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What is an interrupt. An interrupt is a feature of processors that allows the user to grab hold of the processors attention. So if you’re running a piece of code just doing calculations, I end that you have an outside piece of hardware that needs to immediately talk to the processor, to tell it something that’s what you’d use an interrupt for more technically speaking, it’s a pin that is read On every clock cycle, so that when there is a specific change on that pin the lot of the space in memory it’s in the code is reading is store. That address is stored to the stack pointer and then it jumps to a pre described location by that by that interrupt, set up so that’s. What we’re going to talk about today now that’s the interrupt function. That’S, the basics of interrupts external pin, interrupts, are interrupts on that pin. There are other interrupts on the Arduino like serial, interrupts that let the communication work flawlessly there are timing, interrupts that let you do some of the cool timing functions that are doing this taken care of for you that sort of thing these are external, pin, interrupts and There are two pins on the Arduino that do that. We’Ll talk about that in just a second external pin interrupts, have a couple of states that they read. Oh, therefore, there’s, rising and it’s this edge on the square wave, so it’s, going from logic low to logic. High and it’s just this edge right here: there’s falling logic, high, going to logic, low and again it’s, just this edge, there’s logic, low, it’s, just being held low and then change state it’s, going from low to high or high to low doesn’t matter.

If there’s any change, it changes so that’s it so you’re able to read the change whenever there’s, a change that you want to read: rising falling change, state low state, whatever you can jump to a specific line in code, so that’s what we’re going to do now? If you watched my Arduino project challenge with the reaction timer, you can use interrupts for that, so that’s what we’re going to do. So what this is is it’s the code I had for the Arduino project challenge I’ve gone ahead and modified it though, and I’m going to go ahead and take you through what it’s doing so in order to use interrupts. You have to use a method called attach interrupt and there are two pins. I said on the Arduino Uno at least those are pins and pins. Three but they’re not called pins and pins 3 when you’re talking about attach interrupts their 0 and 1 0 for 2 and 1 for 3 it’s a little confusing, but ok um. If you’re using a mega, you have four pins to choose from or sorry you’ve got an additional 4 to choose from pins, 21, 20, 19 and 18, but I’m only got noonah. So those are the only two. I care about the four states: low change, rising and falling, and the function which is technically referred to the ott as the ISR, the interrupt service routine. We’Ll talk about that. When we talk about go back into the code note, these are often important inside the attached function, delay, won’t work and the value returned by Millie’s will not increment.

So Millie’s is the current status of the millisecond timer for the amount of time your programs been running and your serial data received, while in function may be lost here’s. Why, as I mentioned, delay Milly serial data, all use interrupts calling interrupts within interrupts is bad. It doesn’t work confuses the processor, gets crazy. So got to remember that, oh and if you’re using a global variable which is referenced within the ISR you’re going to interrupt function, you have to declare it as volatile. Why? What happens if you’ve got two functions, trying to access the same variable at the same time, that’s bad, so volatile kind of takes care of that, and it takes care of a few other things that are a little complicated and make my head hurt. Trying to think about okay let’s get into the code, so I’ve gone ahead and defined a variable start that keeps track of when we started timing. I set up the serial I set. Pin 13 is the led to let you know when to push the button I’m using attached. Input 0, which is technically, is to actually pin 2 on the board. I’Ve set that as an input, I’ve said it I’ve set the pull up. So I don’t have to worry about floating junk I’ve seeded, a random number generator with the amount of time it’s taken to get to this point. I wait a random amount of time from 1 to 3 seconds.

After that, wait. I turn on the LED. Now is the time you’ve got to push the button, so here I’ve attached the interrupt that’s just a time, saving measure to make sure the data is more accurate. Someone commented on that so that’s I’m, trying to make this a little more accurate for them, so attach interrupt 0 that’s pin to react. That’S the function as defined down here void react that’s. Really all you have to do. You just got to name a function and the edge falling now I say falling because I’ve used the pull up when I use the pull up, it’s pulled it up to 5 volts logic high and when I push the switch it’s going to pull it down to Ground so it’s going to go from logic high to logical up. I set the start timer. So when I push the button, it jumps to react now when I’m not pushing the button it’s within the main loop and it’s just looping and it’s just going to wait for that interrupt when that happens. The variable Finn is reads the current time and then it prints it out and it does that calculation here now I can hear what you’re saying you’ve called Millie’s within your interrupt function, yes and here’s. Why I can do that it’s not in the function. Does Millie’s doesn’t increment within the function? So if I written I’ve written Millie’s here and if I would also rate Millie’s at the bottom of the function, I wouldn’t see a change by calling it at the top I’m reading the state it was at as it jumped into the function.

It might be off by a fraction of a millisecond but that’s okay, because all I have here is millisecond accuracy, so that’s, where that’s, okay and serial print I’m, not reading into the data I’m, just printing out which isn’t that big a deal because I’m already within The that interrupt function, everything’s good okay, so I go ahead and upload this to the board got ta, pull it over here, open up the serial monitor like I’m song, push it and there’s my reaction time, 334 milliseconds and that’s it. Now. There is a little issue. I wanted to address and I’m going to see if I can make it there, it is okay, this is called bouncing. Let me explain to you what bouncing is. I am NOT a computer and I’m going to assume that, if you’re watching this, you are not a computer either, meaning that when I push this switch, there is no guarantee that when I push it, it’s not going to go all the way down the first time Or this manufacturing of the switch is not going to cause it to bounce up and down a couple times. So what you’re seeing here is a jump from five volts too low and then a little bit of a jump back up above the logic threshold and then back down and a little bit up above the logic threshold again and then back down that’s bouncing now. This is all happening within a fraction of a couple.

Milliseconds it’s not happening over a long span, so what we can do is we can actually take care of that using an external timing variable or because this is a single shot program. We can do it another way now, when I say single shot. Give me what I mean is I push this button? I want the program to end. I don’t care about the interrupts anymore, so I can actually just say: detach, interrupts, interrupts on pin zero or it’s our interrupts, pin 0 and 2. Now, if I do this again, I won’t see that so this is just one way. If you don’t care about interrupts anymore, you just need it for a single shot. So if I run the CN wait for the LED there, it is no issue run. It again. Wait for the LED, no issue that’s one way, if I get rid of this, this is where I’ve got a delay that is set that volatile follow volatile long last button. Push that’s, not push, can I say volatile without it getting angry with me. Yes, I can. Okay, okay, so volatile last button push. So what I can say is if the difference between the lat, the time we’re at now push is greater than some threshold say: 100 milliseconds, then I can go ahead and do this stuff last button push equals Finn and that’s updating the timer. So if I upload this again and I wait for the LED to light up up still get a bit of a bouncing issue, let’s see if I can fix that.

If an last button push is greater than honored down in Princeton let’s make this bigger see. If that fixes it and certain monitor led area now, the issue is, I can still push the button and have it work so that’s where that detach interrupts helpfulness comes in hope, but I don’t really, if you want it to be able to repeat that again, I’m, Showing you the detach, interrupts works for this, because I only care about the interrupts for one instance D at one calculation. This is helpful if you need that interrupts more than once. If you need to keep calling a button, push to grab its attention or some state change to grab its attention, this is a better means to calculate it. So that’s a good way to do it that’s a way to remove de belt that’s a way to remove bouncing so that’s, really it when it comes to external pin, interrupts I’ll, see if I can write something on timing interrupts for the Arduino it’s going to be A little trickier because normally that’s an AVR C thing but I’ll see if I can make it work for this so I’m, going to post this code, I’m going to start posting uh my codes and notes. I probably won’t post the notes for this, because it’s not really necessary, but if you want to, I can but oh yeah, I’ll post the codes for this I’ll. Put that link in the description and if you again, if you guys, have any suggestions on what you’d like me to cover topics, questions suggestions, love to hear it put them in the comments.

I’Ll read them, you know I do so that’s it.


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