Switching to ESP-IDF For PWM Waveform Control

I used ESPHome and Home Assistant to quickly experiment with parameters for ESP32 chip’s LEDC peripheral for generating PWM (pulse-width modulated) signals, seeing how they looked under a cheap oscilloscope. But for actually driving a segmented LCD, I will need even better control over signal behavior. It is an issue of timing: I need to toggle between high and low states for each common segment pin to generate an alternating signal, and I have two common segments to cycle through. In order to avoid flickering the LCD, this cycle needs to occur at least several tens of times a second.

The tightest control over timing I could get with ESPHome appears to be the on_loop automation, which is generally triggered roughly every 16 milliseconds. This translates to roughly 62Hz which, if I could complete the entire cycle, would be sufficient. But performing all of those toggles within a single on_loop would be too fast for our eyes to see, so we can only take one step in the cycle per on_loop. In order to toggle both high and low on consecutive on_loop, that cuts me down to 31Hz. Then there are two common segments, which cuts it further to 15Hz. I need something faster.

Until I have other tools in my toolbox, the “something faster” I have on hand require going to Espressif’s ESP-IDF SDK. PlatformIO makes ESP-IDF easier to work with, and I’ve had experience with this arena. My starting point (chosen because I’ve done similar things before) is to write a FreeRTOS task dedicated to toggling voltage levels by changing PWM parameters. In between steps of the cycle, I use a FreeRTOS wait (vTaskDelay) to send this task into the background until the next step. This mechanism allows finer control over timing than the ~16ms of on_loop, though it is only slightly better at 10ms by default. Repeating the math above, that works out to 25Hz which would at least be as good as 24fps film. But that is not the limit. Once I’m working within ESP-IDF, I have the option to get even finer timing control. I can get a little bit faster by reconfiguring FreeRTOS tick rate via ESP-IDF’s menuconfig tool. And for ultimate timing control I can start working with hardware timers.

I whipped up a test program to generate a staircase pattern. From 0% duty cycle, to 50% duty cycle, to 100%, then 50%, and repeat with 0%. Running at 20ms per step in the cycle, the timing looks solid. I can easily move this to 10ms and still have a solid square wave.

The 50% PWM value looked almost good enough without a capacitor. (Left) I have a huge pile of 300pF capacitors on hand, so I tried one and the waveform looked much better. (Right.) This is good enough for me to move forward with wiring this signal into a segmented LCD.

Source code for this experiment is available on GitHub.

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