NRFX PWM Pin Polarity

[Konstantin Klitenik]

Hello,

I'm using NRFX PWM directly because I need something more custom than zephyr's PWM driver provides. I'm trying to understand what .pin_polarity field in the config does. It does not seem to invert the polarity of the signal.

All .pin_polarity seems to be doing is setting the pin value after PWM has stopped.

.pin_polarity = {true, true, false, false}

.pin_polarity = {false, false, false, false}

-------------------

nrf_pwm_values_individual_t pwm_vals[] = { { 800, 0, 0, 800 },

{ 0, 800, 0, 800 },

{ 0, 0, 0, 800 },

{ 0, 0, 0, 800 } };

nrfx_pwm_t pwm_instance = NRFX_PWM_INSTANCE(HEARTBEATPWM_INST_IDX);

nrfx_pwm_config_t config = {

.output_pins = { 22, 23, NRF_PWM_PIN_NOT_CONNECTED, NRF_PWM_PIN_NOT_CONNECTED },

.pin_inverted = { false, false, false, false },

.irq_priority = NRFX_PWM_DEFAULT_CONFIG_IRQ_PRIORITY,

.base_clock = NRF_PWM_CLK_16MHz,

.count_mode = NRF_PWM_MODE_UP,

.top_value = 800, // 50 µs period (16MHz / 1 prescaler)

.load_mode = NRF_PWM_LOAD_WAVE_FORM,

.step_mode = NRF_PWM_STEP_AUTO,

};

status = nrfx_pwm_init(&pwm_instance, &config, pwm_handler, &pwm_instance);

nrf_pwm_sequence_t seq = { .values.p_individual = pwm_vals,

.length = NRF_PWM_VALUES_LENGTH(pwm_vals),

.repeats = 0,

.end_delay = 0 };

nrfx_pwm_simple_playback(&pwm_instance, &seq, 100, NRFX_PWM_FLAG_LOOP);

-------------------

Is the only way to invert polarity by setting MSB of the PWM to 1 (eg: 800 | (1 << 15))?

Also, I'm a little confused why the default "active" state is LOW. Is that expected?

[Kenneth]

Hello,

I would expect the state of the PWM pins to occur when you call nrfx_pwm_init(), since I can see it's then pwm_configure()->pins_configure() is applied.

You refer to .pin_polarity, I assume you means .pin_inverted. I notice your config is missing the .skip_gpio_cfg = false.

If you also have the pwm node in the devicetree I expect you need to use nordic,invert; for the pin control of the pins.

Kenneth

[Konstantin Klitenik]

Sorry, yes. .pin_polarity meant to be .pin_inverted. I've also since added .skip_gpio_cfg = false and .skip_psel_cfg = false. 

We do have PWM0 in the device tree. That's a separate instance controlling something else. This particular example is using PWM1 and that PWM is NOT defined in the device tree.

Also, binding file for nordic,invert says:

nordic,invert:

type: boolean

description: |

Invert pin polarity (set the active state to low).

which would imply that normal (non-inverted state) should be active high, which is not what I'm seeing above.

[Kenneth]

Hello again,

Think I understand the problem now, see documentation for the PWM:
"PWM parameters are organized into a sequence containing at least one half word (16 bit). Its most significant bit[15] denotes the polarity of the OUT[n] while bit[14:0] is the 15-bit compare value."
From: https://docs.nordicsemi.com/bundle/ps_nrf9151/page/pwm.html#ariaid-title3 

So please try to or with | 0x8000.

Kenneth

[Konstantin Klitenik]

I read that as well and doing that gets me what I want. I'm just confused because the documentation says it should default to active high but what I'm seeing is that it default to active low (hence the need to invert).

I would expected it to work the opposite, based on the documentation.

And if the inversion is done with setting bit 15, what's the purpose of .pin_invert field?

[Konstantin Klitenik]

Any update here?

[Kenneth]

Hi,

I should likely look into the details here, but on a high level:

You can't compare the zephyr pwm driver with the nrfx pwm driver, the configuration and setup is completely different.

When using the zephyr pwm driver it will setup the interface as configured by devicetree as part of the pre-kernel before running to main, in which case the pwm will be controlled according to the zephyr pwm driver.

If you are using nrfx instead, then the pins are completely untouched by zephyr, it's you that need to manually setup the pins to behave as you want, that include potentially setting pins as output pins with desired level, normally this will occur when you call nrfx_pwm_init, until then the pins are likely floating input input pins. The pwm sequence itself will only occur when calling the nrfx_pwm_simple_playback(), in which case the pwm will behave directly according to pwm hardware.

Kenneth