Lab 8

The MC68HC11 Timer

Before the Lab

The main timer of the MC68HC11 can be used to generate an interrupt when a certain amount of time elapses. It can be used to generate a pulse or a square wave with a specified duration. The timer can also be used to measure the duration of a pulse or count external pulses. 

To use an input-capture function to detect a specific edge on a timer input pin you need to set the control bits (EDGxB, EDGxA) in the control register TCTL2 according to the table given in the reference manual page 10-17 of the MC68HC11 Reference Manual. The ICxF status bits in the TFLG1 register are automatically set to one each time a selected edge is detected at the corresponding input-capture pin. Writing clears these status pins to the TFLG1 register with a one in the corresponding data bit position.  When the ICxI bit in the TMSK1 register is zero, the corresponding input-capture interrupt is inhibited. When this bit is set to one, the corresponding input-capture interrupt is enabled and a hardware interrupt request is generated whenever the corresponding ICxF bit is set to one.

To program the automatic pin action to occur for an output-compare function you need to set the control bits (OMx, OLx) in the control register TCTL1 according to the table given in the reference manual page 10-31. The OCxF status bits in the TFLG1 register are automatically set to one each time the corresponding output-compare register matches the free running timer. Writing clears these status pins to the TFLG1 register with a one in the corresponding data position. When the OCxI bit in the TMSK1 register is zero, the corresponding output compare interrupt is inhibited. When this bit is set to one, the corresponding output-compare interrupt is enabled and a hardware interrupt request is generated whenever the corresponding OCxF bit is set to one.

More information on the timer is available in the MC68HC11 Reference Manual. Read with care through chapter 10 in this manual, especially sections 10.3 and 10.4. This device has some subtleties that will trip you up if you are not thoroughly familiar with its operation.

You need to write the following programs:

1. A program that will generate a square wave from pin 5 of port A. The wave should have a variable duty cycle. You will have to specify the width of the high and the low portion of the waveform. You need to set the timer to generate an interrupt when the specified time elapses and to set your ISR to reload the timer with the required duration. Your program should be initialized to be able to take over the timer interrupt. Don't forget to clear the 0C3F in the ISR.

2. A program that will rotate a single lit LED from right to left. Use the timer to accurately control the time an LED will be kept lit (e.g. 500 ms). When the light reaches the boundary of the 8 LEDs just make the sequence wrap around. You might need to refer to your code in Lab 6.

3. A program that will generate a sine wave with a specified frequency on one of port C's pins.

4. A program that will measure the period of a signal from a function generator and continuously display it on the screen (for that you will need to use the monitor subroutine called _out2bdsp at location $ffcc). To do this you will connect the i/p signal to pin 2 of port A. You will need to set up an ISR that will be called whenever a leading (or trailing) edge is detected. Each time the ISR is called it reads the content of the input-capture latch. By computing the difference between two edges, the period of a pulse can easily be determined.

In the Lab

Before you run any of your programs, a word about connections. These exercises will require access to the header pins on the HC11-VDK board. DO NOT, UNDER ANY CIRCUMSTANCES, USE THE GORILLA CLIPS TO ATTACH TO THE HEADERS. ATTACH ALL CONNECTIONS VIA WIRE-WRAP. Your T.A. will show you how. Once you are through making your measurements, REMOVE THE WRAPS from the board. Loose wires could accidentally short out. Wire is cheap. IC's are not.

1. Load and run your first program. Observe the output on an oscilloscope. Is it what you expected? Note the different durations carefully. Can you see any deviation from what you expected? Why? (Or why not?) What are the limits of your program? What are the limits of the high and low portions of your signal? Why?
2. Load and run program two. Ensure that the timing is as designed.
3. Load and run program three. What is the smallest and largest signal frequency that you can generate? Where are the limitations coming from?
4. Load and run program four. You need to explain to your T.A. the meaning of your results.

After the Lab

Clean up your programs to hand in to the TA. Answer all the questions appearing above. Calculate the longest period that you can measure with the last program.

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