CE-2800 Embedded Systems Software
Lab 3: Bouncing Light

Objectives

• Become familiar with assembly language program design and instructions
• Use assembly language branch instructions to implement conditional logic
• Use assembly language instructions to initialize and use the Stack Pointer for subroutines.
• Use assembly language to implement and call subroutines.

Assignment

In this lab, you will create a "bouncing LED" program (did you ever see the "Knight Rider" TV series? It's OK if you didn't - it was stupid). Specifically, your program must:

1. Begin by lighting LED 0.
2. After waiting for exactly X ms, light LED 1 and turn LED 0 off. X is the number of 10*(number of chars in your MSOE email name) milliseconds. Thus, if your email name has 7 chars, the delay must be 70ms.
3. Continue "moving" the lighted LED every X milliseconds. After LED 7 is on, start moving the lighted LED position in the opposite direction in the next step.
4. When LED 0 is reached, start moving back in the other direction once again.
5. Repeat this forever.

Here is a flowchart to help you visualize the logical structure of the preceding sequence:

Design constraints:

• Use the program structure you used in the previous lab, with the various sections clearly delineated and commented.
• Never allow all the LEDs to be off at any time (not including the Power LED), except for a few microsecond delay as the circuitry is activated and deactivated.
• Use bit-logic instructions (LSL, LSR) to manipulate the bit contents of a single register that represents which LED is lit.
• Use a time delay loop (like the one discussed in class), but use a triply-nested loop based on the DelayLoop sample (on the course website) to achieve the exact required delay.  You'll have to adjust the values of C1, C2, and C3 to achieve this, as well as adding a few NOP instructions. Note: Minimize the use of the NOP instruction in "fine-tuning" your loop.
• You must implement the delay loop in a subroutine - NOT embedded in the "main" body of your program. When computing values for C1, C2, and C3, don't forget to adjust the formula to include the number of cycles it takes to RCALL the subroutine and RET from it!
• You will need to implement several labels for jump targets (always use labels for jump targets; never use raw addresses). Use meaningful names for your labels.
• Define meaningful names for the registers you use, and subsequently refer to the registers by these names in your program.
• Comment your code heavily.

Hints:

You can use the DelayLoop sample from the course website as a starting point; however, you will have to incorporate the delay code as a subroutine from the "main" body of code.

Use the LSR instruction to move the LED position to the "right" (for example, from LED 7 to LED 6). After each shift, use a branch instruction that tests to see if the bit you shifted was shifted into the Carry bit. (Note: the DelayLoop sample code uses a branch instruction that tests the Zero bit, not the Carry bit, so you cannot use that code directly) Similarly, to move to the left, use the LSL instruction, each time testing to see if the bit was shifted into the Carry bit.

As an option, you may implement and use additional, separate subroutines to handle moving the LEDs to the right and left. You must however, still implement a single delay subroutine.

Demonstration

You must demonstrate your working program on your board, with the oscilloscope measuring the period of either LED 1 or LED 6.  Develop and debug your program in AVRStudio first (its' easier that way). It may be helpful to set the AVRStudio Simulator clock speed to 8MHz, and adjust your timing loop to result in a delay that is twice that required when it runs on your 16MHz board.

Lab Submission (due 11:00pm, Tuesday, January 3)

For your submission, prepare a report in Microsoft Word format.

Be sure that you answer the following question in your report.

• What values for constants C1, C2, and C3 are needed to result in the required delay loop?
• Show how these values, when used in the formula found in the DelayLoop comments, correspond to the required delay time in milliseconds.
• How many NOP instructions did you have to add?

Copy/paste your program's instructions from the .lst file generated by the Assembler into your report.

Include both the Word report and your program (.asm file) in your submission.

Upload your submission through Blackboard (assignment "Lab 3").

Be sure to keep copies of all your files, in case something gets lost.

Grading

Your lab grade will be determined by the following factors:

Program - comments and formatting are important aspects of assembly language programming! And it has to work correctly.

Timeliness of submission as stated in the course policies.