> Am 10.09.2016 um 09:08 schrieb H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>: > > Hi, > >> Am 10.09.2016 um 05:17 schrieb Matthijs van Duin <matthijsvanduin@xxxxxxxxx>: >> >> 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 > > Yes and no. > > 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. So if you run PWM @ 500Hz and 100% there is 1ms 1 and 1 ms 0. was too early in the morning: should be PWM @ 500Hz and 50%. > But this translates to 1.5 ms no power and 0.5ms power which is 50% of the intended > current. At PWM at 100% with the current PWM driver we still get a DC/DC control of 1.95ms 1 and 0.05ms 0 which translates into 0.55ms no power and 1.45 ms power which is only 75% of the desired maximum. > > This gives some "reduction" factor to all PWM duty cycle values, but the 100% > case is the most noticeable one. > > 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. > > This effect becomes smaller if the PWM frequency is reduced and 83Hz seems more > reasonable (although still a little arbitrary) than the current value. (BTW: for > the Pyra we already use 83Hz). > >> in the pwm-omap-dmtimer driver instead? > > Yes, it probably should be fixed as well but it does not completely solve > the backlight control issue due to the DC/DC converter's behaviour. > >> >> 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 > > BR, > Nikolaus > > _______________________________________________ > http://projects.goldelico.com/p/gta04-kernel/ > Letux-kernel mailing list > Letux-kernel@xxxxxxxxxxxxxxx > http://lists.goldelico.com/mailman/listinfo.cgi/letux-kernel -- 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