🔬 Lab 13 Timers#

📌 Objectives#

  • Students should be able to write software module that generates periodic interrupts to run user-specific tasks.

  • Students should be able to generate PWM signals using a timer module.

  • Students should be able to use PWM outputs to adjust the motor speeds.

Note

Make it work, make it right, make it fast. – Kent Beck

📜 Synopsis#

This lab aims to write software that can adjust the applied power to the two motors. You will create PWM outputs on the P2.6 and P2.7 pins connected to the EN pins of the two TI DRV8838 motor drivers. The period of both PWM outputs should be fixed at 20 ms (50 Hz). However, the software should be able to independently set the duty cycle of the EN pin to each motor from 0 to 999 (0 to 99.9%). At 50 Hz, the motor will not respond to individual highs and lows; instead, the motors will respond to the average level. More specifically, the delivered power will be:

\[ P = VI \frac{d}{100} \]

where \(V\) is the voltage, \(I\) is the current, and \(d \in [0, 99.9]\) is the duty cycle in percent (%).

Duty cycles are typically expressed as a percentage of ON time, which is a real number between 0.0% and 100.0%. However, in microcontrollers, it is preferable to avoid floating point numbers and their computations. Therefore, we will use a 16-bit integer between 0 and 1,000 to represent a duty cycle between 0.0% and 100.0%. We will use the unit permille, which means one out of every one thousand (mille). Another advantage of using permille is that we can express 1,000 different values using a 16-bit integer, whereas we can only have 100 different integer values if we use a percentage.

💻 Procedure#

Setup#

Before proceeding, review the solution for TimerA1_Init posted on Gradescope to confirm you have the accurate code. If the solution isn’t available on Gradescope, consult your instructor to go over your code.

Write Timer functions in TimerA1.c.#

  1. Open TimerA1.h and TimerA1.c and read them thoroughly.

  2. Carefully examine Program13_1 to understand (i) how TimerA1 is initialized and (ii) how the semaphore is employed to coordinate between the foreground and background threads.

  3. Write the TimerA1_Stop() and TA1_0_IRQHandler() functions, as discussed in Lecture 13 and referenced in Valvano’s textbook.

  4. Demonstrate Program13_1 as shown in the video below. Ensure that the red LED blins at 5 Hz, the blue LED blinks at 2.5 Hz, and the LCD updates the elapsed time at a rate of 5 Hz. Your demo should also show the timer interrupt being enabled by pressing a switch and disabled by pressing a bump sensor.


Video Credit: C24 Chanon Mallanoo

Important

Why do we employ a function pointer?

We want to enable interrupt service routines to execute a range of user-defined tasks without the need for rewriting them each time a new task arises. As an illustration, when we have different tasks for TimerA1 to execute, we don’t have to rewrite the TimerA1.c file. Instead, we can create functions that carry out these tasks and simply pass their addresses to TimerA1.

Write functions in PWM.c and Motor.c.#

  1. Thoroughly review the documentation provided in PWM.h and PWM.c to understand the functions you will implement.

  2. Note that the PWM period for both motors is set at 20 ms (50 Hz). When calling PWM_Init34 from Motor.c, you should use a period value of 15,000, corresponding to the PWM period of 20 ms. It is not the Timer period, as explained in Lecture 13 Slide 17.

  3. Keep in mind that the duty cycles passed into PWM_DutyRight and PWM_DutyLeft are in permille. The code handles the conversion to the Timer period, so you don’t need to perform this calculation when calling these functions in Motor.c.

  4. Read Motor.h and Motor.c thoroughly.

  5. Implement the functions in PWM.c to generate two PWM outputs controlling the robot’s wheels. Both PWM signals operate at 50 Hz (20 ms).

  6. Demo Program13_2 as shown in the video below. Use motor functions defined in Motor.c to maneuver the robot and ensure you run at least one iteration in the while loop.


    Video Credit: C26 Peter Choi, Royal Military College of Canada

Write functions in Lab13_Timersmain.c#

  1. Write Program13_3().

  2. Combine the PWM and Timer_A1 periodic interrupt functionality into one software system that controls the robot like Program 13.2, but uses the periodic interrupt to track the time elapsed.

  3. While the Timer interrupt runs in the background, the foreground thread will display the motor functions on the LCD.

  4. As shown in the component block diagram below, PWM.c is not directly accessible to Program13_3; It is encapsulated in Motor.c. Program13_3 will only use the functions in Motor.c to control the motors.

    ../_images/Lab13_ComponentBlockDiagram.png

  5. Demo Program13_3 as shown in the video below.


    Video Credit: C26 Peter Choi, Royal Military College of Canada

🚚 Deliverables#

Deliverable 1#

  • [6 Points] Demo Program13_1(). Use TimerA1 to blink the red LED at 5 Hz while the blue LED blinks at 2.5 Hz and the LCD updates the elapsed time at 5 Hz. Your demo should also show the timer interrupt being enabled by pressing a switch and disabled by pressing a bump sensor. During the demo, explain what you are demonstrating.

Deliverable 2#

  • [7 Points] Demo Program13_2(). Use motor functions defined in Motor.c to move the robot. You must run at least two iterations in the while loop. During the demo, explain what you are demonstrating.

Deliverable 3#

  • [7 Points] Demo Program13_2 as shown in the video below. Use motor functions defined in Motor.c to maneuver the robot and ensure you run at least two iterations in the while loop. During the demo, explain what you are demonstrating.

Deliverable 4#

  • [9.5 Points] Push your code to your repository using git. Write comments in your code.

This lab was originally adapted from the TI-RSLK MAX Solderless Maze Edition Curriculum and has since been significantly modified.