Lab 6 { External Interrupts Solution




Complete the following objectives:

  • Understand when interrupts can be used, and how they are used.

  • Demonstrate how a previous lab’s implementation can be improved by mak-ing use of external interrupts.

  • Learn about some of the interrupt facilities that are available on the AT-mega128 microcontroller.

  • Explore the ATmega128 datasheet to learn how to con gure and enable speci c interrupts on your mega128 microcontroller board.

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Figure 1: Sample Input to External Interrupt

  1. Microcontrollers often provide several di erent ways of con guring interrupt triggering, such as level detection and edge detection. Suppose the signal shown in Figure 1 was connected to a microcontroller pin that was con gured as an input and had the ability to trigger an interrupt based on certain signal conditions. List the cycles (or range of cycles) for which an external interrupt would be triggered if that pin’s sense control was con gured for: (a) rising edge detection, (b) falling edge detection, (c) low level detection, and (d) high level detection. Note: There should be no overlap in your answers, i.e., only one type of interrupt condition can be detected during a given cycle.


To complete this prelab, you may nd it useful to look at the full ATmega128 datasheet. If you consult any online sources to help answer the prelab questions, you must list them as references in your prelab.

  1. In computing, there are traditionally two ways for a microprocessor to listen to other devices and communicate: polling and interrupts. Give a concise overview/description of each method, and give a few examples of situations where you would want to choose one method over the other.

  1. Describe the function of each bit in the following ATmega128 I/O registers: EICRA, EICRB, and EIMSK. Do not just give a brief summary of these reg-isters; give speci c details for each bit of each register, such as its possible values and what function or setting results from each of those values. Also, do not just directly paste your answer from the datasheet, but instead try to describe these details in your own words.

  1. The ATmega128 microcontroller uses interrupt vectors to execute particular instructions when an interrupt occurs. What is an interrupt vector? List the interrupt vector (address) for each of the following ATmega128 interrupts: Timer/Counter0 Over ow, External Interrupt 5, and Analog Comparator.


Most modern computing systems use interrupts to communicate with peripheral devices. Interrupts can be very bene cial because they allow a processor to continue executing useful instructions until a peripheral device indicates it needs attention.

Using interrupts can also be tricky, as they can sometimes result in lot of over-head (i.e., time that must be spent, but is not spent doing anything productive). When an interrupt request comes in, the processor has to stop what it is doing, save its current place in the program (including storing any in-use or otherwise special variables), and then it can service the interrupt. Once the event that has caused an interrupt as been handled, the processor must take the time to reload any stored variables, and then it can nally resume what it was doing before the interrupt occurred. This process, which is the cost of servicing an interrupt in this manner, is referred to as a context switch.

Depending on how long it takes to service an interrupt, and depending on how frequently the event causing the interrupt occurs, the processor may not be able to spend much time on its original task before another interrupt occurs. For example, if a peripheral device wants the processor to store a single byte

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Lab 6 { External Interrupts

ECE 375

of data every couple of clock cycles, it may try to interrupt the processor every time another byte is ready. This would cause the processor to spend all of its time storing variables, servicing the interrupt, and reloading variables, only to immediately be interrupted again since the next byte is ready. Since this scenario is very clearly undesirable, many modern computers use coprocessors and peripheral controllers (like a DMA controller, for example) to handle frequent requests.

Despite the potential downsides of using interrupts, there are of course still situations where their use is preferred, such as handling infrequent events which do not justify spending any time busy-waiting.


For this lab, you need to write a short assembly program that causes your TekBot to move forward. Then, when either the right or left whisker is hit, it will need to react by backing up for 1 second, turning away for 1 second, and then moving forward again. The TekBot counts how many times each whisker is triggered and the LCD displays two counters for left/right whiskers. Be sure that your counters should be capable of counting more than a single digit. You will lose points if your counters can only go up to 9.

As you can probably tell, this is the same BumpBot behavior that you saw previously in both Lab 1 and Lab 2. Polling was used to detect whisker hits in these prior labs, but this time you must use external interrupts to detect a falling edge on either of the whisker inputs. You need to use INT0, INT1, INT2, and INT3 for a right whisker input, a left whisker input, clearing a right whisker counter, and clearing a left whisker counter, respectively.

You must write your code so that your TekBot can only be interrupted by bumper hits when it is moving forward (i.e., not while currently in the middle of any HitRight or HitLeft behavior). Additionally, you must not allow bumper hit interrupts to queue up while you are in the middle of any HitRight or HitLeft behavior. For example, if you hit the right bumper rst and then hit the left bumper while the TekBot is still performing its HitRight behavior, the TekBot must not go directly into HitLeft once HitRight has nished.

A skeleton le has been provided to assist you; also, you can reuse some code from BasicBumpBot.asm. To demonstrate you have completed the implementa-tion portion of this lab, show your TA the BumpBot operation, and explain how your code was written to meet the additional requirements mentioned above.


A full lab write-up is required for this lab. When writing your report, be sure to include a summary that details what you did and why, explains any problems you may have encountered, and answers the study questions given below. Your write-up and code must be submitted by the beginning of next week’s lab. Remember, NO LATE WORK IS ACCEPTED.

Study Questions

  1. As this lab, Lab 1, and Lab 2 have demonstrated, there are always multiple ways to accomplish the same task when programming (this is especially true for assembly programming). As an engineer, you will need to be able to justify your design choices. You have now seen the BumpBot behavior implemented using two di erent programming languages (AVR assembly and C), and also using two di erent methods of receiving external input (polling and interrupts).

Explain the bene ts and costs of each of these approaches. Some important areas of interest include, but are not limited to: e ciency, speed, cost of context switching, programming time, understandability, etc.

  1. Instead of using the Wait function that was provided in BasicBumpBot.asm, is it possible to use a timer/counter interrupt to perform the one-second delays that are a part of the BumpBot behavior, while still using external interrupts for the bumpers? Give a reasonable argument either way, and be sure to mention if interrupt priority had any e ect on your answer.


With the basic BumpBot behavior, a TekBot can sometimes get stuck in a loop if it encounters a corner: continually backing up, hitting the right whisker, backing up again, hitting the left whisker, then the right whisker again, then the left whisker again, and so on.

To complete the challenge for this lab, add a \memory” to your TekBot so that it can detect this problem. When the TekBot has hit alternating whiskers ve times, it should reverse like normal, but then turn around 180 degrees (i.e., turn for a few seconds), and then resume forward motion to get out of the corner.

In addition, correct the simple problem of the TekBot hitting the same object multiple times in a row. In this scenario, the TekBot hits one of its whiskers, backs up and turns away as usual, but does not turn far enough and hits the

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Lab 6 { External Interrupts

ECE 375

same object with the same whisker. Add code to detect this scenario, so that

the TekBot will back up and turn away twice as long as normal whenever the

same whisker is hit twice in a row.

Add these changes to your code, and make sure your additions are well-

documented. Demonstrate the improved behavior to your TA to receive challenge

credit for this lab, and be sure to submit your challenge code as a separate le.

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