Messages in this thread | | | Date | Mon, 30 Mar 2020 17:12:31 +0200 | From | Clemens Gruber <> | Subject | Re: (EXT) Re: [PATCH 1/4] pwm: pca9685: remove unused duty_cycle struct element |
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Hi,
On Wed, Feb 26, 2020 at 06:03:02PM +0100, Matthias Schiffer wrote: > As it turns out, this driver is broken in yet another way I didn't find > before: For changing the global prescaler the chip needs to be put into > sleep mode, but the driver doesn't follow the restart sequence > described in the datasheet when waking it back up. In consequence, > changing the period of one PWM does not only modify the period of all > PWMs (which is bad enough, but can't be avoided with this hardware), > but it also leaves all PWMs disabled...
I am unable to reproduce this: If I set a specific duty cycle on a channel and then change the period, the channel stays active. I can see the brightness of an LED decrease if I increase the period.
This is expected, because after the SLEEP bit is set, we wait for 500usecs and then write to the LED ON/OFF registers. This leaves the channel enabled with the new period (but with old duty_ns value => different ratio)
A few years ago, I played around with the idea of remembering the duty_ns to period_ns ratio and setting it accordingly after a period change, possibly also with a shortcut of setting the RESTART bit if the ratio did not change. Maybe after the switch to the atomic API, this would be a nice improvement.
Best regards, Clemens
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