add new sketch for RTC running in MIX mode with Alarm

This examples is configuring the RTC in MIX mode
(binary + calendar BCD) and set Alarm B (if exists)
few ms after the current time, whenAlarm A is set
in calendar mode.
Mainly used on stm32wl55 devices.

examples/mixRTCAlarm/mixRTCAlarm.ino

@@ -0,0 +1,97 @@
+/*
+  mode Mix RTC alarm
+
+  This sketch shows how to configure the alarm A & B of the RTC in MIX mode
+
+  Creation 12 Dec 2017
+  by Wi6Labs
+  Modified 03 Jul 2020
+  by Frederic Pillon for STMicroelectronics
+  Modified 03 Jul 2023
+  by Francois Ramu for STMicroelectronics
+
+  This example code is in the public domain.
+
+  https://github.com/stm32duino/STM32RTC
+*/
+
+#include <STM32RTC.h>
+
+/* Get the rtc object */
+STM32RTC& rtc = STM32RTC::getInstance();
+
+/* Change these values to set the current initial time */
+const byte seconds = 06;
+const byte minutes = 22;
+const byte hours = 16;
+
+/* Change these values to set the current initial date */
+const byte day = 25;
+const byte month = 6;
+const byte year = 23;
+
+uint32_t timeout;
+
+void setup()
+{
+  Serial.begin(115200);
+
+  // Select RTC clock source: LSI_CLOCK, LSE_CLOCK or HSE_CLOCK.
+  rtc.setClockSource(STM32RTC::LSE_CLOCK);
+
+  /* Configure the MIX mode */ 
+  rtc.setBinaryMode(STM32RTC::MODE_MIX);
+
+  rtc.begin(true, STM32RTC::HOUR_24);
+
+  rtc.setTime(hours, minutes, seconds);
+  rtc.setDate(day, month, year);
+
+  /* wait for a while */
+  delay(200);
+
+  Serial.printf("Start at %02d:%02d:%02d.%03d\r\n",
+          rtc.getHours(), rtc.getMinutes(), rtc.getSeconds(), rtc.getSubSeconds());
+
+  /* Attach the callback function before enabling Interrupt */
+  rtc.attachInterrupt(alarmAMatch);
+
+  /* Program the AlarmA in a 12 seconds */
+  rtc.setAlarmDay(day);
+  rtc.setAlarmTime(hours, minutes, seconds + 12);
+  rtc.enableAlarm(rtc.MATCH_DHHMMSS);
+  Serial.printf("Set Alarm A in 12s (at %02d:%02d:%02d)\r\n",
+          rtc.getAlarmHours(), rtc.getAlarmMinutes(), rtc.getAlarmSeconds());
+
+#ifdef RTC_ALARM_B
+  /* Program ALARM B in 400ms ms from now (keep timeout < 1000ms) */
+  timeout = rtc.getSubSeconds() + 400;
+
+  rtc.attachInterrupt(alarmBMatch, STM32RTC::ALARM_B);
+  rtc.setAlarmSubSeconds(timeout, STM32RTC::ALARM_B);
+  rtc.enableAlarm(rtc.MATCH_SUBSEC, STM32RTC::ALARM_B);
+  Serial.printf("Set Alarm B (in %d ms) at %d ms\r\n", 400,
+  rtc.getAlarmSubSeconds(STM32RTC::ALARM_B));
+#endif
+
+}
+
+void loop()
+{
+
+}
+
+void alarmAMatch(void *data)
+{
+  UNUSED(data);
+  Serial.printf("Alarm A Match at %02d:%02d:%02d\r\n",
+          rtc.getHours(), rtc.getMinutes(), rtc.getSeconds());
+}
+
+void alarmBMatch(void *data)
+{
+  UNUSED(data);
+  Serial.printf("Alarm B Match at %d ms\r\n", rtc.getSubSeconds());
+}
+
+