18-348 Lab #7

Spring 2016

NOTE: Lab 7 consists of two components (Lab 7 Part A and Lab 7 Part B).

Relevant lectures:
- Part A: Lecture 12. Time; Timer/Counters; Watchdog Timers
- Part B: Lecture 13. Interrupts (SWI)

Links to all files referenced in the lab and prelab can be found in the Files section at the end of this document.


Pre-Lab Lab Support Materials

Pre-Lab 7 - Part A:



Watchdog Timers

The MC9SC128 has a hardware watchdog timer called the Chip Operating Properly (COP) timer.  Documentation for the COP is somewhat difficult to find in the datasheet, but the below discussion and sections, 9.4.5 and 9.5.2 of the datasheet should get you through it.

NOTE: For this lab we shall assume the course module oscillator clock operates at 16 MHz, and the bus clock at 8 MHz.

The COP timer is enabled using the COPCTL register.  Section describes the bit settings for this register. 

Reset sequence:  The watchdog is reset by writing the value 0x55 to the ARMCOP register, followed by a write of 0xAA.  Writing any value other than 0x55 or 0xAA will force an immediate COP reset.  Other sequences of 0x55 and 0xAA are permitted, but will not reset the COP timer.  The COP timer begins counting from the time of the reset, not the end of the original COP timer period.

Reset action:  When the COP reset occurs, the entire chip is reset.  After the reset, the chip determines the source of the reset.  If the reset occured because of the COP, the system will call the code that begins at the COP reset vector.  The vector location for the COP is $FFFA.   So $FFFA must contain the address of the code that should be run when a COP reset occurs.  You can see this implemented in the lab_7_watchdog.asm file used in the lab.  Note that the COP timer is disabled after the reset and must be re-enabled.


Part 1:    Watchdog timer

  1. QUESTION #1: Analyze the code below to determine the number of clock cycles from POINT1 to the next time POINT 1 is reached, in terms of the value of LOOP_PARAMETER.  Note that this is the time between COP resets.  Your solution shall have the form:
    You may use any method you wish to compute the timing.
                ;this block loads the initial loop counter value from
                ;the dip switch.
                XGDX ;swap D and X to preserve current value
                LDD #LOOP_PARAMETER ;set the loop counter
                XGDX ;swap back           
                DBNE X, waitLoop

                ;COP reset requires the $55 / $AA write sequence
                LDAB #$55
                STAB ARMCOP
                LDAB #$AA
                STAB ARMCOP
                ;POINT 1
                ;increment the display by rotating the A register
                ADCA #0
                STAA PTT
                BRA loopForever

  2. QUESTION #2: Read discussion section of this lab, and sections, 9.4.5 and 9.5.2 of the MC9S12 data sheet.  Assume that COPCTL is set to $C4.  Determine the period (in ms) of the COP timer.
  3. QUESTION #3: Using the formula you derived above, compute the minimum and maximum values of LOOP_PARAMETER that will allow the loop to run without causing a reset on the board.  In other words, what is the largest and smallest value of LOOP_PARAMETER that will reset the COP timer within the window, based on the code and the settings of COPCTL.

Part 2:  Timer

This part of the prelab is to familiarize you with the timer control registers.

Match the list of possible entries below each table to the blanks in the table.  Review section 15.3.2 of the MC9S12 data sheet for information about the registers referenced below:

Register bits

Table A.1:  Timer control register parameters

Put each of these values into Table A.1 above.  HINT:  use each value only once.

For the various values of PR  (aka TSCR2:[2:0]), compute the length of a timer tick in microseconds. Note that a timer tick is the time it takes the timer to increment once (not the "rollover" time). Assume the bus clock is 8Mhz.  Fill these into Table A.2 below:

PR Value
Timer Tick Length (us)








Table A.2:  Prescaler values

Pre-Lab 7 - Part B:



As discussed in class, the SWI instruction can be used to implement a breakpoint debugging tool in almost any system that keeps programs in RAM, even if it doesn't have hardware debugging support. As it turns out, the course processor keeps its programs in flash memory and does have hardware debugging support, but we still want you to get a feel for SWI as a way to ease into how interrupts work. So, in this lab we'll be compiling an SWI instruction into the midst of a program and using that instruction as a run-time monitoring device.

What we want you to build is a run-time monitor that uses SWI to display current register values on LEDs. You'll use a button to select which byte from which register to display. You might even find this capability useful in future projects as a run-time monitoring mechanism.

For any further information on interrupts, please refer to the lecture notes or Chapter 5 of the MC9S12 data sheet.


Write an assembly language program that will demonstrate basic functionality needed without being a complete implementation. In particular, you will read inputs from and write outputs to memory locations rather than switches and LEDs. We encourage you to use the simulator to convince yourself that your program works, but we do not expect you to get it working on real hardware until the lab section. (If you choose to use the simulator or do the bonus, you will find the instructions from the lab section helpful.)

The program you write must meet the following requirements:

Bonus (Optional):
Create a test program that sets distinct values in all the registers in a main program, then exercises the SWI to check for proper operation of all selection values. The program shall compile and run with the simulator so that proper operation can be determined in the simulator by listing the values of those memory locations.

Each iteration of the main testing loop:

  1. Shall change value in the selector memory location (to select a new register)
  2. Shall execute an SWI instruction
  3. Shall copy the result memory location to the appropriately named variable in memory they declare.
  4. All possible selection values shall be tested over some multiple iterations of the loop.

Hand-in Checklist: (85 + 8 points)

All non-code submissions shall be in a single PDF document.

Part A:

  1. (10 points)  Answer to question #1 from part 1.
  2. (10 points)  Answer to question #2 from part 1.
  3. (5 points)  Answer to question #3 from part 1.
  4. (10 points) Fill in Table A.1 above.
  5. (10 points) Fill in Table A.2 above.

Part B:

  1. (40 points) Program listing as a text file with a .asm extension. Name this file prelab_7_swi_andrewID.asm (with your appropriate andrew ID in the file name). The only thing we want in this text file is the actual SWI service routine as commented assembly language so that we can combine it with a test program to execute it.
  2. (Bonus: 8 points) Optional. Simulator project that tests operation of SWI service routine as described above. For credit please turn in complete project.

Refer to the LAB FAQ for more information on lab handin procedures and file type requirements.  You MUST follow these procedures or we will not accept your submissions.

Lab 7 - Part A

Goal:  To implement timers and watchdog timers on the MC9S12 module.


This discussion covers techniques for time-keeping using the crystal oscillators and timers.  This technique involves using a state machine to sample and detect timer rollover.  Time is kept in a fixed-point fractional format.

rollover - the timer value (read from TCNT) is a up-count value.  That is, it starts at 0 and is incremented by 1 for every timer tick.  When the timer reaches its maximum value, 2^16-1 or 65535, the next increment will return it to 0.  This event is known as rollover.

Suppose the timer is running at 8 MHz.  (Setting the prescaler to 0).  This means that timer tick period is:

Since the timer value (TCNT) is a 16-bit value, a full timer rollover will be:

We accumulate time in a 32-bit value (unsigned long), where we represent 1 second as 0x10000.  That is to say, the upper 16 bits represent whole seconds, while the the lower 16 bits represent fractional seconds.  Using the period for timer rollover that we calculated in (2) above, we compute the representation of 8.192 ms in our fixed point notation:

  So we add 0x219 to our counter value every time we detect a rollover.  But how do we detect a counter rollover?  The simplest way would be to monitor the value (TCNT==0) in a loop.  But if we have other processing to do in our loop, we might miss this.  Instead, we use the state machine pictured below to monitor the MSB of TCNT.

Timer State Machine
Figure 1:  Timer Rollover Detection State Machine

Every time the state machine enters the TCNT_ROLLOVER state, we increment our counter value by the value we computed in (4).

The only thing we must ensure is that we sample TCNT at least twice during the timer period computed in (2)  This restricts the amount of processing we can do in the monitoring loop.


Part 1:

  1. Download the lab_7_watchdog.asm file.  Rename it to lab_7_watchdog_gxx.asm.
  2. Wire the board according to the instructions at the top of the file.
  3. Start a new assembly project using the 348 assembly stationery and replace the main file with lab_7_watchdog_gxx.asm.
  4. Run the program and find the minimum and maximum values of PTAD that do not cause a watchdog reset.

Note: The value of PTAD is used for the most significant byte of the loop counter.  This means that changing the PTAD input changes the loop counter by $100,  so the loop counter resolution is quite coarse.

Part 2:

For this part of the lab, you will implement a stopwatch according to the time-keeping techniques described in the discussion above and the lecture notes from lecture 12.

Note:  For your reference, a call to lcdSimpleWrite() requires no more than 17 ms to complete.

  1. Make the following connections on the board (note that Port T and Port AD are not used, so this wiring is compatible with the wiring for Part 1).
  2. Download the lab_7_stopwatch_skeleton.c file.  Rename it to lab_7_stopwatch_gXX.c.
  3. Create a new C project, and replace main.c with the renamed skeleton file.  Add the lcd_lib files to your project.
  4. Add the modclock files to your project to change the CPU module clock speed up to 8 MHz.
  5. Repeat the calculations from the Discussion section above to compute the fractional time which must be added to your software time count after each timer rollover.

You shall implement a stopwatch program that meets the following requirements:

  1. The stopwatch shall start counting time from 0 when PB1 is pressed.
  2. The stopwatch shall stop counting time when PB2 is pressed.
  3. The stopwatch program shall display the count in the format "minutes:seconds" on the LCD panel.
  4. The module timer prescaler shall be configured so the timer runs at 0.5 MHz.
  5. The module timer shall be free running (i.e. you may read TCNT but never reset it).
  6. The stopwatch shall monitor the TCNT value for rollover using the state machine from Figure 1.
  7. The stopwatch clock shall be accurate within 1 second per 2 minutes elapsed time.

Test your program using a stopwatch or a watch with a second hand.  To receive full credit, your device must remain synchronized (maintaining time to within 1 second of) a reference timepiece for 2 minutes (120 seconds).  You may tune the fractional time parameter added after each rollover to adjust your clock accuracy.  You will have to justify any deviation from the value you computed in step 4.

Bonus: Part 3 (Optional):

This bonus part is based on Part 1 above.  In the original lab_7_watchdog.asm file, the value of PTAD is used for the most significant byte of the loop counter.  This means that changing the PTAD input changes the loop counter by $100,  so the loop counter resolution is very coarse.  Modify the lab_7_watchdog.asm file so that the value of PTAD is used for the least significant byte.  Hard-code the most significant byte to the maximum PTAD value you identified in part 1.  Use this program to identify the exact maximum loop counter value.  Save this program file as lab_7_watchdog2_gxx.asm.


  1. List the minimum and maximum PTAD values for part 1.
  2. Are your results from Part 1 consistent with your computations from the prelab?  If not, what was the source of the discrepancy? (100 words maximum)
  3. Compute the margin for PTAD. Do this by computing both the minimum and maximum using the formula:  100% * [(COP_time - loop_time) / COP_time  ].
  4. Show your calculations similar to those in the discussion section of the lab for how you computed the fractional time value you use to increment the time variable in your program.  You shall number the equations in your calculations consistent with the calculations in the discussion section.
  5. Did you modify/tune the fractional time value used to increment your counter?  If yes, explain why you chose to modify it. (100 words maximum)
  6. Predict the accuracy by determining the number of seconds the clock will show after 10,000 seconds (use the techniques shown in lecture 12, but with the values for this lab).
  7. BONUS: From part 3, record the hard-coded most significant byte that the maximum PTAD value that allowed the loop to run without reset.  Note that these two values together comprise the LOOP_PARAMETER from the prelab.  Did you find the same value you computed in your prelab?  Why or why not? (100 words maximum)
  8. BONUS: Using the setup from part 2, test your clock against real digital stopwatch (for example, a stopwatch mode on a wristwatch).  Start both at the same time and let them run for at least one hour (the longer the better within reason). Stop both at the same time and compute the time error of your clock (e.g., drifted 13 seconds in 8 hours is a 0.045% drift rate). Is this as close as you could get or using this information should you slightly adjust your values? (We're not asking you to do the adjustment and re-run the experiment; just tell us what adjustment, if any, you would make.)

Part A - Demo Checklist: (50 + 5 points)

  1. (20 points)  Demo the watchdog program to the TA.  You must demonstrate the minimum and maximum values by showing that the program functions normally (LEDs chase) at MIN_VALUE and MAX_VALUE and fails at (MIN_VALUE - 1) and (MAX_VALUE + 1).
  2. (30 points)  Demo the stopwatch program to the TA.  To receive full credit, you device must remain synchronized with a reference timepiece for 2 minutes (120 seconds) and behave according the requirements above.
  3. Bonus: (5 points) (Optional) Demo lab_7_watchdog2_gxx.asm.  Show that the program works at MAX_VALUE and fails at MAX_VALUE + 1.  In order to receive credit for any of the bonus (including the bonus questions), you must successfully demo the bonus program to the TA.

Lab 7 - Part B



In this lab, you will take one of the prelab SWI handlers with suitable modifications and get it to work on the course hardware. (You can use either prelab submission, combine them, make changes, or start over from scratch as you desire. There is no bonus or penalty based on which partner's prelab is used as the starting point.)


Part 1:

  1. Connect LEDs and Switches so that you have 8 LEDs and 8 Switches available. We strongly suggest you do this in a way that does not conflict with the hardware requirements for the other half of this lab.
  2. Modify the code described in the prelab to read selection values from switches and write result values to the LEDs (with "1" bits displayed as an "on" LED). Name this file lab_7_swi_gXX.asm.
  3. Add an assembly language main loop that sets the various registers to distinct values and repeatedly executes an SWI within the loop. Use these values:
        A              $11
        B              $22
        X              $3344
        Y              $5566
        CCR             any value, but tell the TA what it is
        SP              any value, but tell the TA what it is

BONUS (Optional):
As a separate program, modify the SWI monitoring capability so that you can display and modify any one of up to 256 predetermined memory locations while a C program is executing. The C program shall be one of your choosing that executes SWI once every time through its main loop. The bytes being displayed and modified shall be the bytes of a 64-element 8 bit integer C array. Name this file lab_7_swi_bonus_gXX.c.

Part B - Demo Checklist: (50 + 5 points)

  1. (50 points): Demonstrate to the TA that the appropriate register values display when you press buttons. The TA may ask you to change the value in a register via editing and recompiling your program to verify correct operation.
  2. (Bonus 5 points): Optional. Demonstrate to the TA that your bonus memory display and modification function works properly.

Hand-in Checklist: (175 + 17 points)

All non-code submissions shall be in a single PDF document.

Part A:

  1. (5 points) List any problems you encountered in the lab and pre-lab, and suggestions for future improvement of this lab. If none, then state so to get these points.
  2. (20 points)  Submit your lab_7_watchdog_gxx.asm file.  The coding style sheet must be followed to receive full credit.
  3. (30 points)  Submit your lab_7_stopwatch_gXX.c file.  The coding style sheet must be followed to receive full credit.
  4. (65 points) Answer questions 1-6 in part A above.
  5. Bonus: (6 points) (optional)  Submit lab_7_watchdog2_gxx.asm.
  6. Bonus: (6 points) (optional) Answer bonus questions 7-8 in part A above.

Part B:

  1. (5 points) List any problems you encountered in the lab and pre-lab, and suggestions for future improvement of this lab. If none, then state so to get these points.
  2. (50 points) Submit code files for your SWI monitoring routine and associated main program. Name the file lab_7_swi_gXX.asm. Code must be fully commented to receive full credit.
  3. (Bonus: 5 points) Submit code files for your SWI memory display and modification routine and associated main program. Name this file lab_7_swi_bonus_gXX.c. Code must be fully commented to receive full credit.

Refer to the LAB FAQ for more information on lab handin procedures and file type requirements.  You MUST follow these procedures or we will not accept your submissions.

Hints and Suggestions:

Part A:

FILES for this lab:

Part A:

Part B:

Relevant reading:

Also, see the course materials repository page.

Change notes for 2016: