ESP32-C3: LEDC output during light sleep / low power mode

[TorinCB]

Hello all,

My goal is simple: to get an ESP32-C3 LEDC peripheral to output a PWM signal during light sleep.

The technical reference manual states that this is possible if FOSC_CLK is selected as the common clock source for the LEDC peripheral. This is achieved in ESP-IDF by setting `ledc_timer_config_t.clk_cfg` to `LEDC_USE_RTC8M_CLK` (if you dig into the HAL, you'll see that this results in `APB_CLK_SEL` being set to 2, which is the value for FOSC_CLK according to the manual).

My simple test program below sets up the LEDC peripheral and then toggles between power-on and light sleep. (I have it hooked up to a power profiler, so I can definitely tell that light sleep is achieved.)

Code: Select all

extern "C" {

void app_main()
{
    const ledc_timer_config_t timer_cfg = {
        .speed_mode = LEDC_LOW_SPEED_MODE,
        .duty_resolution = LEDC_TIMER_14_BIT,
        .timer_num = LEDC_TIMER_0,
        .freq_hz = 1000,
        .clk_cfg = LEDC_USE_RTC8M_CLK,
    };
    ESP_ERROR_CHECK(ledc_timer_config(&timer_cfg));

    const ledc_channel_config_t chan_cfg = {
        .gpio_num = 8,
        .speed_mode = LEDC_LOW_SPEED_MODE,
        .channel = LEDC_CHANNEL_0,
        .intr_type = LEDC_INTR_DISABLE,
        .timer_sel = LEDC_TIMER_0,
        .duty = 0,
        .hpoint = 0,
        .flags = {
            .output_invert = 0,
        },
    };
    ESP_ERROR_CHECK(ledc_channel_config(&chan_cfg));

    ESP_ERROR_CHECK(ledc_set_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_0, 1 << 13));
    ESP_ERROR_CHECK(ledc_update_duty(LEDC_LOW_SPEED_MODE, LEDC_CHANNEL_0));

    while (true) {
        // full power 1 sec
        vTaskDelay(pdMS_TO_TICKS(1000));

        // low power 1 sec
        ESP_ERROR_CHECK(esp_sleep_enable_timer_wakeup(1000000));
        ESP_ERROR_CHECK(esp_light_sleep_start());
    }
}

}

I've tried with IO7 and IO8 (on this board, there are limited exposed free GPIO).

I find consistently that the output is perfect during power-on, but remains at 0V during light sleep.

Can anyone please spot what I may have missed?

My board is using a ESP32-C3-MINI-1 module. We utilise a 32.768 kHz external crystal as our RTC source (to allow BLE modem sleep during light sleep), but for the purposes of this, I've disabled it in sdkconfig.

[St0fF64]

Code: Select all

ledc_sleep_mode_t::LEDC_SLEEP_MODE_KEEP_ALIVE

could be spilled into the `ledc_channel_config_t` structure's `sleep_mode` member, before configuring the channel.

P.S.: that's why I like C++ - you really get a warning about uninitialized structure members.