tests/periph_pm: use button as a wake-up source

[benpicco]

Contribution description

To test if GPIO interrupts can wake the CPU from sleep, configure BTN0 (if available) as a wake-up source.

Pressing the buttong should wake up the CPU.

Testing procedure

On samr21-xpro enter set 0 to enter the deepest sleep mode.
You will find that the CPU does not react to UART input anymore.
Press the button on the board.

A Button pressed. message should be printed and the CPU should return to the previous operation mode.

Issues/PRs references

During #12615 it was discussed why GCLK0 wasn't used for the EIC controller. My speculation for the use of the 32kHz clock was that it might still work in deep sleep mode.
This tests confirms that assumption.

With

--- a/cpu/sam0_common/periph/gpio.c
+++ b/cpu/sam0_common/periph/gpio.c
@@ -188,7 +188,7 @@ int gpio_init_int(gpio_t pin, gpio_mode_t mode, gpio_flank_t flank,
        internal or external 32 kHz */
     GCLK->CLKCTRL.reg = (EIC_GCLK_ID |
                          GCLK_CLKCTRL_CLKEN |
-                         GCLK_CLKCTRL_GEN_GCLK2);
+                         GCLK_CLKCTRL_GEN_GCLK0);
     while (GCLK->STATUS.bit.SYNCBUSY) {}
 #else /* CPU_FAM_SAML21 */
     /* enable clocks for the EIC module */

the CPU won't wake up from Deep Sleep when the button is pressed.

A drawback of the slow clock is that fast interrupts are lost. This is a real problem I observed with the at86rf215 driver.
A solution would be to switch the EIC clock to GCLK2 when entering Deep Sleep.

[keestux]

Tested with sodaq-one. Indeed it wakes up when pressing the button (after set 0)

Repeated the test using GCLK0 in gpio.c, then it does not respond to the button press.

[vincent-d]

I did a quick test on a nucleo board, it works as expected.

ACK