new example to demonstrate the seconds interrupt

enable the second interrupt and display time
each second, thanks to its callback function

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

Author: FRASTM

Diff for: examples/RTC_Seconds/RTC_Seconds.ino

@@ -0,0 +1,100 @@
+/*
+  RTC_Seconds
+
+  This sketch allows to test STM32RTC Seconds IRQ.
+
+  Creation 25 nov 2021
+  by FRASTM for STMicroelectronics
+
+  This example code is in the public domain.
+
+  https://github.com/stm32duino/STM32RTC
+
+*/
+
+#include <STM32RTC.h>
+
+/* use led to display seconds */
+#if defined(LED_BUILTIN)
+#define pin  LED_BUILTIN
+#endif
+
+/* Get the rtc object */
+STM32RTC& rtc = STM32RTC::getInstance();
+
+/* Change these values to set the current initial time
+
+   format: date: "Dec 31 2017" and time: "23:59:56"
+   by default use built date and time
+*/
+/* Change these values to set the current initial time */
+const byte seconds = 0;
+const byte minutes = 5;
+const byte hours = 11;
+
+/* Change these values to set the current initial date */
+/* Monday 25 Nov. 2021 */
+const byte weekDay = 4;
+const byte day = 25;
+const byte month = 11;
+const byte year = 21;
+
+bool toggling = false; // changed each second by the CallBack function
+
+static STM32RTC::Hour_Format hourFormat = STM32RTC::HOUR_24;
+static STM32RTC::AM_PM period = STM32RTC::AM;
+
+void setup()
+{
+  Serial.begin(115200);
+  while (!Serial) {}
+
+#if defined(LED_BUILTIN)
+  // configure pin in output mode
+  pinMode(pin, OUTPUT);
+  digitalWrite(pin, HIGH);
+#endif /* LED_BUILTIN */
+
+  Serial.print("RTC Init ");
+
+  // Select RTC clock source: LSI_CLOCK, LSE_CLOCK or HSE_CLOCK.
+  // By default the LSE is selected as source for better accuracy.
+  rtc.setClockSource(STM32RTC::LSE_CLOCK);
+
+  // initialize RTC 24H format
+  rtc.begin(hourFormat);
+
+  // Set the time
+  rtc.setHours(hours, period);
+  rtc.setMinutes(minutes);
+  rtc.setSeconds(seconds);
+
+  // Set the date
+  rtc.setWeekDay(weekDay);
+  rtc.setDay(day);
+  rtc.setMonth(month);
+  rtc.setYear(year);
+
+  Serial.print("with seconds Alarm \r\n");
+  rtc.attachSecondsInterrupt(rtc_SecondsCB);
+}
+
+void loop()
+{
+  if (toggling) {
+   Serial.printf("%02d:%02d:%02d \r", rtc.getHours(), rtc.getMinutes(), rtc.getSeconds());
+  } else {
+   Serial.printf("%02d %02d %02d \r", rtc.getHours(), rtc.getMinutes(), rtc.getSeconds());
+  }
+#if defined(LED_BUILTIN)
+  digitalWrite(pin, toggling);
+#endif /* LED_BUILTIN */
+  delay(1000);
+}
+
+/* callback function on each second interrupt */
+void rtc_SecondsCB(void *data)
+{
+  UNUSED(data);
+  toggling = !toggling;
+}