Support sleep for NRF52 boards by suspending the main task

This uses a FreeRTOS semaphore to block the main loop task and resume
it on RX radio IRQ or timeout.

Using the Arduino core functions suspendLoop() and resumeLoop() can
lead to deadlocks which can only be avoided by disabling IRQs and
possibly missing incoming packets.

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

Author: fschrempf

src/helpers/NRF52Board.cpp

@@ -19,6 +19,18 @@ static void disconnect_callback(uint16_t conn_handle, uint8_t reason) {
 
 void NRF52Board::begin() {
   startup_reason = BD_STARTUP_NORMAL;
+  rx_wait_sem = xSemaphoreCreateBinary();
+  xSemaphoreGive(rx_wait_sem);
+  xSemaphoreTake(rx_wait_sem, portMAX_DELAY);
+}
+
+void NRF52Board::sleep(uint32_t secs) {
+  MESH_DEBUG_PRINTLN("Suspending loop for powersaving, set wakeup timer in %u seconds", secs);
+  xSemaphoreTake(rx_wait_sem, pdMS_TO_TICKS(secs * 1000));
+}
+
+void NRF52Board::onRXInterrupt() {
+  xSemaphoreGive(rx_wait_sem);
 }
 
 void NRF52BoardDCDC::begin() {

src/helpers/NRF52Board.h

@@ -6,6 +6,9 @@
 #if defined(NRF52_PLATFORM)
 
 class NRF52Board : public mesh::MainBoard {
+private:
+  SemaphoreHandle_t rx_wait_sem;
+
 protected:
   uint8_t startup_reason;
 
@@ -14,6 +17,8 @@ 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;
 };
 
 /*