nrfx_pwm_simple_playback() on nRF9160?

[Jake]

Hi, 

I'm using nRF9160 and want to use PWM with the automatic duty sequence feature using DMA, to make an LED "pulse" for example. 

Looking at the documentation for the registers to control PWM, they seem to be the same on nRF9160 as nRF52, so I have tried to use the nrfx library with the nrfx_pwm_simple_playback() function. Though the library seems to be aimed at nRF52, it is included in zephyr and compiles and runs without crashing just fine on nRF9160. 

GPIO and PWM are working in this project, just the nrfx_pwm_simple_playback() isn't working as expected. The output on the scope is showing an unchanging wave at ~10kHz with a duty of around 95%, I have no idea where this configuration comes from. The output from the pins do not seem to be following the sequence I have configured, and I am wondering if this isn't actually supposed to work, or whether my code is just incorrect. 

Is this library expected to work on nRF9160? If not, is there a similarly easy way to get the right result on nRF9160?

I'm using the 1.0.0 DK with SDK 1.5.0 and modem firmware 1.2.3. I've included some code for review. 

Thank you for your help!

nrfx_pwm_config_t conf = {
    .output_pins   = { 
        leds[0].led_pin,    /*  Extracted from DTS, P0.16 for easy scope probing */
        leds[1].led_pin,    /*  Extracted from DTS, P0.3 */
        leds[2].led_pin,    /*  Extracted from DTS, P0.4 */
        leds[3].led_pin     /*  Extracted from DTS, P0.5 */
     },                                   
    .irq_priority  = NRFX_PWM_DEFAULT_CONFIG_IRQ_PRIORITY,
    .base_clock    = NRF_PWM_CLK_1MHz,                    
    .count_mode    = NRF_PWM_MODE_UP,                     
    .top_value     = 1000,                                
    .load_mode     = NRF_PWM_LOAD_COMMON,                 
    .step_mode     = NRF_PWM_STEP_AUTO,                   
    .skip_gpio_cfg = false 
};

nrfx_pwm_init(&_nrfx_pwm_dev, &conf, unused_dummy_handler, NULL);

static nrf_pwm_values_common_t seq_values[32];
for (uint8_t i = 0; i < 8; i++)
{
    seq_values[i] = (i * 2 + 1) * 62;
    seq_values[15-i] = (i * 2 + 1) * 62;
    seq_values[16+i] = (i * 2 + 1) * 62;
    seq_values[31-i] = (i * 2 + 1) * 62;
}

nrf_pwm_sequence_t seq = { .values.p_common = seq_values, .length = 32, .repeats = 1, .end_delay = 0 };

nrfx_pwm_simple_playback(&_nrfx_pwm_dev, &seq, 1, 0);

[Simon]

Could you check out the NCS tutorial part 2 section 3.2: https://devzone.nordicsemi.com/nordic/nrf-connect-sdk-guides/b/getting-started/posts/nrf-connect-sdk-tutorial---part-2-ncs-v1-4-0#h22sjwht26buofm6h31gzxdos802c0 

It will show how to use PWM with the nrf9160DK and NCS v1.4.0 (not too different from NCS v1.5.0). It may be helpful to complete part 0 and 1 Your first “Hello World” first

Best regards,

Simon

[Jake]

Hi Simon, that's not so useful unfortunately. I already have PWM working just fine and am currently using a timer to periodically change the duty cycle following a sequence. My question was how to use DMA to achieve this whilst saving power, nrfx_pwm_simple_playback() is supposed to do this but doesn't seem to be working on nRF9160. 

[Simon]

Okay, sorry. I was too quick to answer. I'll look into it We/Thu.

[Jake]

Hi Simon, any idea on this? Could you find out whether nrfx_pwm_simple_playback() is currently expected to work on nRF9160? 

[Simon]

I can see that there is a sample using nrfx_pwm_simple_playback() here: https://devzone.nordicsemi.com/f/nordic-q-a/35541/nrfx-pwm-driver-timing/137057#137057 (built for the nRF5 SDK). Try modifying your sample according to that.

If you still don't get it to work, could you upload your sample here, then I can test it on an nRF52 DK and figure out whether the issues your're seeing is due to the sample or the particular board.

[Jake]

Hi Simon, 

I've made a stripped down empty project using that code from MartinBL, and am trying to run it on NRF52 DK now (to check). Weirdly, even this isn't working! nrfx_pwm_init() is always returning an error because it was already previously initialised (somehow).

I noticed when debugging, if I place a breakpoint inside nrfx_pwm_init(), it will always hit that point BEFORE even entering main() in main.c. Very strange. I am thinking this is something to do with Zephyr executing a whole bunch of stuff before any of my code runs; how can I find out or stop this being pre-initialised? This is preventing me from being able to run nrfx_pwm_init() myself and load in the right config. 


The picture shoes the breakpoint being hit BEFORE main() in main.c is entered... the state is NRFX_DRV_STATE_UNINITIALIZED. Later when I try to run nrfx_pwm_init() myself, the state is NRFX_DRV_STATE_INITIALIZED so the init fails. 

I've attached my test project for you to try yourself. Using SDK 1.5.0 and nrf52dk_nrf52832 as the board. 

If you have any insight into what is going on here, that would be really useful :) Thank you!

test_proj.zip

[Simon]

You should not mix the Zephyr layers and the nfx layers. If you're using the nrfx drivers you should not set CONFIG_PWM in the prj.conf file.

The function nrfx_pwm_init() will be run by the kernel before main. This is because the driver zephyr/drivers/pwm/pwm_nrfx.c (translation layer between zephyr api and nordic specific api) will get added (or, CONFIG_PWM will add the folder where pwm_nrfx.c is located, and then it will get added) when CONFIG_PWM is set and a Nordic board is used. DEVICE_DT_DEFINE will make it such that pwm_nrfx_init()-->nrfx_pwm_init() will run before main.

I would recommend you to look at the sample zephyr/samples/boards/nrf/nrfx to get an intuition on how to go about using the nrfx api.

Best regards,

Simon

[Jake]

Ahh I understand. That's perfect, thank you! Just switching to CONFIG_NRFX_PWM=y has made the init work. Now the playback is exiting cleanly but then the system is restarting immediately after... I'll look into your links and see what I'm missing. 

Thank you :) 

[Simon]

A wild guess is that the restart is caused by PWM interrupt that is not handled, and you need to do something like this: https://github.com/nrfconnect/sdk-zephyr/blob/v2.4.99-ncs2/samples/boards/nrf/nrfx/src/main.c#L35-L37.

Setting CONFIG_RESET_ON_FATAL_ERROR=y and CONFIG_LOG=y might prevent the restart and give you more information about the fault.

Please get back to me if you don't figure this out and I'll look into it.

[Jake]

That was exactly the issue, thanks Simon! I've moved back to nRF91 with this and it is working now :) Thanks again