On 10 September 2016 at 09:08, H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> wrote: > Reducing the PWM frequency is good by itself since it should not be unnecessarily > fast and helps to make the PWM to "average current" translation more linear. > > The non-linear effect is that the PWM controlled DC/DC converter reacts almost > immediately to a 1->0 control transition but needs some time (ca. 0.5ms) to recover > on a 0->1 transition. DT already allows for compensation of many non-linearities by specifying the duty cycle of each brightness increment. Though, as you observed, there's one limitation it cannot fix here: > If we just fix the PWM generator to output a steady 1 signal at 100%, we have a > very significant change if we switch to 99%, depending on PWM frequency. Specifically the next-to-brightest step (assuming 0.5ms off-time) would be: 75% @ 500 Hz 90% @ 200 Hz 95% @ 100 Hz 96% @ 83 Hz Note that perceptually the distance to 100% might be smaller due to non-linear response of the eye. That's my experience with pwm controlled leds anyway, which may or may not apply to backlights (though with my laptop's backlight I never really have use for the distinct steps at the brightest end while those at the darkest end seem disproportionally large). > This effect becomes smaller if the PWM frequency is reduced and 83Hz seems more > reasonable (although still a little arbitrary) than the current value. While 500Hz is perhaps a bit high, 83Hz actually seems very low to me. Searching a bit around yielded 175 Hz as common frequency for CCFL backlights and higher for LED backlights (source: http://www.tftcentral.co.uk/articles/pulse_width_modulation.htm). (I may be reacting a bit twitchy here due to having encountered dimmed LED lighting that was flickering obnoxiously for me while noone else noticed this.) Matthijs -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
