PWM event generation

RE: PWM event generation

ovrebekk — Wed, 25 Nov 2020 09:59:00 GMT

Hi Enrique

I use the GPIOTE and PPI peripherals to connect the PWM events to pins, allowing me to track them on an oscilloscope.

If you want to do the same you can use the following code to set it up:

    NRF_GPIOTE->CONFIG[0] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            (32 + 10) << GPIOTE_CONFIG_PSEL_Pos;
    NRF_GPIOTE->CONFIG[1] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            (32 + 11) << GPIOTE_CONFIG_PSEL_Pos;
    NRF_GPIOTE->CONFIG[2] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            (32 + 12) << GPIOTE_CONFIG_PSEL_Pos;
    NRF_GPIOTE->CONFIG[3] = GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos |
                            GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos |
                            (32 + 13) << GPIOTE_CONFIG_PSEL_Pos;

    NRF_PPI->CH[0].EEP = (uint32_t)&NRF_PWM0->EVENTS_PWMPERIODEND;
    NRF_PPI->CH[0].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[0];
    NRF_PPI->CH[1].EEP = (uint32_t)&NRF_PWM0->EVENTS_LOOPSDONE;
    NRF_PPI->CH[1].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[1];
    NRF_PPI->CH[2].EEP = (uint32_t)&NRF_PWM0->EVENTS_SEQEND[0];
    NRF_PPI->CH[2].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[2];
    NRF_PPI->CH[3].EEP = (uint32_t)&NRF_PWM0->EVENTS_SEQEND[1];
    NRF_PPI->CH[3].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[3];
    NRF_PPI->CHENSET = 0xF;

The GPIO's used in this case is P1.10-P1.13, mapped to PWMPERIODEND, LOOPSDONE, SEQEND[0] and SEQEND[1] respectively.

Best regards
Torbjørn

RE: PWM event generation

spw — Tue, 24 Nov 2020 17:27:47 GMT

I am curious about:
- What is the frequency you used to the PWM?
I used a 1 MHZ frequency with a top_count of 500.

- How did you count the number of events?
I used a timer counter with a PPI trigger interaction. Did you use the same or other approach?

I will review the code on my end, maybe there is something I missed or I can provide further info about it.

Regards,

Enrique.

RE: PWM event generation

ovrebekk — Tue, 24 Nov 2020 16:31:55 GMT

I tried running the complex playback function, and the results seem to be as expected:

nrf_drv_pwm_complex_playback(&m_pwm0, &seq0, &seq1, 4, 0);

Since I run the loop four times I see the SEQEND0 and SEQEND1 events happening 4 times one after the other, followed by the LOOPSDONE event happening once at the very end.

The PWMPERIODEND event happens at a much higher rate, as it follow the PWM frequency.

If I run the nrfx_pwm_simple_playback(..) function, more similar to your example, I see that the SEQEND1 and LOOPSDONE events occur repeatedly at the same rate, while the PERIODEND event occurs X times as often, where X is the length of the sequence.

So I am a bit unsure how you got the results you listed earlier. In particular it seems odd that the PERIODEND event would happen less frequent than the SEQEND0 and SEQEND1 events. This shouldn't really be possible...

Best regards
Torbjørn

RE: PWM event generation

spw — Mon, 23 Nov 2020 18:39:03 GMT

Yes, I have seen that figure. It's the one I used to assume when the events was supposed to be generated. Thanks for confirming that my assumptions were correct, I was a bit worry to have misunderstood the figure.

I know the SEQEND events are generated at reading of the last byte, that makes sense. It seemed to me that such detail would not affect to the question, since that should not affect the amount of events generated.

Thanks for taking a look to it.

regards,
spw

RE: PWM event generation

ovrebekk — Mon, 23 Nov 2020 09:49:00 GMT

Have you checked figure 6 in the PWM peripheral documentation?

Essentially the SEQEND events are generated when the last byte is read from the corresponding sequence registers, which happens before the playback is completed.

This allows you to safely rewrite the data stored in the sequence buffer, knowing that the PWM peripheral is done reading from it.

[quote user=""]

NRF_PWM_EVENT_LOOPSDONE

Generated when both sequences (0 and 1) have being finished the number specified by playback_count.
NRF_PWM_EVENT_PWMPERIODEND

Generated each time top_count is reached.

[/quote]

Yes, that sounds correct.

Regarding the number of events you see I think I will need to reproduce this myself. I will do some tests tomorrow when I have access to the necessary tools.

Best regards
Torbjørn