Dim brightness m5stack

How-to
1

Some people are using m5stack for their projects, one question that I had since the beginning was how to dim my lcd brightness. For micropython you can use:

from machine import Pin, PWM

pwm2 = PWM(Pin(32), duty=40)

You can use a value other than 40.

Hope this is usefull to someone!
PS: use at you own risk

2

As far as I can tell, M5 is using the exact same technique for ther lobo-derived micropython fork. Their display class actually has a brightness method that uses PWM on the light pin.

I just checked the schematics, applying a PWM signal from pin 32 will dim the backlight leds. The circuit seems to be designed to be used that way.

One thing though: I found that when soft resetting the M5Stack, one has small issues with PWM. I actually have to do this:

backlightPwm = PWM(Pin(32))
backlightPwm.deinit()
backlightPwm.init(duty=128) # or any other duty

It seems that (like many other hardware things) PWM persists across soft-resets and the simple creation of a PWM object is insufficient to restart/reset the pulsation. This is quite logical, since a soft reset only resets microPython (and very few other ancilary items, like lvgl’s ILI9341). Thus the deinit/init sequence. Since this seems to work, I did not investigate further…

3

I frequently see this MicroPython feature being brought up. Do you happen to know the reason for doing it this way? Is it for performance reasons?

4
  • Soft-reset (ctrl-D in a REPL): restarts the microPython virual machine. Some cleanup can be done before that restart (the lvgl ili9341 driver does so). The chip is not restarted. Near instantaneous.
  • Hard reset: reboots the microcontroller. On my machine with 4Mb PSRam that can take quite a few seconds.

When toying/developping, it is enough to restart the microPython engine in order to be able to import modules anew. My work cycle is:

  1. Import the test module I am working with from the REPL
  2. If there is an issue, edit, ctrl-D on the console and back to 1.

The biggest time consumer being the initial compilation when modules are imported. MicroPython does not save the bytecode. If I am satisfied with a module, I either cross-compile to create the bytecode or even freeze it in the firmware.

5

Thanks for the explanation. I can see why soft reset is valuable when hard reset takes so long in comparison (which is somewhat strange to me, coming from the STM32 world where I can reset the processor and be running an application in 0.5 seconds).

6

Here, the thing is that pesky PSRam that seems to be checked at every restart… Furthermore the RTOS takes time to start up…