Fix with PWM for brightness slowing devices down #6894
Conversation
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I was just reading through all of that code today and eyeing with suspicion that unconditional call in |
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I wonder if this is also going to fix #6838. It certainly looks like it! |
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I’m beginning to suspect that since disabling auto refresh restores the ability to control brightness, the SAMD51 TCC timer used for the display backlight PWM is shared with the display auto refresh timing somehow. |
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To check #6838, I tested with a Feather ESP32-S3 TFT with this fix. Now the display is dark until the brightness is set explicitly, but the brightness can now be controlled. I don't think this is a timer issue; the code that @gamblor21 removed forced the brightness to 1.0 on every refresh. But now there's a newly exposed bug that the brightness is not set correctly on initial startup. |
I think this is not the case, given observations by @gamblor21 in adafruit/Adafruit_CircuitPython_Display_Shapes#57 that creating an independent PWM object does not cause the same delays. I think it's more likely that unconditionally updating the PWM object on each |
Fix for CircuitPython v8.0.0-beta.2: display_shapes.polygon color fill is rendering very slowly
#6734 removed support for auto brightness left a call in
displayio_display_backgroundto set the brightness to a 1.0f fixed value. In the previous code another function call checked that auto-brightness was set to True and skipped any work if it was not true and limited to how often the brightness was set.With the previous PR always calling
common_hal_displayio_display_set_brightnessto 1.0f it was both preventing the brightness from being set (using the same auto-brightness behavior that no longer exists) and having to change the PWMOut object was slow the lower the frequency of the PWM timer. Most boards with built in displays were set to 50KHz so it was barely noticeable. The Titano was set to 500Hz so the problem became apparent. But there is a measurable speed-up even when the frequency was set to the higher value.This PR does not fix the brightness from not being set properly. This would be one source of error but apparently others exist still.
But this PR does fix a slowdown with any board using PWM to control the display brightness.