cpu/sam0_common: implement periph_gpio_read_output

[benpicco]

Contribution description

The GPIO peripheral of the sam0 MCUs have a feature where they can read back the output level of a GPIO as if the GPIO was in input. (As opposed to the currently implemented mode where gpio_read() just returns the configured register value).

This can be useful to detect hardware defects.
The drawback is slightly higher power draw (according to the data sheet, didn't measure), so guard this behind the periph_gpio_read_output, just like periph_gpio_fast_read.

Testing procedure

The GPIO test application tests/periph/gpio has been updated to make use of the feature when available.

2025-12-16 17:22:39,054 # > init_out 1 5
2025-12-16 17:22:41,031 # > read 1 5
2025-12-16 17:22:41,033 # GPIO_PIN(1.05) is LOW

2025-12-16 17:22:43,160 # > set 1 5
2025-12-16 17:22:44,309 # > read 1 5
2025-12-16 17:22:44,311 # GPIO_PIN(1.05) is HIGH

2025-12-16 17:22:48,471 # > clear 1 5
2025-12-16 17:22:49,971 # > read 1 5
2025-12-16 17:22:49,973 # GPIO_PIN(1.05) is LOW

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

403c748 tests/periph/gpio: use periph_gpio_read_output when available

Success	Failures	Total	Runtime
10950	0	10950	09m:13s

Artifacts

• Documentation preview