Messages in this thread | | | Date | Sat, 10 Sep 2016 05:17:36 +0200 | From | Matthijs van Duin <> | Subject | Re: [PATCH] ARM: dts: omap3-gta04: reduce panel backlight PWM frequency to 83Hz |
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On Mon, Sep 05, 2016 at 11:16:38AM +0200, H. Nikolaus Schaller wrote: > This helps to get 100% intensity closer to "always on". > > It compensates for an effect of dmtimer which at 100% still emits short > "off" impulses and the startup-time of the DC/DC converter makes > backlight intensity not reach full scale. The lower the PWM frequency > is, the smaller is this effect.
Sounds to me like you're working around something that should be fixed in the pwm-omap-dmtimer driver instead?
Looking at the (baremetal) dmtimer pwm code I wrote ages ago, which supports fully off to fully on, I do seem to be handling both endpoints in a special way. A rough conversion of my code into C:
// period in timer cycles void pwm_init( volatile struct dmtimer *timer, u32 period, bool invert ) { assert( period >= 2 ); timer->if_ctrl = 2; // reset timer, configure as non-posted timer->reload = -period; timer->trigger = 0; timer->config = 0x1043 | invert << 7; // pwm initially disabled }
// value in timer cycles, 0 <= value <= period void pwm_set( volatile struct dmtimer *timer, u32 value ) { if( value == 0 ) { timer->config &= ~0x800; // disable pwm return; } u32 period = -timer->reload; if( value >= period ) timer->match = 0; else timer->match = value - period - 1; timer->config |= 0x800; // enable pwm }
At the time I used a scope to check the exact behaviour of dmtimer pwm on a dm814x. My notes mention (when pwm enabled): match < reload output on continuous match == reload output on 1 cycle, off period-1 cycles match == -2 output on period-1 cycles, off 1 cycle match == -1 output freezes
Hope this helps
Matthijs
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