new example to configure RTC alarm in BIN mode to wakeup from sleep

In this sketch the RTC is configured in BINary only mode
(for the stm32 devices that supports it)
An Alarm A is set to wakeup the system from low power
deep sleep.

Signed-off-by: Francois Ramu <francois.ramu@st.com>

examples/binAlarmTimedWakeup/binAlarmTimedWakeup.ino

@@ -0,0 +1,78 @@
+/*
+  binAlarmTimedWakeup
+
+  This sketch demonstrates the usage of Internal Interrupts to wakeup a chip in deep sleep mode.
+  when the RTC is configured in BINary mode (BIN_ONLY)
+
+  In this sketch:
+  - RTC in BINARY only for the stm32 that supports this mode
+  - RTC date and time are configured.
+  - Alarm is set to wake up the processor each 'atime' and called a custom alarm callback
+  which increment a value and reload alarm with 'atime' offset.
+
+  This example code is in the public domain.
+*/
+
+#include "STM32LowPower.h"
+#include <STM32RTC.h>
+
+/* Get the rtc object */
+STM32RTC& rtc = STM32RTC::getInstance();
+
+/* Change this value to set alarm match offset in millisecond */
+/* Note that STM32F1xx does not manage subsecond only second */
+static uint32_t atime = 600;
+static uint32_t time_ts; /* value to ge the binary time */
+
+// Declare it volatile since it's incremented inside an interrupt
+volatile int alarmMatch_counter = 0;
+
+void setup() {
+  // Select RTC clock source: LSI_CLOCK, LSE_CLOCK or HSE_CLOCK.
+  rtc.setClockSource(STM32RTC::LSI_CLOCK);
+  // Select the STM32RTC::MODE_BIN
+  rtc.setBinaryMode(STM32RTC::MODE_BIN);
+  rtc.begin(true); /* reset the RTC else the mode is not changed */
+
+  pinMode(LED_BUILTIN, OUTPUT);
+
+  Serial.begin(115200);
+  while (!Serial) {}
+  Serial.println(" Start !");
+
+  // Configure low power
+  LowPower.begin();
+  LowPower.enableWakeupFrom(&rtc, alarmMatch, &atime);
+
+  rtc.getEpoch(&time_ts);
+
+ // Configure first alarm in 2 seconds then it will be done in the rtc callback
+  rtc.setAlarmEpoch(0, STM32RTC::MATCH_SUBSEC, (time_ts + 2000), STM32RTC::ALARM_A);
+}
+
+void loop() {
+  Serial.print("Alarm Match: ");
+  Serial.print(alarmMatch_counter);
+  Serial.println(" times.");
+  Serial.flush();
+  digitalWrite(LED_BUILTIN, HIGH);
+  LowPower.sleep();
+  digitalWrite(LED_BUILTIN, LOW);
+  LowPower.sleep();
+}
+
+void alarmMatch(void* data)
+{
+  // This function will be called once on device wakeup
+  // You can do some little operations here (like changing variables which will be used in the loop)
+  // Remember to avoid calling delay() and long running functions since this functions executes in interrupt context
+  uint32_t _millis = 1000;
+
+  if (data != NULL) {
+    _millis = *(uint32_t*)data;
+  }
+
+  rtc.getEpoch(&time_ts);
+  alarmMatch_counter++;
+  rtc.setAlarmEpoch(0, STM32RTC::MATCH_SUBSEC, (time_ts + _millis), STM32RTC::ALARM_A);
+}