Solved–Lab 3 { Introduction to AVR Simulation with Atmel Studio –Solution

Description

SECTION OVERVIEW

Complete the following objectives:

• Look at a sample AVR assembly program.

• Learn how to use the Atmel Studio simulator to step through the sample program.

• Learn how to interact with the Processor, I/O View, and Memory windows.

• Begin learning about basic assembly program structures (such as loops).

PRELAB

To complete this prelab, you may nd it useful to look in the AVR Starter Guide. If you consult any online sources to help answer the questions below, you must list these sources as references in your prelab.

1. What are some di erences between the debugging mode and run mode of the AVR simulator? What do you think are some bene ts of each mode?

2. What are breakpoints, and why are they useful when you are simulating your code?

3. Explain what the I/O View and Processor windows are used for. Can you provide input to the simulation via these windows?

4. The ATmega128 microcontroller features three di erent types of memory: data memory, program memory, and EEPROM. Which of these memory types can you access by using the Memory window of the simulator?

(a) Data memory only

(b) Program memory only

(c) Data and program memory

(d) EEPROM only

(e) All three types

PROCEDURE

Introduction

Writing assembly code requires a lot of attention to detail. Despite your best intentions, it is easy to introduce subtle bugs into your program, especially when you are rst learning assembly language programming. Often, you will try to run your code on your mega128 board, observe that it isn’t working, but have a di cult time identifying which portion of your program isn’t working as in-tended. In other words, simple mistakes can result in behavior that is signi cantly di erent than what you intended, which is often confus-ing or frustrating. Therefore, it is very useful to know how to to simulate your code, so that you can provide \controlled” input and observe that each portion of your code is working as you intended.

In this lab, you will learn to use the simulator to step through a sample pro-gram, observing and recording key values along the way. Be sure to write down your answers to all bolded questions as you proceed through the lab, as you will need them at the end of the lab.

Creating Breakpoints

To get started, complete the following steps to establish a new project with the sample program included:

1. Open Atmel Studio, and create a new Assembler project.

2. Download the sample assembly program given on the lab webpage (Lab3Sample.asm), and include it into your project.

3. Make sure that the sample program builds without any errors.

In the sample program you just downloaded, there are several lines that have the comment \SET BREAKPOINT HERE”. Before you simulate this sample code, you will need to add a breakpoint to every one of these lines. Do the following for each line that has a \SET BREAKPOINT HERE” comment:

• Right-click on the line of code, and then select Breakpoint ! Insert Breakpoint.

• If done correctly, a red dot will appear to the left of the instruction, and the entire line will be highlighted in red.

For this lab to work correctly, it is very important that all required breakpoints are set. Once you have completed this step, and veri ed that all required break-points are in place, you are ready to begin simulating the sample code.

©2020 Oregon State University Winter 2020 Page 1
Lab 3 { Introduction to AVR Simulation with Atmel Studio ECE 375

Preparing for Simulation

To start up the simulator, select Debug ! Start Debugging and Break from the Atmel Studio menu bar (or just press Alt+F5).

Since this is your rst time launching the simulator for this particular Atmel Studio project, you will be prompted to con gure your .asmproj le with the message \Please select a connected tool and interface and try again.” Press Continue, and then open the dropdown menu below the text \Selected debug-ger/programmer”, and click on Simulator.

Then, you can press Ctrl+S to save this change to the project le, and then switch back over to the code editor tab, which should still have the sample program open. Finally, you can select Debug ! Start Debugging and Break again and the simulator will properly start.

Once the simulator has started, you should see several new view tabs in the right-side window of Atmel Studio.

• If you do not see \Processor Status” anywhere, go to Debug ! Windows ! Processor Status to bring it up. It should auto-dock in the right-side window, if not you can drag the window over to dock it manually.

• If you do not see \I/O” anywhere, go to Debug ! Windows ! I/O to bring it up. Again, it should dock automatically, but you can dock it manually if needed.

• If you do not see any \Memory” windows open, go to Debug ! Win-dows ! Memory ! Memory 1 to bring one up. This memory view is traditionally docked in the lower-right panel of Atmel Studio.

Before you continue, have your TA verify that you have set all required breakpoints, and that you have opened the Processor Status, I/O, and Memory windows correctly.

Simulating the Sample Code { Part 1

1. We began simulation by pressing \Start Debugging and Break”, so the sim-ulator is currently in debugging (line-by-line) mode, paused at the very rst instruction of our program (it should be rjmp INIT). This line is highlighted in yellow, which means it is the next instruction to be executed.

2. Since the simulator hasn’t actually run any lines of code yet, take this op-portunity to observe the default/initial values of some of the I/O registers you have already seen in the previous lab.

• What is the initial value of DDRB?

• What is the initial value of PORTB?

• Based on the initial values of DDRB and PORTB, what is Port B’s default I/O con guration?

3. Press Debug ! Step Into (F11) to execute the rst instruction and step forward to the next line. Observe that the ow of the program has moved to the line directly following the INIT label.

4. Next, press Debug ! Continue (F5) to enter run mode. The simulator will run until it encounters the next breakpoint.

5. At this point (Breakpoint #1), the simulator has just nished executing the 4 lines of code that initialize the stack pointer. The stack pointer’s value is displayed (in hexadecimal representation) in the Processor Status window.

• What 16-bit address was the stack pointer just initialized to?

6. Press Continue again. The program has now advanced another two lines (to Breakpoint #2), and the general purpose register r0 has just been initialized to a certain value. The contents of general purpose registers like r0 are also displayed in the Processor Status window.

• What are the current contents of register r0?

7. Press Continue again. The program ow has continued for several lines, and then paused again at the end of the rst iteration of a loop structure (Breakpoint #3), which began at the LOOP label. The next instruction to be executed will test whether to move the program ow back up to LOOP (i.e., it tests whether the loop will continue). Press Continue again. The simulator is still pointing to the same instruction, which means that another iteration of the loop was run. Keep pressing Continue until the simulator stops at the rst breakpoint outside of the loop (Breakpoint #4).

• How many times did the code inside the loop structure end up running?

• Which instruction would you modify if you wanted to change the number of times that the loop runs?

• What are the current contents of register r1?

8. Press Continue again. The program ow is now within another loop struc-ture (Breakpoint #5). Keep pressing Continue until the program stops at the rst breakpoint outside of the LOOP2 loop (Breakpoint #6).

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Lab 3 { Introduction to AVR Simulation with Atmel Studio ECE 375

• What are the current contents of register r2?

9. Press Continue to advance to the nal breakpoint (Breakpoint #7).

• What are the current contents of register r3?

Inserting Values into Memory

Before you advance the simulator beyond Breakpoint #7, you will need to man-ually insert some values into the Data Memory. Go to the Memory window, and select data IRAM from the \Memory:” dropdown. You should now see a table of two-digit hexadecimal values (bytes), aligned so that the rst entry (top left) is the contents of the rst location in Data Memory (at address $0100). The entry to its right is the contents of location $0101, then to its right is $0102, etc.

The Memory window allows you to type values directly into memory, as long as the simulation is currently paused in line-by-line mode. Before stepping through any more of the sample code (i.e., before beginning the FUNCTION subroutine), complete the following steps:

1. Enter the value you observed in r0 into location $0100.

2. Enter the value you observed in r1 into location $0101.

3. Enter the value you observed in r2 into location $0102.

4. Enter the value you observed in r3 into location $0103.

Simulating the Sample Code { Part 2

Once you have placed the correct values into the correct data memory locations, you can resume stepping through the sample code.

1. Press Step Into (F11) to move into the FUNCTION subroutine.

• What is the value of the stack pointer now that your program ow has moved inside of a subroutine?

2. Press Step Out (Shift+F11), which will run the rest of the subroutine all at once and pause at the rst instruction after the function call (which happens to be the end of MAIN).

• What is the nal result of FUNCTION? (What are the hexadec-imal contents of memory locations $0105:$0104?)

STUDY QUESTIONS / REPORT

For this lab, a full report write-up is not required. Instead, simply submit your answers to the bolded questions that were asked throughout the lab. For your convenience, the questions that you need to answer are shown below:

1. What is the initial value of DDRB?

2. What is the initial value of PORTB?

3. Based on the initial values of DDRB and PORTB, what is Port B’s default I/O con guration?

4. What 16-bit address (in hexadecimal) is the stack pointer initialized to?

5. What are the contents of register r0 after it is initialized?

6. How many times did the code inside of LOOP end up running?

7. Which instruction would you modify if you wanted to change the number of times that the loop runs?

8. What are the contents of register r1 after it is initialized?

9. What are the contents of register r2 after it is initialized?

10. What are the contents of register r3 after it is initialized?

11. What is the value of the stack pointer when the program execution is inside the FUNCTION subroutine?

12. What is the nal result of FUNCTION? (What are the hexadecimal contents of memory locations $0105:$0104)?

CHALLENGE

The FUNCTION subroutine featured in the sample code accepts two 16-bit values as parameters, and also returns a 16-bit value as its result. To complete the challenge for this lab, provide detailed answers to the following questions:

1. What type of operation does the FUNCTION subroutine perform on its two 16-bit inputs? How can you tell? Give a detailed description of the operation being performed by the FUNCTION subroutine.

2. Currently, the two 16-bit inputs used in the sample code cause the \brcc EXIT” branch to be taken. Come up with two 16-bit values that would cause the branch NOT to be taken, therefore causing the \st Z, XH” instruction to be executed before the subroutine returns.

3. What is the purpose of the conditionally-executed instruction \st Z, XH”?

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