nRF5 SDK15.0 - PWM Driver: Just set a fixed duty cycle?

[awneil]

The PWM Driver has all sorts of fancy stuff for dealing with clever sequences - but I can't see anything to just set a simple, steady, fixed duty cycle!

How does one just set a fixed duty cycle?

What I actually want is to be able to fade an LED up to a certain brightness; so what I really want is to be able to specify an increasing sequence, and then a steady duty cycle to last indefinitely once the fade-up sequence completes.

Is there a simple way to do that?

[awneil]

Edvin said:

If you look in the description of the sec_values, you see that the first bit will be the polarity of the PWM signal, as described in nrf_pwm.h, line 248 (in SDK15.0.0).

What it doesn't tell you is how it sets the polarity - does 1 set active high, or active low?

[Edvin]

I agree. That is also why I didn't specify which is which. But you can do this with a simple test. Set it to e.g. 1/10th of 0x7FFF ≈ 0x0800. Then check if the light is on for 1/10th, or off for 1/10th of the period. Testing this shows that with sec_value of 0x0800 the LED is on for a very short amount of time. Since the LED is ON when the pin is LOW, this means that if the msb is 0, then the PWM signal will go from low to high when 0x0800 ticks = 2048 ticks has passed. Then it stays high until the .top_value is reached.

Note that you will see a similar behavior if you set the value to 0x8000 (=0b1000 0000 0000 0000) + 0x7FFF-0x0800 = 0xF7FF.

This is because the signal will start high, and stay high until the value reaches 0xF7FF-0x8000 = 0x77FF ticks, and switch to LOW (LED ON) for the rest of the period, which is 0x8000 ticks.

But to summarize:

msb = 0, signal goes low to high on seq_values.

msb = 1, signal goes high to low on seq_values.

Best regards,

Edvin

[awneil]

Edvin said:

But you can do this with a simple test

Sadly, there seems to be far too much of that with the Nordic parts.

Documentation is supposed to tell us these things - not leave us puzzled and having to experiment & reverse-engineer it all!

Edvin said:

the signal will start high

So what is the NRF_DRV_PWM_PIN_INVERTED flag supposed to do in the config.output_pins[] settings?

I doesn't seem to make any difference ...

[awneil]

Now I see:

• nrf_pwm_sequence_t.repeats sets the number of times to repeat each step within the sequence;
   
• playback_count sets the number of times to repeat the entire sequence.

Of course, with only a single step in the sequence, they both end up as the same effect!

EDIT

To further clarify, nrf_pwm_sequence_t.repeats is the number of repeats after the first iteration; ie,

• repeats = 0 runs each step once;
• repeats = 1 runs each step once, plus 1 repeat;
• repeats = 2 runs each step once, plus 2 repeats;
• repeats = n runs each step once, plus n repeats

[awneil]

Quoting from code doesn't seem to work on this forum?

        .top_value    = 32768, // = 0x8000

causes nrf_drv_pwm_init() to Assert - the maximum allowed value is 7FFF.