![]() ![]() ![]() Load and run the sketch and you should see that LED D2 now has to pause until the tick_2s_isr() completes before it flashes. Comment out the lines with "interrupts()" and "noInterrupts()" like this: Now what would happen if tick_2s_isr() did not enable interrupts so there was no way for the function to be reentered? Try it. The interrupt ISR is said to be reentrant. When interrupts are enabled the timerIsr() is able to service every interrupt even though it calls a function that takes a long time to process. But the tick_2s_isr() function contains a time consuming loop which takes much longer than 100ms so how is it that the interrupt routine can keep flashing D2? It can do this because tick_2s_isr() enables interrupts using the interrupts() function. This routine is called every 2 seconds as a result. When it reaches zero the function tick_2s_isr() is called. The interrupt service routine timerIsr() counts down a timer variable starting at 20. When the sketch initializes and setup() is run, the ATmega328's timer 1 is set to interrupt every 100ms and every time it interrupts the state of LED D2 is toggled making it flash on and off 5 times a second. If you don't have an oscilloscope you can build yourself one very cheaply using my book as a guide: Sound Card Oscilloscope: Build Better Electronics Projects. If you you speed up the flashing rate by adjusting the values of "TIMER_US" and "TICK_COUNTS" you can observe the pulses on an oscilloscope. In_long_isr = false // Indicates exit of the long isr In_long_isr = true // Indicates we are in the long isrĭigitalWrite( LED1, digitalRead( LED1 ) ^ 1 ) // Toggle LED 1įor (i = 0 i < 400000 i++) // Delay because delay() will not work here If (in_long_isr) // Prevent reentering this code Called every time the count gets to 2S Called every time the hardware timer 1 times out.ĭigitalWrite( LED0, digitalRead( LED0 ) ^ 1 ) // Toggle LED 0 ![]() timerIsr() 100 milli second interrupt ISR() Timer1.attachInterrupt( timerIsr ) // attach the ISR routine here Timer1.initialize(TIMER_US) // Initialise timer 1 PinMode(LED0, OUTPUT) // Set the pins as outputs Volatile bool in_long_isr = false // True if in long interrupt Volatile long tick_count = TICK_COUNTS // Counter for 2S #define TICK_COUNTS 20 // 2S worth of timer ticks #define TIMER_US 100000 // 100mS set timer duration in microseconds #include // Header file for TimerOne library Note: Download and rename to remove the hyphen before installing. Requires the TimerOne library downloadable from LED D1 will flash every 4 seconds and LED D2 will flash 5 times a second Load the sketch onto your Arduino Uno and let it run. This demonstration is based on The ATmega328's Timer 1 so you should download and install the TimerOne library as below.Ĭopy and paste the Arduino interrupt demo sketch below This would have been an interesting exercise but it isn't really what this guide is all about and there is a convenient code library available that does just what we need. It is perfectly possible to write code that directly interfaces with the hardware including the internal ATmega328 timers on the Arduino. Things you need to play with timer interruptsīuy an Arduino Uno now and start building smart electronics projectsĭownload and install the Arduino Timer One library Provide accurate timing in conjunction with hardware timers.Free up your main loop for other things instead of sitting in delay functions.Provide a fast response to external inputs and user interface.In general most tasks can be accomplished without resorting to interrupts but if you do then the use of interrupt service routines can clean up your code and add another dimension to your programmes. Why use interrupts in your Arduino projects ![]() It is true that interrupt use can lead to unpredictable code behaviour that is difficult to debug but they can also greatly enhance your projects if you follow a few simple rules. That is interrupts that are allowed to interrupt themselves. Not only will I show you how to get a regular timed interrupt to work but I will also demonstrate the advantages of reentrant routines. I'm going to show you how you can take advantage of internal timer interrupts on the Arduino using a couple of LED's and a simple code sketch. Most of these sketches or programmes concentrate on getting the job done in the main loop() function and rarely venture into the world of interrupts. There are countless examples of Arduino sketches on the Internet covering all but the most obscure and complicated projects. ![]()
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