14| - ROT4 - |GND B13| - ROT3 - |GND B12| - COL3 -------------------
Also I programmed small PC-based app named OSHStudio, which allows you to choose a function for each pin. Here is how the initial pinout looks in OSHStudio, when you click «Get Config from Device»:
You can configure pins choosing its function in comboboxes:
When buttons wired in matrix, columns have to be connected to Button Matrix COLUMNS pins, rows — to Button Matrix ROWS pins. Single separate buttons (not matrix), can be connect to «Single Button +3,3V» or «Single Button GND». If you connect button to «Single Button +3.3V» than connect the other side of button to +3.3V, if you connect button to «Single Button GND» than connect it to GND. Here is an example of matrix wiring:
You can test your buttons in the «Buttons» tab of OSHStudio:
In this tab you can also configure POV Hat switches. When activated first Hat Switch used first 4 buttons of matrix, 2nd Hat Switch — from 2nd 4 buttons and so on.
The firmware supports full step, half step and quarter step encoders. Rotary encoders can be wired in «chained» or «single» configuration. Chained config means you wire side encoders’ pins (PINA & PINB) together and central pin become «control» pin. Central pin identified your encoder type — 1/1 (full step), 1/2 (half step) or 1/4 (quarter step). You can wire different type encoders in one chain.
Also encoders can be connected in single configuration. In this case you should configure side pins according to the type of encoder and central pin have to be connected to +3.3V.
Encoders can also be tested in the «Buttons» tab.
Virtual button is «clicked» when shaft is changing position.
Analog pins ADC0 — ADC6 used to connect potentiometers or other analog inputs. Also you can calibrate axises in OSHStudio, and calibration values in this case will be stored in MCU itself. After calibration an axis is «expanded» by the controller, so PC always sees axis as 4096 steps.
I recommend set to «Not used» all unused pins 🙂
You can find pictures in better resolution, code and firmware itself in the github project.