FreeRTOS Interrupt Management

Interrupts are events that the OS scheduler can not understand. This event is made available to the scheduler by a semaphore (like a queue of length 1).

There are external interrupts by attachInterrupt and alarm interrupts by RTC library as interrupt processing in GR-ROSE SDK. Semaphores are used to link these interrupts with tasks.

The following is an example of task processing to notify when an alarm interrupt of RTC library occurs

#include <Arduino.h>
#include <RTC.h>
#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"
RTC rtc;
int g_year    = 2019;
int g_mon     = 5;
int g_day     = 29;
int g_hour    = 9;
int g_min     = 20;
int g_sec     = 55;
int g_week    = RTC_WEEK_WEDNESDAY;
void rtcAlarmISR();
SemaphoreHandle_t xBinarySemaphore;
void vHandlerTask(void *pvParameters);
void vPeriodicTask(void *pvParameters);
void setup(){
  pinMode(PIN_LED1, OUTPUT); // led for alarm
  digitalWrite(PIN_LED1, LOW); //turn off
  rtc.setDateTime(g_year, g_mon, g_day, g_hour, g_min, g_sec, g_week);
  rtc.setAlarmTime(g_hour, g_min + 1, g_week);
  xBinarySemaphore = xSemaphoreCreateBinary();
  xTaskCreate( vHandlerTask, "Handler", 512, NULL, 3, NULL );
  xTaskCreate( vPeriodicTask, "Periodic", 512, NULL, 2, NULL );
void loop(){
  delay(100); // entering Blocked State
void rtcAlarmISR()
  xSemaphoreGiveFromISR( xBinarySemaphore, pdFALSE );
void vHandlerTask(void *pvParameters){
    xSemaphoreTake(xBinarySemaphore, portMAX_DELAY);
    Serial.println("Got semaphore for alarm ");
    digitalWrite(PIN_LED1, HIGH);// led for alarm
void vPeriodicTask(void *pvParameters){
    rtc.getDateTime(g_year, g_mon, g_day, g_hour, g_min, g_sec, g_week);
    Serial.print(g_year, DEC);Serial.print("/");
    Serial.print(g_mon, DEC); Serial.print("/");
    Serial.print(g_day, DEC); Serial.print(" ");
    Serial.print(g_hour, DEC); Serial.print(":");
    Serial.print(g_min, DEC); Serial.print(":");
    Serial.println(g_sec, DEC);

Output result:
2019/5/29 9:20:55
2019/5/29 9:20:56
2019/5/29 9:20:57
2019/5/29 9:20:58
2019/5/29 9:20:59
Got semaphore for alarm
2019/5/29 9:21:0
2019/5/29 9:21:1

In the above example, the alarm interrupt is "rtcAlarmISR ()". This is not managed as an OS, and making this process longer affects the scheduling of the OS.

In the alarm interrupt, only xSemaphoreGiveFromISR () is executed to "Give" the semaphore. In the task "vHandlerTask", wait until the semaphore can be "Take" by xSemaphoreTake ().

By doing this, interrupt processing can be minimized and tasks can be executed under OS scheduling.