48 lines
4.1 KiB
Markdown
48 lines
4.1 KiB
Markdown
# Joystick demo
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# Lessons Learned
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* I still don't trust C++
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* How to drive an WS2812 RGB LED strip from the ESP32 using the Freenove library
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* How the RMT peripheral on the ESP32-S3 works and why it is involved when driving WS2812 LED strs
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* How to select pin assignments on the ESP32 without relying on the tutorial
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* How to calibrate joysticks
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## Not trusting C++
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To be fair, I give C++ more hate than it deserves. But the way some people write it really does the language no favors.
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The WS2812 tutorial(s) use a global variable with an initializer.
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I chose to use a heap allocation rather than use a global scope class initializer. Calling code outside of a function in a C-derived language just feels wrong to me. The objects live in the data segment, whcih is nice, but the compiler will generate some magic unspecified initializer function wherein it will call the constructors on the object, before it calls `main()`. But the order of those initializers is not guaranteed, so if A depends on B, you may be shit out of luck. And I especially don't trust global initializers that seem to be touching hardware. The whole thing just feels *wrong*.
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<center><img alt="it's wrong" width="320" src="cartmanitswrong.gif"/></center>
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Still, I could have done better. I was lazy and used `new` to create a new object on the heap. This is fine for the simple demo but for a real product I would have statically allocated the memory in the data segment and then pointed the `new()` operator at it somewhere in `setup()`.
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```arduino
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```
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## Selecting pin assignments: read the datasheets
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Up until now I've been using whatever pins the tutorial chose because, well, I had enough to figure out while getting my fundamental electronics lessons. But ...
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<center><img alt="read the datasheet" width="320" src="readthedatasheet.jpg"/></center>
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## WS2812s
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The WS2812 is a series of LEDs on a strip, which can be controlled by sending a compact stream of data to the strip, and each LED takes its own data before forwarding on the rest. This allows you to control each LED individually in a theoretically endless string of LEDs with very little hardware overhead. In the particular example here, the WS2812 strip is arranged as an octagon of LEDs on a square PCB package with one `V+` line, one `GND` line, and one `S`(ignal) line.
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Oh, and you control it using the **infrared remote control module** on the ESP32.
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<center><img alt="wot" width="320" src="../05-photoresistors/wot.jpeg"/></center>
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The WS2812 is controlled with a wierd single line protocol that's really efficient but very sensitive to timing. The whole thing works off of PWM signals on the wire, and there is no clock synchronization. You could do it yourself on a GPIO pin manually sending the PWM signals but it is very easy to get wrong. So the RMT module, which was originally designed to convert data to pulses and back again for sending over infrared, is abused for this purpose instead. We can just send the data and the CPU goes along its merry way.
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## Calibrating joysticks
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I implemented calibration logic in my code, but it was a bit lazy, and to be honest, it doesn't do what real calibration typically does. But by the time I got here, I was just ready to move on to the next project. When you press the calibration button, the calibration routine starts which continually samples over a period of time. It creates a deadzone according to the lowest and highest values seen on each axes during this time. When future joystick reads occur, anything within this deadzone are counted as being at the center of the axis. Really I did this to account for potential wobble in the joystick I hooked to the arduino, but that was unnecessary, it was remarkably stable in the center.
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A real calibration routine is more advanced and allows the center to actually move. As it samples, it figures out where the center of the sample average range is, and future joystick reads return the axis movement relative to that center location. My calibration routine here can't do that. I considered changing this, but again, that was outside of the original design goal for the calibration routine.
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