ESP8266 TimerInterrupt Library

This library enables you to use Interrupt from Hardware Timers on an ESP8266-based board.

Why do we need this Hardware Timer Interrupt?

Imagine you have a system with a mission-critical function, measuring water level and control the sump pump or doing something much more important. You normally use a software timer to poll, or even place the function in loop(). But what if another function is blocking the loop() or setup().

So your function might not be executed, and the result would be disastrous.

You'd prefer to have your function called, no matter what happening with other functions (busy loop, bug, etc.).

The correct choice is to use a Hardware Timer with Interrupt to call your function.

These hardware timers, using interrupt, still work even if other functions are blocking. Moreover, they are much more precise (certainly depending on clock frequency accuracy) than other software timers using millis() or micros(). That's necessary if you need to measure some data requiring better accuracy.

Functions using normal software timers, relying on loop() and calling millis(), won't work if the loop() or setup() is blocked by certain operation. For example, certain function is blocking while it's connecting to WiFi or some services.

The catch is your function is now part of an ISR (Interrupt Service Routine), and must be lean / mean, and follow certain rules. More to read on:

Attach Interrupt

Important Notes:

1. Inside the attached function, delay() won’t work and the value returned by millis() will not increment. Serial data received while in the function may be lost. You should declare as volatile any variables that you modify within the attached function.

2. Typically global variables are used to pass data between an ISR and the main program. To make sure variables shared between an ISR and the main program are updated correctly, declare them as volatile.

Installation

Use Arduino Library Manager

The suggested and easiest way is to use Arduino Library Manager. Search for ESP8266TimerInterrupt, then select / install the latest version.

Manual Install

1. Navigate to ESP8266TimerInterrupt.
2. Download the latest release ESP8266TimerInterrupt-master.zip.
3. Extract the zip file to ESP8266TimerInterrupt-master directory
4. Copy whole folder to Arduino libraries' directory such as .Arduino/libraries/ESP8266TimerInterrupt-master.

More useful Information

The ESP8266 timers are badly designed, using only 23-bit counter along with maximum 256 prescaler. They're only better than UNO / Mega. The ESP8266 has two hardware timers, but timer0 has been used for WiFi and it's not advisable to use. Only timer1 is available. The timer1's 23-bit counter terribly can count only up to 8,388,607. So the timer1 maximum interval is very short. Using 256 prescaler, maximum timer1 interval is only 26.843542 seconds !!!

The timer1 counters can be configured to support automatic reload.

New from v1.0.2

Now with these new 16 ISR-based timers, the maximum interval is practically unlimited (limited only by unsigned long miliseconds) The accuracy is nearly perfect compared to software timers. The most important feature is they're ISR-based timers Therefore, their executions are not blocked by bad-behaving functions / tasks. This important feature is absolutely necessary for mission-critical tasks.

The ISR_Timer_Complex example will demonstrate the nearly perfect accuracy compared to software timers by printing the actual elapsed millisecs of each type of timers. Being ISR-based timers, their executions are not blocked by bad-behaving functions / tasks, such as connecting to WiFi, Internet and Blynk services. You can also have many (up to 16) timers to use. This non-being-blocked important feature is absolutely necessary for mission-critical tasks. You'll see blynkTimer Software is blocked while system is connecting to WiFi / Internet / Blynk, as well as by blocking task in loop(), using delay() function as an example. The elapsed time then is very unaccurate

Supported Boards

• ESP8266

Usage

How to use:

//These define's must be placed at the beginning before #include "ESP8266TimerInterrupt.h"
#define TIMER_INTERRUPT_DEBUG      1

#include "ESP8266TimerInterrupt.h"

#ifndef LED_BUILTIN
#define LED_BUILTIN       2         // Pin D4 mapped to pin GPIO2/TXD1 of ESP8266, NodeMCU and WeMoS, control on-board LED
#endif

volatile uint32_t lastMillis = 0;

void ICACHE_RAM_ATTR TimerHandler(void)
{
  static bool toggle = false;
  static bool started = false;

  if (!started)
  {
    started = true;
    pinMode(LED_BUILTIN, OUTPUT);
  }

  #if (TIMER_INTERRUPT_DEBUG > 0)
  if (lastMillis != 0)
    Serial.println("Delta ms = " + String(millis() - lastMillis));
  lastMillis = millis();
  #endif
  
  //timer interrupt toggles pin LED_BUILTIN
  digitalWrite(LED_BUILTIN, toggle);
  toggle = !toggle;
}

#define TIMER_INTERVAL_MS        1000 

// Init ESP8266 timer 0
ESP8266Timer ITimer;


void setup()
{
  Serial.begin(115200);
  Serial.println("\nStarting");
  
  // Interval in microsecs
  if (ITimer.attachInterruptInterval(TIMER_INTERVAL_MS * 1000, TimerHandler))
    Serial.println("Starting  ITimer OK, millis() = " + String(millis()));
  else
    Serial.println("Can't set ITimer correctly. Select another freq. or interval");

}

void loop()
{
  
}

TO DO

1. Search for bug and improvement.

DONE

For current version v1.0.2

1. Basic hardware timers for ESP8266.
2. Fix compatibility issue causing compiler error while using Arduino IDEs before 1.8.10 and ESP8266 cores 2.5.2 and before
3. More hardware-initiated software-enabled timers
4. Longer time interval
5. Similar features for Arduino (UNO, Mega, etc...) and ESP32

Contributing

If you want to contribute to this project:

• Report bugs and errors
• Ask for enhancements
• Create issues and pull requests
• Tell other people about this library

Copyright

Copyright 2019- Khoi Hoang