GPIOTE PWM config question

[Tim]

Hi, in the (not great) mBed PWM driver, when setting up GPIOTE to toggle the output pin they do this:

/** @brief Function for initializing the GPIO Tasks/Events peripheral.
 */
void gpiote_init(PinName pin, uint8_t channel_number)
{
    // Connect GPIO input buffers and configure PWM_OUTPUT_PIN_NUMBER as an output.
    NRF_GPIO->PIN_CNF[pin] = (GPIO_PIN_CNF_SENSE_Disabled << GPIO_PIN_CNF_SENSE_Pos)
                            | (GPIO_PIN_CNF_DRIVE_S0S1 << GPIO_PIN_CNF_DRIVE_Pos)
                            | (GPIO_PIN_CNF_PULL_Disabled << GPIO_PIN_CNF_PULL_Pos)
                            | (GPIO_PIN_CNF_INPUT_Connect << GPIO_PIN_CNF_INPUT_Pos)
                            | (GPIO_PIN_CNF_DIR_Output << GPIO_PIN_CNF_DIR_Pos);
    NRF_GPIO->OUTCLR = (1UL << pin);
    // Configure GPIOTE channel 0 to toggle the PWM pin state
    // @note Only one GPIOTE task can be connected to an output pin.
    /* Configure channel to Pin31, not connected to the pin, and configure as a tasks that will set it to proper level */
    NRF_GPIOTE->CONFIG[channel_number] = (GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos) |
                                         (31UL << GPIOTE_CONFIG_PSEL_Pos) |
                                         (GPIOTE_CONFIG_POLARITY_HiToLo << GPIOTE_CONFIG_POLARITY_Pos);
    /* Three NOPs are required to make sure configuration is written before setting tasks or getting events */
    __NOP();
    __NOP();
    __NOP();
    /* Launch the task to take the GPIOTE channel output to the desired level */
    NRF_GPIOTE->TASKS_OUT[channel_number] = 1;

    /* Finally configure the channel as the caller expects. If OUTINIT works, the channel is configured properly.
       If it does not, the channel output inheritance sets the proper level. */
    NRF_GPIOTE->CONFIG[channel_number] = (GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos) |
                                         ((uint32_t)pin << GPIOTE_CONFIG_PSEL_Pos) |
                                         ((uint32_t)GPIOTE_CONFIG_POLARITY_Toggle << GPIOTE_CONFIG_POLARITY_Pos) |
                                         ((uint32_t)GPIOTE_CONFIG_OUTINIT_Low << GPIOTE_CONFIG_OUTINIT_Pos); // ((uint32_t)GPIOTE_CONFIG_OUTINIT_High <<
                                                                                                             // GPIOTE_CONFIG_OUTINIT_Pos);//

    /* Three NOPs are required to make sure configuration is written before setting tasks or getting events */
    __NOP();
    __NOP();
    __NOP();
}

Can someone explain why they set pin 31 (which doesn't exist) to low using GPIOTE? What won't work if you only do the final CONFIG? Also what do the mean by If OUTINIT works? Does it not always work?

[Tim]

Ah I think I understand a bit now. Maybe. When they connect GPIOTE to the pin it will immediately set the pin to the internal GPIOTE level, which may be high, and therefore cause an unwanted pulse. Therefore to get the internal GPIOTE level low they disconnect it from any pins and trigger the HiToLo task. Then the internal level is low so when the reconnect it to the target pin it will stay low. This is just a guess but it makes sense to me.