Linear decrease of advert rate to conserve battery

Start advertising aggressively when powered on then
slow down linearly over 75 seconds. This will conserve
battery by not advertising rapidly the whole time we
are seeking a connection. The slowest rate is
approximately once every 4.5 seconds to balance
responsiveness and battery life.

We use a fixed advertising duration of 5 seconds and start
with a 62.5 ms advertising interval. Every 5 seconds
(the advertising duration) we step down to a larger
advertising interval (slower advertising). We continue
to decrease the advertising interval linearly for
75 seconds from the start of advertising. At 75 seconds
we have an advertising interval of 4.44 seconds which we
keep until connected. A reboot will restart the sequence.

When we receive a disconnect event we restart the sequence
with fast advertising and then slow down as described above.
Note that we are not using the BLE high duty cycle setting to
change the advertising rate. The rate is managed by repeatedly
setting the minimum and maximum intervals.

The linear rate of decrease and the slowest interval size
were determined experimentally by the author. The 5.3 Core
spec suggests that you not advertise slower than once
every 1.2 seconds to preserve responsiveness but we
ignored that suggestion.

src/components/ble/NimbleController.cpp

@@ -122,6 +122,8 @@ void NimbleController::StartAdvertising() {
 
   adv_params.conn_mode = BLE_GAP_CONN_MODE_UND;
   adv_params.disc_mode = BLE_GAP_DISC_MODE_GEN;
+  adv_params.itvl_min = advInterval;
+  adv_params.itvl_max = advInterval + 100;
 
   fields.flags = BLE_HS_ADV_F_DISC_GEN | BLE_HS_ADV_F_BREDR_UNSUP;
   fields.uuids128 = &dfuServiceUuid;
@@ -148,6 +150,8 @@ int NimbleController::OnGAPEvent(ble_gap_event* event) {
     case BLE_GAP_EVENT_ADV_COMPLETE:
       NRF_LOG_INFO("Advertising event : BLE_GAP_EVENT_ADV_COMPLETE");
       NRF_LOG_INFO("reason=%d; status=%d", event->adv_complete.reason, event->connect.status);
+      if (advInterval < 7100)
+        advInterval += 500;
       StartAdvertising();
       break;
 
@@ -181,6 +185,7 @@ int NimbleController::OnGAPEvent(ble_gap_event* event) {
       alertNotificationClient.Reset();
       connectionHandle = BLE_HS_CONN_HANDLE_NONE;
       bleController.Disconnect();
+      advInterval = 100;
       StartAdvertising();
       break;
 

src/components/ble/NimbleController.h

@@ -94,6 +94,7 @@ namespace Pinetime {
 
       uint8_t addrType; // 1 = Random, 0 = PUBLIC
       uint16_t connectionHandle = BLE_HS_CONN_HANDLE_NONE;
+      uint16_t advInterval = 100; // multiplied by 0.625ms, must be in 32..16384
 
       ble_uuid128_t dfuServiceUuid {
         .u {.type = BLE_UUID_TYPE_128},