[nrf fromtree] samples: boards: nrf: system_off: Add sample with system clock disabled

Extend the system off samples by adding an option to disable
the system clock. When the system clock is disabled additional
power savings can be observed.
After using the `sys_clock_disable()` function, the GRTC is
turned off making system time-related features unavailable.

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>
(cherry picked from commit 127ba8d)

Author: adamkondraciuk

samples/boards/nordic/system_off/Kconfig

@@ -23,4 +23,9 @@ config LPCOMP_WAKEUP_ENABLE
 	help
 	  Enable system off wakeup from analog comparator.
 
+config SYS_CLOCK_DISABLE
+	bool "Power down system clock before system off"
+	help
+	  System clock and GRTC will be switched off during system off.
+
 source "Kconfig.zephyr"

samples/boards/nordic/system_off/sample.yaml

@@ -47,6 +47,44 @@ tests:
         - "Off count: 0"
         - "Active Ticks:"
         - "Entering system off; press sw0 to restart"
+  sample.boards.nrf.system_off.grtc_off:
+    platform_allow:
+      - nrf54l15dk/nrf54l05/cpuapp
+      - nrf54l15dk/nrf54l10/cpuapp
+      - nrf54l15dk/nrf54l15/cpuapp
+      - nrf54lm20dk/nrf54lm20a/cpuapp
+    extra_configs:
+      - CONFIG_SYS_CLOCK_DISABLE=y
+    harness: console
+    harness_config:
+      type: multi_line
+      ordered: true
+      regex:
+        - "system off demo"
+        - "Retained data not supported"
+        - "System clock will be disabled"
+        - "Entering system off; press sw0 to restart"
+  sample.boards.nrf.system_off.retained_mem.grtc_off:
+    platform_allow:
+      - nrf54l15dk/nrf54l05/cpuapp
+      - nrf54l15dk/nrf54l10/cpuapp
+      - nrf54l15dk/nrf54l15/cpuapp
+      - nrf54lm20dk/nrf54lm20a/cpuapp
+    extra_configs:
+      - CONFIG_APP_USE_RETAINED_MEM=y
+      - CONFIG_SYS_CLOCK_DISABLE=y
+    harness: console
+    harness_config:
+      type: multi_line
+      ordered: true
+      regex:
+        - "system off demo"
+        - "Retained data: valid"
+        - "Boot count: 1"
+        - "Off count: 0"
+        - "Active Ticks:"
+        - "System clock will be disabled"
+        - "Entering system off; press sw0 to restart"
   sample.boards.nrf.system_off.grtc_wakeup:
     platform_allow:
       - nrf54l15dk/nrf54l05/cpuapp

samples/boards/nordic/system_off/src/main.c

@@ -17,6 +17,7 @@
 #include <zephyr/pm/device.h>
 #include <zephyr/sys/poweroff.h>
 #include <zephyr/sys/util.h>
+#include <zephyr/drivers/timer/system_timer.h>
 
 #define NON_WAKEUP_RESET_REASON (RESET_PIN | RESET_SOFTWARE | RESET_POR | RESET_DEBUG)
 
@@ -97,11 +98,15 @@ int main(void)
 		printf("Retained data not supported\n");
 	}
 
+#if defined(CONFIG_SYS_CLOCK_DISABLE)
+	printf("System clock will be disabled\n");
+#endif
 #if defined(CONFIG_GRTC_WAKEUP_ENABLE)
 	int err = z_nrf_grtc_wakeup_prepare(DEEP_SLEEP_TIME_S * USEC_PER_SEC);
 
 	if (err < 0) {
 		printk("Unable to prepare GRTC as a wake up source (err = %d).\n", err);
+		return 0;
 	} else {
 		printk("Entering system off; wait %u seconds to restart\n", DEEP_SLEEP_TIME_S);
 	}
@@ -141,6 +146,9 @@ int main(void)
 	}
 
 	hwinfo_clear_reset_cause();
+#if defined(CONFIG_SYS_CLOCK_DISABLE)
+	sys_clock_disable();
+#endif
 	sys_poweroff();
 
 	return 0;