Support sleep for NRF52 boards by suspending the main task

This uses the core functions suspendLoop() and resumeLoop() to suspend
the main task and put the CPU in a low power idle mode.

The wakeup occurs either through the specified timeout using a timer
interrupt or through an RX interrupt from the radio module.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

Author: fschrempf

src/helpers/NRF52Board.cpp

@@ -4,6 +4,7 @@
 #include <bluefruit.h>
 
 static BLEDfu bledfu;
+static SoftwareTimer sleep_timer;
 
 static void connect_callback(uint16_t conn_handle) {
   (void)conn_handle;
@@ -17,8 +18,27 @@ static void disconnect_callback(uint16_t conn_handle, uint8_t reason) {
   MESH_DEBUG_PRINTLN("BLE client disconnected");
 }
 
+static void resume() {
+  sleep_timer.stop();
+  resumeLoop();
+}
+
+static void sleep_timer_callback(TimerHandle_t xTimerID) {
+  resume();
+}
+
 void NRF52Board::begin() {
   startup_reason = BD_STARTUP_NORMAL;
+  sleep_timer.begin(5000, sleep_timer_callback, NULL, false);
+}
+
+void NRF52Board::sleep(uint32_t secs) {
+  sleep_timer.setPeriod(secs * 1000);
+  suspendLoop();
+}
+
+void NRF52Board::onRXInterrupt() {
+  resume();
 }
 
 void NRF52BoardDCDC::begin() {

src/helpers/NRF52Board.h

@@ -14,6 +14,8 @@ class NRF52Board : public mesh::MainBoard {
   virtual uint8_t getStartupReason() const override { return startup_reason; }
   virtual float getMCUTemperature() override;
   virtual void reboot() override { NVIC_SystemReset(); }
+  virtual void sleep(uint32_t secs) override;
+  virtual void onRXInterrupt() override;
 };
 
 /*