PWM signal was reversed.

RE: PWM signal was reversed.

Hung Bui — Wed, 06 May 2015 13:31:40 GMT

@plskeggs: I believe by "power down" he wanted to mention SystemON with CPU turn off (_WFE()).

Do you still have trouble with PWM ? If possible please attach a project that shows the issue, I can try to test it here.

RE: PWM signal was reversed.

plskeggs — Mon, 04 May 2015 06:50:00 GMT

Hi, Torbjørn, What do you mean exactly by powerdown? If I do this: sd_power_mode_set(NRF_POWER_MODE_CONSTLAT); does this avoid any PWM powerdown issue? I'm using USE_WITH_SOFTDEVICE = 1 and I have 3 PWM channels.

Even with this, and with the Feb 5 2015 Github release of nrf_pwm.c, I still sometimes get an inverted waveform (I'm only looking at channel 0 for now).

I repeat this sequence 10 times a second (with PWM frequency = 488 Hz):

set pwm value to 0 (to float one side of an H-bridge) for 5ms
read two ADC channels
restore PWM value to previous setting
do some calculations (quickly)
set new PWM value

I'm suspecting there is a problem with your code when it writes to TASKS_OUT to "fix" the polarity of the output. I think sometimes it gets it wrong, and inverts the waveform by mistake.

It looks to me like there is a race condition between when the CC[2] is captured and when the TASKS_OUT is written, though I think the safe_margins_present() is supposed to prevent that. I tried increasing the pwm_cc_margin_by_prescaler[] for my prescaler, but this did not help.

Any suggestions?

RE: PWM signal was reversed.

Hung Bui — Fri, 06 Feb 2015 15:07:24 GMT

Hi Nimal, The PWM driver is updated on github, could you please try and let us know the result.

RE: PWM signal was reversed.

nimalhuang — Tue, 03 Feb 2015 16:51:53 GMT

Hi, Torbjørn,

Have you update the PWM driver so that fix the power down

RE: PWM signal was reversed.

ovrebekk — Fri, 30 Jan 2015 09:23:11 GMT

The latest revision of the nRF51x22 has a issue that affects PWM if you use powerdown, and it might be this issue causing the PWM inverting.

Fortunately a workaround exists, and I will try to update the PWM driver today to implement it.

Can you confirm if you are using power down, and also what revision of the nRF51x22 you have?

UPDATE: Sorry for the slow response, I was caught up in other things. Can you please try the attached file and see if it works better? nrf_pwm.c

It now adds the workaround for the timer issue introduced in revision 3 (and explained in the nRF51x22 PAN).

If you can confirm that it solves the issue I will also push an update to the repository.

Regards Torbjørn