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?