Workflow

Switch performance level to PL2.

static void performance_level_switch_test(void)
{
    struct system_gclk_gen_config gclk_conf;

    /* Switch to PL2 */
    system_switch_performance_level(SYSTEM_PERFORMANCE_LEVEL_2);
    system_flash_set_waitstates(2);

    /* Switch GCLK0 to 48MHz */
    system_gclk_gen_get_config_defaults(&gclk_conf);
    gclk_conf.source_clock = SYSTEM_CLOCK_SOURCE_DFLL;
    gclk_conf.division_factor = 1;
    gclk_conf.run_in_standby  = false;
    gclk_conf.output_enable   = true;
    system_gclk_gen_set_config(GCLK_GENERATOR_0, &gclk_conf);
}

Configure GCLK0/GCLK1 output pin and extwakeup pin.

static void config_clock_output_and_extwake_pin(void)
{
    struct system_pinmux_config pin_conf;
    system_pinmux_get_config_defaults(&pin_conf);

    pin_conf.mux_position = CONF_GCLK0_OUTPUT_PINMUX;
    pin_conf.direction    = SYSTEM_PINMUX_PIN_DIR_OUTPUT;
    system_pinmux_pin_set_config(CONF_GCLK0_OUTPUT_PIN, &pin_conf);
    pin_conf.mux_position = CONF_GCLK1_OUTPUT_PINMUX;
    system_pinmux_pin_set_config(CONF_GCLK1_OUTPUT_PIN, &pin_conf);

    pin_conf.direction = SYSTEM_PINMUX_PIN_DIR_INPUT;
    pin_conf.input_pull = SYSTEM_PINMUX_PIN_PULL_UP;
    pin_conf.mux_position = CONF_EXT_WAKEUP_PINMUX;
    system_pinmux_pin_set_config(CONF_EXT_WAKEUP_PIN, &pin_conf);
}

Config external interrupt.

static void configure_extint_channel(void)
{

    struct extint_chan_conf config_extint_chan;
    extint_chan_get_config_defaults(&config_extint_chan);
    config_extint_chan.gpio_pin           = BUTTON_0_EIC_PIN;
    config_extint_chan.gpio_pin_mux       = BUTTON_0_EIC_MUX;
    config_extint_chan.gpio_pin_pull      = EXTINT_PULL_UP;
    config_extint_chan.detection_criteria = EXTINT_DETECT_BOTH;
    extint_chan_set_config(BUTTON_0_EIC_LINE, &config_extint_chan);
    extint_chan_enable_callback(BUTTON_0_EIC_LINE,EXTINT_CALLBACK_TYPE_DETECT);
    system_interrupt_enable_global();
    while (extint_chan_is_detected(BUTTON_0_EIC_LINE)) {
        extint_chan_clear_detected(BUTTON_0_EIC_LINE);
    }
}