David Brownell wrote: > Worth highlighting that this is necessarily a "low quality" > PWM ... in the sense that it's got lots of jitter because > of needing CPU intervention in IRQ context, so it's subject > to delays from both IRQs being blocked and from other timer > driven activities firing first. > Very, very good point. The jitter can be quite low (100's of usecs) on a 200 MHz ARM9 chip, in fact, but you'll eat your battery alive if it's a portable device. And the jitter will also destroy your waveform at high or low duty cycles. > There are lots of applications where that jitter is enough > to preclude using this kind of PWM. I get the feeling that > some of the Linux folk seeing "this PWM thing" are not very > cognizant of such issues ... they've not had to use PWM to > do anything where the jitter matters. (Not that I have; > but I know that such apps are a motivation for most of the > PWM hardware on microcontrollers. A few PWMs plus some > sensors, hall effect or QEI or whatever; then you get a > motor controller.) > For some definitions of "motor controller". :) You might even be able to use a GPIO-as-PWM signal to feed an R/C servo positioner, but the jitter there will be very visual when it occurs. Dimming control of LEDs and incandescent lamps can be very forgiving of jitter, but I was thinking more along the lines of "make this LED blink faster or slower" when I came up with the implementation. b.g. -- Bill Gatliff bgat@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
