Hi Daniel, On 18.09.2014 01:42, Daniel Drake wrote: > Hi, > > I'm using pwm-samsung on Exynos4412 for a variable-brightness LED. > > When the LED is set to maximum brightness via the pwm-leds driver, we > arrive at pwm_samsung_config with duty_ns = period_ns, i.e. 100% duty > cycle. > > This function does: > > /* -1UL will give 100% duty. */ > --tcmp; > writel(tcmp, our_chip->base + REG_TCMPB(pwm->hwpwm)); > > I think that comment is incorrect. If tcmp is written as -1UL then the > LED totally turns off. And there is nothing in the Exynos4412 manual > to suggest that -1UL should be set in the TCMP register for 100% duty.
Looking at Figure 11-3 in 11.3.2 Basic Timer Operation chapter of Exynos 4412 public datasheet [1] (page 659), the calculation above seems correct. The default state of timer output is high and if TCMP is set to a value higher than TCNT, then it will never toggle to low. [1] http://www.samsung.com/global/business/semiconductor/file/product/Exynos_4_Quad_User_Manaul_Public_REV1.00-0.pdf > > If I remove that --tcmp line, so that 100% duty cycle is handled as > tcmp=0, the problem is solved: the LED turns on at max brightness when > the leds subsystem requests so. According to my computations, with tcmp=0 you should get exactly the minimum supported duty cycle (1 / N, where N is the number of ticks of period), not full brightness. Are you sure that you have the right output polarity configured? > > Any ideas? Is this -1UL thing a quirk from older chip versions not > applicable to Exynos4? Comparing few datasheets, the timers seem identical in this aspect. Best regards, Tomasz -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
