35 votes
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How do interrupts work on the Arduino Uno and similar boards?

TL;DR : When writing an Interrupt Service Routine (ISR): Keep it short Don't use delay () Don't do serial prints Make variables shared with the main code volatile Variables shared with main code may ...
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19 votes
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Using millis() and micros() inside an interrupt routine

The other answers are very good, but I want to elaborate on how micros() works. It always reads the current hardware timer (possibly TCNT0) which is constantly being updated by the hardware (in fact, ...
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18 votes

How many interrupt pins can an Uno handle?

There are two types of "pin change" type interrupts. The external interrupts, of which there are two on the Uno. They are called 0 and 1, however they refer to digital pins 2 and 3 on the board. These ...
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14 votes

Why the need to use the volatile keyword on global variables when handling interrupts in Arduino?

First, it's volatile not Volatile. I cover these concepts in my page about Interrupts however to avoid giving a link-only answer I'll repeat the relevant bits. What are "volatile" variables? ...
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11 votes
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How precise can I get Arduino using rotary encoders?

Interrupts are what you would use in this situation. The rates that you have mentioned are slow enough where you probably would be able to count it inside of a loop, however this is not recommended, ...
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11 votes

Attach the Arduino ISR function to the class member

Interrupt Service Routine (ISR) outside a class Let's consider a simple use of interrupts: volatile bool switchChanged; void switchPressed () { switchChanged = true; } // end of switchPressed ...
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9 votes
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Multiple independent LED patterns

Handling multiple patterns at the same time is certainly possible with a platform like Arduino, and there are a number of ways you could go about it. One method I would consider is writing functions ...
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9 votes
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Is volatile needed when variable is accessed from > 1 ISRs, but not shared outside ISRs?

volatile only informs the compiler's code generator that the variable may be modified by something other than the code being generated, so not to assume any copy of it remains accurate. ISR code must ...
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9 votes
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Counting pulses with interrupt

You need to reset IRQCount back to 0 before attaching the interrupt again. Otherwise it will just continue counting from where it stopped last time. I would actually keep the interrupt attached and ...
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9 votes
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How to properly use volatile variables in Arduino?

You need to learn about critical sections. What is probably happening is that the variables are being changed by the interrupt routines mid-way through the calculations. Your 'fix' reduces the time ...
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9 votes

Calling I2C functions inside an ISR

Here is an analogy for you to help you understand why using the IRQ is a good idea. Imagine you are at a conference. There's hundreds of you in the audience, and you are in a question and answer ...
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9 votes
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What is the right way to query an I2C device from an interrupt service routine?

Just because a pin is called "interrupt" doesn't mean that you have to read it using an interrupt input. The INTx pins of the ADXL345 are simple "level" outputs. All the time there is an "interrupt" ...
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8 votes

How many interrupt pins can an Uno handle?

There are two types of interrupts. What the Arduino Playground said: The processor at the heart of any Arduino has two different kinds of interrupts: “external”, and “pin change”. There are only two ...
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8 votes
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What is causing my Uno to wake up?

sleep_enable() allows the MCU to sleep, but it does not cause it to sleep. Call sleep_cpu() after enabling.
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8 votes
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Is `millis()` affected by long ISRs?

After some digging around in the core, it seems like Arduino updates millis() with a 8 bit timer: it uses overflow with a prescaler value of 64. In simpler terms, it has it set up so a certain piece ...
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8 votes
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What's the benefits between different interrupts on Arduino

Imagine that you have the circuit below: simulate this circuit – Schematic created using CircuitLab Let's say that you wanted to catch the user pressing the button. The input signal will go ...
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8 votes

Using millis() and micros() inside an interrupt routine

It is not wrong to use millis() or micros() within an interrupt routine. It is wrong to use them incorrectly. The main thing here is that while you are in an interrupt routine "the clock isn't ...
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8 votes
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Sending serial data in an interrupt

You cannot use Serial inside an interrupt. Transmitting Serial relies on interrupts being available, and from inside an interrupt they aren't. All Serial communication must be done from loop(). So ...
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7 votes
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Are function pointer assignments atomic in Arduino?

Have a look at the code for attachInterrupt() and detachInterrupt() in /Applications/Arduino.app/Contents/Resources/Java/hardware/arduino/cores/arduino/WInterrupts.c (well, that's where they are on a ...
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7 votes

IR Receiver interrupt and arduino sleep mode

This is what I have managed to achieve ... Library IRLib - GitHub Version 1.51 March 2015 Copyright 2013-2015 by Chris Young http://tech.cyborg5.com/irlib/ This library is a major rewrite ...
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7 votes

interrupting an interrupt

the timer will set the Timer Interrupt Flag (TIFR0 register) The ISR will be called, and the flag will be cleared you do some long calculations or something the timer will match OCR0A, and set the ...
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6 votes

Why is that any sensor signal need to be hooked up to the interrupt pins on Andruino boards?

The interrupt pins are used to notify the MCU that a device needs attention. The INTx pins are chosen first because they are easy to program for; each gets a dedicated interrupt vector and hence ISR. ...
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6 votes

Multiple Buttons on Single Analog Pin and Trigger Pin Arduino Uno

Use the analog comparator in the ATmega328P to trigger an interrupt once the input voltage rises enough to indicate a button press. Connect the analog network to both the analog input and D6. If ...
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6 votes

Using millis() and micros() inside an interrupt routine

The quoted phrase is not a warning, it is merely a statement about how things work. There's nothing intrinsically wrong with using millis() or micros() within a properly-written interrupt routine. ...
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6 votes
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Critical sections in Arduino, interrupt consuming data

Warning: I am not familiar with the ESP8266 hardware, and some of my answers below may be only valid on the AVR architecture. The simplest way to synchronize your main code with your interrupt ...
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6 votes
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Using a lot of interrupts on a single Arduino

On a Uno, all of the digital and analog pins can trigger a pin-change interrupt. That gives you 20 inputs straight away. The Mega also has pin change interrupts but not on all its pins. The pins that ...
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6 votes

How to generate hardware interrupt in mpu6050 to wakeup arduino from SLEEP_MODE_PWR_DOWN mode?

Well, this code worked. In this code, MPU's INT PIN is Active Low so generates Ground on motion which is than connected to INT0 or arduino UNO pin 2 which generates interrupt and wakes UP the MCU #...
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6 votes
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ATTiny13 Interrupt Issue

You would want your bool LED_STATE = false; to be volatile qualified like volatile bool LED_STATE = false; otherwise the compiler's optimizer may assume that the value cannot be changing between loads ...
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