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UT.6.03x Embedded Systems - Shape the World - Lab 12: Tuning Fork


In chapter 12, we will learn about interrupts, including edge-triggered interrupt and system timer periodic interrupt. We also focus on NVIC (Nested Vectored Interrupt Controller).

The edge-triggered interrupt is used to detect the change in inputs. This technique is useful for switches.

The system timer periodic interrupt is the SysTick timer. Note that the SysTick timer will interrupt itself after the delay time. This interrupt is very accurate. It’s great for sound output.

Next, let’s take a glance at pulse-width modulation (PWM). PWM is a manipulation on digital output such that the duty cycle is changed when the frequency (or period) stays the same. Let the time interval when the signal is HIGH is H, and the time interval in which it’s LOW is L. The duty cycle is defined by H / (H + L). Manipulating the duty cycle while keeping the frequency from changing is used in DC motor so that we can adjust the speed of the DC Motor easily.

LAB 12:

Lab 12 requires knowledge of the SysTick periodic interrupt and some breadboarding skills.

  1. The switch is positive logic (pull-down resistor). Connect +3.3V to one leg of the switch. On the opposite leg, connect to PA3 first, then decrease the current by plug in the 10-kilohm resistor to GND.
  2. The audio jack we use has 3 legs: Sleeve, Ring, and Tip. The Sleeve is in the middle, and we need to connect it to PA2 through a 1-kilohm resistor. Make sure that the Ring and the Tip are connected to GND.
  3. Plug in and play.









Note that to be able to run and grade the labs, you have to follow the software requirements. Unfortunately, the software and the course don’t support Linux. However, if you follow the official instruction from Texas Instruments, you can upload and run your program on the LaunchPad.

See below for my code, or view it on my GitHub.

 * UTAustinX: UT.6.03x Embedded Systems - Shape the World
 * Lab 12: Tuning Fork
 * File Name: main.c
 * Description: Using SysTick periodic interrupt, creates a frequency of 440Hz
 * just as you do when striking a tuning fork.
 * Author: Phi Luu
 * Location: Portland, Oregon, United States
 * Created: April 16, 2016
 * Updated: June 22, 2017

 * Required hardware I/O connections
 * switch_state connected to PA3
 * Radio Jack connected to PA2

#include "TExaS.h"

// Constant declarations to access port registers
// using symbolic names instead of addresses
// Port A
#define GPIO_PORTA_DATA_R     (*((volatile unsigned long *)0x400043FC))
#define GPIO_PORTA_DIR_R      (*((volatile unsigned long *)0x40004400))
#define GPIO_PORTA_AFSEL_R    (*((volatile unsigned long *)0x40004420))
#define GPIO_PORTA_PUR_R      (*((volatile unsigned long *)0x40004510))
#define GPIO_PORTA_DEN_R      (*((volatile unsigned long *)0x4000451C))
#define GPIO_PORTA_LOCK_R     (*((volatile unsigned long *)0x40004520))
#define GPIO_PORTA_CR_R       (*((volatile unsigned long *)0x40004524))
#define GPIO_PORTA_AMSEL_R    (*((volatile unsigned long *)0x40004528))
#define GPIO_PORTA_PCTL_R     (*((volatile unsigned long *)0x4000452C))
// System Clock
#define SYSCTL_RCGC2_R        (*((volatile unsigned long *)0x400FE108))
// SysTick Timer
#define NVIC_ST_CTRL_R        (*((volatile unsigned long *)0xE000E010))
#define NVIC_ST_RELOAD_R      (*((volatile unsigned long *)0xE000E014))
#define NVIC_ST_CURRENT_R     (*((volatile unsigned long *)0xE000E018))
#define NVIC_SYS_PRI3_R       (*((volatile unsigned long *)0xE000ED20))

// Function prototypes
void InitSound(void);
void EnableInterrupts(void);     // Enable interrupts
void WaitForInterrupt(void);     // low power mode

// Global variables
unsigned short switch_state = 0; // input from the switch is stored here

int main(void) {
    // Setup
    unsigned long num_presses       = 0; // store the number of presses
    unsigned long prev_switch_state = 0; // store the last input of switch

    // use this to avoid miscounting when the switch is HOLD
    // if switch_state != prev_switch_state, then it is considered ONE MORE PRESS
    // if the current input and the last input are the same, then
    // it is a HOLD, and it is NOT counted as ONE MORE PRESS

    // activate grader and set system clock to 80 MHz
    TExaS_Init(SW_PIN_PA3, HEADPHONE_PIN_PA2, ScopeOn);
    InitSound();        // initial steps
    EnableInterrupts(); // enable after all initialization are done

    // Loop
    while (1) {
        // perform other tasks - Read the input over and over again:
        switch_state = GPIO_PORTA_DATA_R & 0x08; // read the switch from PA3

        if ((switch_state != 0) && (switch_state != prev_switch_state)) {
            // if the switch is pressed, and last time it was released:
            // the number of the ACTUAL press increase by 1
        prev_switch_state = switch_state; // save the current input for next time

        if (num_presses % 2 == 0) {
            // if the number of press is even
            // (those presses at the 0, 2nd, 4th, 6th,... time)
            // then turn off PA2
            GPIO_PORTA_DATA_R &= ~0x04;
        } else {
            // if the number of press is odd
            // (those presses at the 1st, 3rd, 5th, 7th,... time)
            // then periodically interrupt the system each 880 Hz
            // the SysTick will automatically
            // trigger itself every (1 s)/(880 Hz) = 1.13636 ms

 * Interrupt Service Routine. Called every 880 Hz
void SysTick_Handler(void) {
    GPIO_PORTA_DATA_R ^= 0x04; // toggle PA2 (Audio Jack)

 * Initializes Port A and SysTick interrupts
void InitSound(void) {
    // Port A Initialization:
    volatile unsigned long delay;

    SYSCTL_RCGC2_R     |= 0x00000001;     // unlock port A clock
    delay               = SYSCTL_RCGC2_R; // allow time for clock to start
    GPIO_PORTA_LOCK_R  |= 0x4C4F434B;     // unlock GPIO Port A
    GPIO_PORTA_CR_R    |= 0x0C;           // allow changes to PA3-PA2
    GPIO_PORTA_AMSEL_R &= ~0x0C;          // disable analog function on PA3-PA2
    GPIO_PORTA_PCTL_R   = 0;              // clear PCTL register on PA3-PA2
    GPIO_PORTA_DIR_R   |= 0x04;           // PA2 output, PA3 input
    GPIO_PORTA_AFSEL_R &= ~0x0C;          // disable alternate function on PA3-PA2
    GPIO_PORTA_PUR_R   &= ~0x0C;          // disable pull-up resistors on PA3-PA2
    GPIO_PORTA_DEN_R   |= 0x0C;           // enable digital I/O on PA3-PA2

    // SysTick Initialization for periodic interrupts:
    NVIC_ST_CTRL_R    = 0;                // disable SysTick during set up
    NVIC_ST_RELOAD_R  = 90908;            // reload value for 440Hz
    NVIC_ST_CURRENT_R = 0;                // clear current
    NVIC_SYS_PRI3_R  &= 0x00FFFFFF;       // priority 0
    NVIC_ST_CTRL_R    = 0x07;             // enable with core clock and interrupts