Update RepeatTimer example

Implement semaphore and mutex to safeguard the ISR.
This is the proper way to handle hardware Interrupts with FreeRTOS. The
more regular approach may/will lead to exceptions

libraries/ESP32/examples/Timer/RepeatTimer/RepeatTimer.ino

@@ -12,17 +12,21 @@
 #define BTN_STOP_ALARM    0
 
 hw_timer_t * timer = NULL;
+volatile SemaphoreHandle_t timerSemaphore;
+portMUX_TYPE timerMux = portMUX_INITIALIZER_UNLOCKED;
 
-void onTimer(){
-  static unsigned int counter = 1;
+volatile uint32_t isrCounter = 0;
+volatile uint32_t lastIsrAt = 0;
 
-  Serial.print("onTimer no. ");
-  Serial.print(counter);
-  Serial.print(" at ");
-  Serial.print(millis());
-  Serial.println(" ms");
-
-  counter++;
+void IRAM_ATTR onTimer(){
+  // Increment the counter and set the time of ISR
+  portENTER_CRITICAL_ISR(&timerMux);
+  isrCounter++;
+  lastIsrAt = millis();
+  portEXIT_CRITICAL_ISR(&timerMux);
+  // Give a semaphore that we can check in the loop
+  xSemaphoreGiveFromISR(timerSemaphore, NULL);
+  // It is safe to use digitalRead/Write here if you want to toggle an output
 }
 
 void setup() {
@@ -31,6 +35,9 @@ void setup() {
   // Set BTN_STOP_ALARM to input mode
   pinMode(BTN_STOP_ALARM, INPUT);
 
+  // Create semaphore to inform us when the timer has fired
+  timerSemaphore = xSemaphoreCreateBinary();
+
   // Use 1st timer of 4 (counted from zero).
   // Set 80 divider for prescaler (see ESP32 Technical Reference Manual for more
   // info).
@@ -48,6 +55,21 @@ void setup() {
 }
 
 void loop() {
+  // If Timer has fired
+  if (xSemaphoreTake(timerSemaphore, 0) == pdTRUE){
+    uint32_t isrCount = 0, isrTime = 0;
+    // Read the interrupt count and time
+    portENTER_CRITICAL(&timerMux);
+    isrCount = isrCounter;
+    isrTime = lastIsrAt;
+    portEXIT_CRITICAL(&timerMux);
+    // Print it
+    Serial.print("onTimer no. ");
+    Serial.print(isrCount);
+    Serial.print(" at ");
+    Serial.print(isrTime);
+    Serial.println(" ms");
+  }
   // If button is pressed
   if (digitalRead(BTN_STOP_ALARM) == LOW) {
     // If timer is still running