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37 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 ...
Nick Gammon's user avatar
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12 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 ...
Nick Gammon's user avatar
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10 votes
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Alternative to polling interrupt flag from main loop?

If the task is pretty short and time sensitive, your best option is to put it in the interrupt handler. If the task is big and not too urgent (like: it could well wait for a millisecond or so), your ...
Edgar Bonet's user avatar
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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 ...
Majenko's user avatar
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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" ...
Majenko's user avatar
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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 ...
Majenko's user avatar
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8 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 #...
bandejiya's user avatar
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8 votes
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Debouncing a button with interrupt

So, I grabbed a Mega and several pushbutton switches (one of which turned out to be a push-on, push-off, much to my bemusement), and ginned up a small Arduino program to illustrate an interrupt-driven ...
starship15's user avatar
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 ...
Gerben's user avatar
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7 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 ...
timemage's user avatar
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7 votes
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Pin Change Interrupt Flag timing on 1284P

When it is entered. The wording is slightly vague but I think "when the interrupt routine is executed" means the commencement of execution. Otherwise multiple pin-change interrupts could be ...
Nick Gammon's user avatar
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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 ...
Nick Gammon's user avatar
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6 votes
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Arduino Interrupt example not working

I think you misunderstood this example: it is not expected to automatically blink! The LED is suposed to change state at every change of level on pin 2. This supposes that you wired something (e.g. a ...
jfpoilpret's user avatar
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6 votes
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Which Arduinos support ATOMIC_BLOCK?

This answer covers: Which Arduinos support ATOMIC_BLOCK? And how can I duplicate this concept in C with __attribute__((__cleanup__(func_to_call_when_x_exits_scope))) and in C++ with class constructors ...
Gabriel Staples's user avatar
6 votes
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Using interrupts as flags/latches without any ISR

The answer to your question is: yes, you can use an interrupt flag even if the interrupt itself is disabled. The flag is raised whenever the peripheral detects the relevant event. You can check it at ...
Edgar Bonet's user avatar
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5 votes
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AVR timer overflow interrupt not working

You are misunderstanding what an overflow interrupt is. That is called when the timer overflows (ie. it reaches TOP which in the case of an 8-bit timer is 255). Since you are counting to 124 that will ...
Nick Gammon's user avatar
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5 votes
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Managing 2 ultrasonic sensors without blocking the other

I strongly advice you to use interrupts and make sure the sensors are in a cylindrical cone in order to prevent interference to each others as much as possible. Here is code snippet which can give ...
Sener's user avatar
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5 votes
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How to set the sampling frequency in Arduino without using a delay

Use the concept of blink without delay. Instead of using delay() to set the timeout you use a timestamp and check micros() against that to see whether your interval has passed: const unsigned long ...
ratchet freak's user avatar
5 votes

Constant ADC sampling rate on ESP8266 or ESP32

You have several options for controlling the sampling rate. Software only This is the simplest, and the previous answer got it almost right. Here is my take: const SAMPLE_INTERVAL = 500; // 500 ...
Edgar Bonet's user avatar
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5 votes

What is the right way to query an I2C device from an interrupt service routine?

Majenko's answer is useful — you can simply forego the use of interrupts for this and poll the pin directly in your main loop. Especially on AVR where the external pins are exposed as registers which ...
hobbs's user avatar
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5 votes

How do I code delay inside a ISR?

An ISR should be kept as short as possible, since while it runs, no other interrupts can be handled (including the one powering the delay() function. Thus you cannot delay that easy inside an ISR and ...
chrisl's user avatar
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5 votes
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Save Watchdog ISR Return Address in EEPROM

GCC has a handy function __builtin_return_address(depth) which should do what you want. To get the return address of the current function (which should be the same whether it's a normal function or ...
Majenko's user avatar
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5 votes

Where am I going wrong in modifying Timer0 and the ISR interrupts in this example?

Don't do serial prints inside an ISR. Ever. They require interrupts to work, and inside the ISR interrupts are off. For a while the attempts to print will fill up a buffer which is emptied by another ...
Nick Gammon's user avatar
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4 votes

Detecting which pin called the ISR Uno

You only have two external interrupts on the Uno, so a simple solution would be to call an "intermediate" routine, which remembers the button number. eg. attachInterrupt(digitalPinToInterrupt(2),...
Nick Gammon's user avatar
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4 votes
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Waking Arduino from sleep by holding button for more than 3 seconds

You won't manage it without external circuitry. The problem is the Arduino has to be awake to know you held the button for 3 seconds. You can only wake immediately you press the button. However you ...
Majenko's user avatar
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4 votes
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Reading FAN tachometer/RPM on 4 wire fan using arduino

There are no interrupts on pin 9 (unless you are using Pin Change interrupts...?). You have to use pin 2 (interrupt 0), or pin 3 (interrupt 1) (also, on the Mega: 18 (interrupt 5), 19 (interrupt 4), ...
Majenko's user avatar
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4 votes
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Arduino continuously reading ADC value using interrupt

reset the resultNumber to 0 after you have written it to serial. and turn interrupts back on void loop () { while (resultNumber < MAX_RESULTS) { } for (int i = 0; i < MAX_RESULTS; i++)...
ratchet freak's user avatar
4 votes

Calling I2C functions inside an ISR

I started writing this as a comment, but the more I wrote the more I needed space to complete my thoughts, so here it is as answer. In my opinion, reacting to a user interaction should NOT be put in ...
frarugi87's user avatar
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