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UT.6.03x Embedded Systems - Shape the World - Lab 10: Traffic Light


Chapter 10 is the most difficult chapter so far. It covers finite-state machine. I have never learned about finite-state machines before, so it was quite challenging for me to let everything sink it.


PLL allows us to speed up or slow down the clock. There is a trade-off between speed and power: If the clock is sped up, the board can do more works as well as consume more energy. On the other hand, if the clock is slowed down, the board uses less energy, which means it cannot do as many works.

PLL has 2 fields; XTAL and SYSDIV2, which are correspondingly represented by the two registers: SYSCTL_RCC_R and SYSCTL_RCC2_R.

To config the PLL, we need to config SYSCTL_RCC_R and SYSCTL_RCC2_R like below:


PLL gives us 2 things:

  1. Higher precision external crystal

  2. Flexibility to choose our speed. Again, slow speed = less power consumption and high speed = more power consumption.

We don’t use PLL configuration in this lab, but it is good to know how to configure the PLL and change a 16-MHz clock into a 80-MHz clock. For more detail, visit the datasheet of the EK-TM4C123GH6PM, page 1366.


Since the PLL changes the system clock to 80 MHz, each tick will take 12.5ns (or 12.5e-9 seconds or 1.25e-8 seconds) to complete. In order to write a standard SysTick delay function, add the following lines:

 * Delays the program
 * @param  delay  count value
 * @assumption    80-MHz clock
 * @notes         delay = DELAY_TIME_SEC / 12.5 / 0.000000001
void SysTickWait(unsigned long delay) {
    NVIC_ST_RELOAD_R  = delay - 1; // number of counts to wait
    NVIC_ST_CURRENT_R = 0;         // overwrite the CURRENT register

    // wait until COUNT is flagged:
    while ((NVIC_ST_CTRL_R & 0x00010000) == 0) {}

We would delay the program according to the value of delay: delay = DELAY_TIME_SEC / 12.5 / 0.000000001

For example, to delay 1ms (recommended), the delay value would equal to 80000. Apply the standard delay function using SysTick timer, we can create a precise delay function by delay 1ms a number of times.

 * Delays the program some milliseconds
 * @param  time_ms  milliseconds to delay
 * @assumption          80-MHz clock
void SysTickWait1ms(unsigned long time_ms) {
    unsigned long i;

    for (i = 0; i < time_ms; i++) {
        // count = 0.001 / 12.5 / 0.000000001 = 80000
        // equivalent to 1 millisecond

Lastly, the COUNT flag of the SysTick can be cleared by a reading of the register or a writing of any value to the CURRENT register. I made a mistake on this because I didn’t know that a read of the register can clear the COUNT flag. Again, for more information, feel free to read the datasheet, page 1366.


I suggest visiting this page to learn structs in C.

We will declare a struct which contains the output, the delay time, and the state transition.


The most abstract content in this chapter is FSM. To fully describe an FSM, we need 5 things:

  1. Set of inputs
  2. Set of outputs
  3. Set of states
  4. State transition graphs or matrix
  5. Output determination

There are two well-known types of FSM, which are Moore FSM and Mealy FSM.

Instead of if statement, FSM use arrays containing different states to branch the flow of code. The application of the FSM will be mentioned below.

By the way, I also learned a new thing from the quiz of the chapter on SysTick Timer. Since SysTick is a 24-bit timer, the longest time one can wait for it is: 2^24 * 12.5 nanoseconds = 0.2097152 seconds.

If an FSM has n digital inputs, then there are possible 2n input patterns.

LAB 10:

The challenge in the lab is to apply the contents we have learned from the course up to this time as well as debugging skills to implement a software for the traffic light system.

  1. Ports Initialization:
    I named ALL the necessary registers for ports initialization. Do it carefully. Activate the clock, then allow time for the clock to start. Unlock GPIO for each port by the magic number 0x4C4F434B, then allow changes to ports and bits you use by setting the committed register. Disable the analog functionality since we won’t use it. Disable the PCTL register. Config the direction register for I/O (Input = 0 and Output = 1). Disable alternate function and pull-up resistors. Finally, enable digital I/O for ports and bits we are going to use.
    Important, remember to set PCTL = 0. I mistakenly set PCTL (port control) to be equal to another number, and the automatic grading machine aborted my program.

  2. SysTick Initialization:
    Initialize the SysTick timer, then create a standard counting function and use that standard counting for main delay function.

  3. The hardest part - FSM declaration:
    The struct must have at least 4 elements, and we must output to the roads before indicating anything on the pedestrian. Every state must have a wait time. The last element, also the most important construction of an FSM, is state transition array. Draw a table with possible inputs, then write possible outputs based on the input and the current state. For references, I made this table below. Remember that the FSM declaration and the table are EXACTLY the same things.

The order of the code segments in while (1) infinite looping must be:
roads outputs ~> pedestrian outputs ~> wait ~> get inputs ~> state transition






Schematic - Kind of messy:


Real board arrangement:



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 go to my project folder on GitHub.

 * UTAustinX: UT.6.03x Embedded Systems - Shape the World
 * Lab 10: Traffic Light
 * File Name: main.c
 * Description:
 *     Simulates a traffic light system with three modes:
 *     Going South, Going West, and Pedestrians.
 * Compatibility: EK-TM4C123GXL
 * Author: Phi Luu
 * Location: Portland, Oregon, United States
 * Created: March 31, 2016
 * Updated: June 23, 2017

 * Required hardware I/O connections
 * West's Red-Yellow-Green connected to PB5-PB4-PB3
 * South's Red-Yellow-Green connected to  PB2-PB1-PB0
 * Pedestrian's Red connected to PF1
 * Pedestrian's Green connected to PF3
 * West's switch connected to PE0
 * South's switch connected to PE1
 * Pedestrian's switch connected to PE2

#include "TExaS.h"
#include "tm4c123gh6pm.h"

// Constant declarations to access port registers
// using symbolic names instead of addresses
// Port F
#define GPIO_PORTF_DATA_R     (*((volatile unsigned long *)0x400253FC))
#define GPIO_PORTF_DIR_R      (*((volatile unsigned long *)0x40025400))
#define GPIO_PORTF_AFSEL_R    (*((volatile unsigned long *)0x40025420))
#define GPIO_PORTF_PUR_R      (*((volatile unsigned long *)0x40025510))
#define GPIO_PORTF_DEN_R      (*((volatile unsigned long *)0x4002551C))
#define GPIO_PORTF_LOCK_R     (*((volatile unsigned long *)0x40025520))
#define GPIO_PORTF_CR_R       (*((volatile unsigned long *)0x40025524))
#define GPIO_PORTF_AMSEL_R    (*((volatile unsigned long *)0x40025528))
#define GPIO_PORTF_PCTL_R     (*((volatile unsigned long *)0x4002552C))
// Port B
#define GPIO_PORTB_DATA_R     (*((volatile unsigned long *)0x400053FC))
#define GPIO_PORTB_DIR_R      (*((volatile unsigned long *)0x40005400))
#define GPIO_PORTB_AFSEL_R    (*((volatile unsigned long *)0x40005420))
#define GPIO_PORTB_PUR_R      (*((volatile unsigned long *)0x40005510))
#define GPIO_PORTB_DEN_R      (*((volatile unsigned long *)0x4000551C))
#define GPIO_PORTB_LOCK_R     (*((volatile unsigned long *)0x40005520))
#define GPIO_PORTB_CR_R       (*((volatile unsigned long *)0x40005524))
#define GPIO_PORTB_AMSEL_R    (*((volatile unsigned long *)0x40005528))
#define GPIO_PORTB_PCTL_R     (*((volatile unsigned long *)0x4000552C))
// Port E
#define GPIO_PORTE_DATA_R     (*((volatile unsigned long *)0x400243FC))
#define GPIO_PORTE_DIR_R      (*((volatile unsigned long *)0x40024400))
#define GPIO_PORTE_AFSEL_R    (*((volatile unsigned long *)0x40024420))
#define GPIO_PORTE_PUR_R      (*((volatile unsigned long *)0x40024510))
#define GPIO_PORTE_DEN_R      (*((volatile unsigned long *)0x4002451C))
#define GPIO_PORTE_LOCK_R     (*((volatile unsigned long *)0x40024520))
#define GPIO_PORTE_CR_R       (*((volatile unsigned long *)0x40024524))
#define GPIO_PORTE_AMSEL_R    (*((volatile unsigned long *)0x40024528))
#define GPIO_PORTE_PCTL_R     (*((volatile unsigned long *)0x4002452C))
// 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))

// Function prototypes
void EnableInterrupts(void);                    // enable interrupts
void PortsInit(void);                           // ports initialization
void SystickInit(void);                         // SysTick initialization
void SystickWait(unsigned long delay);          // SysTick wait function
void SystickWait1ms(unsigned long wait_time_ms); // SysTick delay function

// Struct declaration
struct FiniteStateMachine {                     // represents a state of the FSM
    unsigned short port_b_out;                  // ouput of Port B for the state (cars output)
    unsigned short port_f_out;                  // output of Port F for the state (pedestrian output)
    unsigned short wait;                        // time to wait when in this state
    unsigned char  next[5];                     // next state array

// Shortcuts to refer to the various states in the FSM array
#define GO_SOUTH        0
#define WAIT_SOUTH      1
#define GO_WEST         2
#define WAIT_WEST       3
#define GO_WALK         4
#define HURRY_WALK_1    5
#define OFF_WALK_1      6
#define HURRY_WALK_2    7
#define OFF_WALK_2      8
#define HURRY_WALK_3    9
#define OFF_WALK_3      10
#define NUM_STATES      11

// FSM declaration
typedef const struct FiniteStateMachine STATE[NUM_STATES] = {
    // 0) Go South
        0x21, 0x02, 3000,
    // 1) Wait South
        0x22, 0x02,  500,
    // 2) Go West
        0x0C, 0x02, 3000,
    // 3) Wait West
        0x14, 0x02,  500,
    // 4) Go Pedestrian
        0x24, 0x08, 3000,
    // 5) Hurry Pedestrian 1
        0x24, 0x02,  250,
        { OFF_WALK_1, OFF_WALK_1, OFF_WALK_1, OFF_WALK_1, OFF_WALK_1 }
    // 6) Off Pedestrian 1
        0x24, 0x00,  250,
    // 7) Hurry Pedestrian 2
        0x24, 0x02,  250,
        { OFF_WALK_2, OFF_WALK_2, OFF_WALK_2, OFF_WALK_2, OFF_WALK_2 }
    // 8) Off Pedestrian 2
        0x24, 0x00,  250,
    // 9) Hurry Pedestrian 3:
        0x24, 0x02,  250,
        { OFF_WALK_3, OFF_WALK_3, OFF_WALK_3, OFF_WALK_3, OFF_WALK_3 }
    // 10) Off Pedestrian 3:
        0x24, 0x00,  250,

// Global variables
unsigned char this_state;   // current state
unsigned char switch_input; // input from switches

int main(void) {
    // Setup
    // activate grader and set system clock to 80 MHz
    TExaS_Init(SW_PIN_PE210, LED_PIN_PB543210, ScopeOff);
    PortsInit();           // Port B, Port F, and Port E initialization
    SystickInit();         // SysTick timer initialization
    this_state = GO_SOUTH; // GO_SOUTH is initial state
    EnableInterrupts();    // enable interrupts for grader

    // Loop
    while (1) {
        // make outputs
        GPIO_PORTB_DATA_R = FSM[this_state].port_b_out; // to cars (port B)
        GPIO_PORTF_DATA_R = FSM[this_state].port_f_out; // to pedestrians (port F)

        // get inputs
        // if no switch is pressed
        if (GPIO_PORTE_DATA_R == 0x00) {
            switch_input = 0; // then it is case 0 of the next[] array...
        }  // ... all LEDs stay the way they are since the last pressing
           // if south switch is pressed
        else if (GPIO_PORTE_DATA_R == 0x02) {
            switch_input = 1; // then it is case 1 of the next[] array...
        }  // ... all LEDs correspond to Go South mode
           // if west switch is pressed
        else if (GPIO_PORTE_DATA_R == 0x01) {
            switch_input = 2; // then it is case 2 of the next[] array...
        }  // ... all LEDs correspond to Go West mode
           // if pedestrian switch is pressed
        else if (GPIO_PORTE_DATA_R == 0x04) {
            switch_input = 3; // then it is case 3 of the next[] array...
        }  // ... all LEDs correspond to Go Pedestrian mode
           // if all switches are pressed
        else if (GPIO_PORTE_DATA_R == 0x07) {
            switch_input = 4; // then it is case 4 of the next[] array...
        }  // ... all LEDs correspond periodically: South, West, Pedestrian
           // change state based on input and current state
        this_state = FSM[this_state].next[switch_input];

 * Initializes port F-B-E pins for input and output
void PortsInit(void) {
    // clock
    volatile unsigned long delay;

    // 1) activate clock for Port F, Port B, and Port E
    SYSCTL_RCGC2_R |= 0x00000032;
    delay           = SYSCTL_RCGC2_R; // allow time for clock to start
    // Port F
    GPIO_PORTF_LOCK_R  = 0x4C4F434B;  // 2) unlock GPIO Port F
    GPIO_PORTF_CR_R   |= 0x0A;        // allow changes to PF3, PF1
    GPIO_PORTF_AMSEL_R = 0x00;        // 3) disable analog function
    GPIO_PORTF_PCTL_R  = 0x00;        // 4) PCTL GPIO on PF3, PF1
    GPIO_PORTF_DIR_R  |= 0x0A;        // 5) PF3, PF1 are outputs
    GPIO_PORTF_AFSEL_R = 0x00;        // 6) disable alternate function
    GPIO_PORTF_PUR_R   = 0x00;        // disable pull-up resistor
    GPIO_PORTF_DEN_R  |= 0x0A;        // 7) enable digital I/O on PF3, PF1
    // Port B
    GPIO_PORTB_LOCK_R  = 0x4C4F434B;  // 2) unlock GPIO Port B
    GPIO_PORTB_CR_R   |= 0x3F;        // allow changes to PB5-PB0
    GPIO_PORTB_AMSEL_R = 0x00;        // 3) disable analog function
    GPIO_PORTB_PCTL_R  = 0x00;        // 4) PCTL GPIO on PB5-PB0
    GPIO_PORTB_DIR_R  |= 0x3F;        // 5) PB5-PB0 are outputs
    GPIO_PORTB_AFSEL_R = 0x00;        // 6) disable alternate function
    GPIO_PORTB_PUR_R   = 0x00;        // disable pull-up resistor
    GPIO_PORTB_DEN_R  |= 0x3F;        // 7) enable digital I/O on PB5-PB0
    // Port E
    GPIO_PORTE_LOCK_R  = 0x4C4F434B;  // 2) unlock GPIO Port E
    GPIO_PORTE_CR_R   |= 0x07;        // allow changes to PE2-PE0
    GPIO_PORTE_AMSEL_R = 0x00;        // 3) disable analog function
    GPIO_PORTE_PCTL_R  = 0x00;        // 4) PCTL GPIO on PE2-PE0
    GPIO_PORTE_DIR_R   = 0x00;        // 5) PE2-PE0 are inputs
    GPIO_PORTE_AFSEL_R = 0x00;        // 6) disable alternate function
    GPIO_PORTE_PUR_R   = 0x00;        // disable pull-up resistor
    GPIO_PORTE_DEN_R  |= 0x07;        // 7) enable digital I/O on PE2-PE0

 * Initializes SysTick timer
void SystickInit(void) {
    NVIC_ST_CTRL_R    = 0;        // disable SysTick during set up
    NVIC_ST_RELOAD_R  = 0xFFFFFF; // maximum value to RELOAD register
    NVIC_ST_CURRENT_R = 0;        // overwrite to CURRENT to clear it
    NVIC_ST_CTRL_R    = 0x05;     // enable CLK_SRC bit and ENABLE bit

 * Delays the program
 * @param  delay  count value
 * @assumption    80-MHz clock
 * @notes         delay = Time_To_Delay_In_Seconds / 12.5 / 0.000000001
void SystickWait(unsigned long delay) {
    NVIC_ST_RELOAD_R  = delay - 1; // number of counts to wait
    NVIC_ST_CURRENT_R = 0;         // overwrite the CURRENT register

    // wait until COUNT is flagged:
    while ((NVIC_ST_CTRL_R & 0x00010000) == 0) {}

 * Delays the program some milliseconds
 * @param  wait_time_ms  milliseconds to delay
 * @assumption          80-MHz clock
void SystickWait1mss(unsigned long wait_time_ms) {
    unsigned long i;

    for (i = 0; i < wait_time_ms; i++) {
        // count = 0.001 / 12.5 / 0.000000001 = 80000
        // equivalent to 1 millisecond


In order to make the automatic grading machine run faster, adjust the delay time of each state to be less than 100 milliseconds.


On the real board, the delay should be long for realistic perception, about 500ms to 3000ms.