power down accelerometer unless a watch face requests it

watch-faces/sensor/accelerometer_status_face.c

@@ -54,6 +54,13 @@ void accelerometer_status_face_setup(uint8_t watch_face_index, void ** context_p
 void accelerometer_status_face_activate(void *context) {
     accel_interrupt_count_state_t *state = (accel_interrupt_count_state_t *)context;
 
+    // get the current data rate from Movement
+    state->old_rate = movement_get_accelerometer_background_rate();
+    if (state->old_rate == LIS2DW_DATA_RATE_POWERDOWN) {
+        // if accelerometer was powered down, power it up at the lowest possible rate
+        movement_set_accelerometer_background_rate(LIS2DW_DATA_RATE_LOWEST);
+    }
+
     // never in settings mode at the start
     state->is_setting = false;
 
@@ -126,5 +133,9 @@ bool accelerometer_status_face_loop(movement_event_t event, void *context) {
 }
 
 void accelerometer_status_face_resign(void *context) {
-    (void) context;
+    accel_interrupt_count_state_t *state = (accel_interrupt_count_state_t *)context;
+
+    // restore old data rate. This only does something if old_rate was POWERDOWN.
+    // If the old rate was anything other than POWERDOWN, we didn't change anything and this will be a no-op.
+    movement_set_accelerometer_background_rate(state->old_rate);
 }

watch-faces/sensor/accelerometer_status_face.h

@@ -38,6 +38,7 @@
 typedef struct {
     uint8_t new_threshold;
     uint8_t threshold;
+    lis2dw_data_rate_t old_rate;
     bool is_setting;
 } accel_interrupt_count_state_t;
 

watch-faces/sensor/activity_logging_face.c

@@ -73,6 +73,8 @@ void activity_logging_face_setup(uint8_t watch_face_index, void ** context_ptr)
     if (*context_ptr == NULL) {
         *context_ptr = malloc(sizeof(activity_logging_state_t));
         memset(*context_ptr, 0, sizeof(activity_logging_state_t));
+        // At first run, tell Movement to run the accelerometer in the background. It will now run at this rate forever.
+        movement_set_accelerometer_background_rate(LIS2DW_DATA_RATE_LOWEST);
     }
 }