EE 474 Lab 1: Input/Output Interfacing and Traffic Light Controller, FSMs


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1. Familiarity with IAR workbench software development environment
2. Programming the EK-TM4C123 board
3. Learning basics of general-purpose input and output (GPIO)
4. Performing simple digital I/O input (interfacing push button) and output (interfacing LED)
5. Understanding the power of pointers in C and how to use them in addressing GPIO registers
6. Using the datasheet to extract information necessary for addressing the LaunchPad parts.
7. Understand debouncing
8. Designing a finite state machine (FSM) controller
What you need for the lab
1. The EK-TM4C123 Launchpad (
2. TM4C123 data sheet (
3. IAR workbench or other IDE
4. IDE installation guide (
5. Tutorial video (
Since we use different versions of the IDE in this video, please consult the above IDE installation
guide if you find any differences.
6. Oscilloscope and some cables
7. LEDs (
8. Push buttons
9. Debouncing (
10. Lab write up specifications
1. IAR workbench is installed on the lab machines in the lab and the installation instructions
are posted on Canvas. If you want to download the software on your laptop please note that IAR
works only on Windows OS so if your computer runs Linux or Mac OS, you will need to install a
virtual machine. You are free to use other IDEs such as Keil uVision or Code Composer but make
sure that the TM4C123 is supported (the board is supported by Keil 4.7 but not Keil 5).
2. The free version of IAR limits your data and code to 32 KB, which is not an issue for the lab
assignments in this course.
TM4C123GXL board
Figure 1shows an overview of the different parts of the Launchpad, refer to the user manual (posted
on canvas) for more information on the board’s features
EE 474 Introduction to Embedded Systems____________________________________ ____University of Washington
Figure 1 TM4C123G LaunchPad
The RGB user LED consists of three LEDs Red, Green, and Blue and are connected to the processor
through Port F (as shown in Figure 2 and Figure 3)
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Figure 2 Three LEDs
Figure 3 PortF
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To turn on an LED, the following operations must be performed:
1. Configure the RCGCGPIO register to turn on the GPIO port F by enabling its clock. By default, the
clock to all peripherals, including GPIO ports, are turned off in order to improve the energy
2. Configure the GPIODEN register to set Port F as digital (by default, the mode of all GPIO pin is
3. Configure the GPIODIR register to set the PF1, PF2, and PF3 pins as outputs
4. Configure the GPIODATA register to set the output value of the corresponding GPIO pin to 1. Port
F is one byte (8 bits) wide so to turn on the red LED (as an example), PF1 must be set to 1 so the
value of the port is 0x02 (hexadecimal).
The same steps apply for reading user switches except that GPIODIR register should be configured to set
pins PF0 and PF4 as inputs. Please note that PF0 has special considerations. It is configured as a GPIO by
default but is locked and can be reprogrammed by unlocking the pin in the GPIOLOCK register and uncommitting it by setting the GPIOCR register. Therefore you need to configure GPIOLOCK, GPIOCR, and
GPIOPUR as well.
Section A. Input/Output Interfacing
Example Program
The program shown in figure 4 is a program that turns on the red LED.
Figure 4 sample program for onboard LED
Note that it includes the tm4c123gh6pm.h file which is included in a library specific to the TM4C123GXL
board. This can be downloaded from (available on canvas
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too). The “installing header files” document on canvas refers to installing the header files for the TIVA
series and shows how to set the path in IAR. You can follow the same steps for setting the path of the
TM4C123GXL library.
Section A Required Tasks:
1. Try the program of figure 4 to turn on the red LED on your board.
2. Modify the program to exclude using macros definitions and eliminate the use of the TIVA and
Launchpad libraries. You need to directly address the registers based on their base and offset
addresses as described in the datasheet.
3. Expand the program to light up all LEDs. They should be flashing continuously.
4. Modify the program to define and use your own macros for better readability. You do not need to
include the LaunchPad libraries.
5. Write another C program to incorporate the user input from the switches. You should turn on the
red light when user switch 1 is pressed and turn on the green light when the user switch 2 is
6. Use an oscilloscope to show the voltage output of the LED and the voltage signal of the pin
connected to the onboard pushbutton. Find out the latency between the onboard buttons pressed
and LED lighting up.
Section B. Traffic Light Controller, FSMs
Button Debouncing
Check the correct pinout for the push buttons. Wire one pin of a button to the ground, and the other pin to
PA5 in the TIVA. Note here, PA5 is a positive-logic button input, where 1 means pressed, and 0 means not
pressed. You may use the breadboard and jumper wires from EE 215.
Please try the program of figure 5 to interface a switch to TIVA.
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Figure 5 sample code for push button
Commonly, we would encounter a bouncing issue when a push button is used. When it happens, the GPIO
pins would treat a single press as multiple presses. There are multiple ways to denounce the button,
either by hardware approach or software approach. See the debouncing document in the Canvas for
more details (
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Figure 5 push button bounce
A common approach is using a delay. Essentially, when you press or not press a button, the contact is not
immediately read. We just wait a few moments before reading the new value. Please try this code below:
// SysCtlDelay(7000000); // uncomment me if you need debouncing
LEDs emit light when the current passes through them. LEDs have polarity, the current flows from the
anode (shorter lead) to the cathode (longer lead) to activate them. The LEDs won’t be damaged if you
reversed the direction, but they won’t be turned on.
The brightness of a LED depends on the current flows through it. If the current is more than 8mA, it is not
safe to connect it directly to a GPIO pin on the board. We suggest you use a 220Ω current limiting resistor,
which connects to the cathode of the LEDs. The hardware configuration of this LED is shown in Figure 6.
Note +5 V is VBUS on TIVA. Please try the code above to interface an LED. You need to modify the pin
from PA2 to PE0 to match the hardware configuration. Alternatively, you could connect the pin 1 of
U3a7406 (or any generic 7406 gates found in the kit) to PA2 on TIVA to match the sample code.
Figure 6 LED hardware setup
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Figure 7 sample code for LEDs
Traffic Light Controller
We provide three different LEDs and two push buttons in this lab. One red LED (for stop), one green LED
(for go), and one yellow LED (for warn). The suggested pins are:
LEDs/ switch Possibility 1 Possibility 2 Possibility 3
Button for start/stop PA2 PB2 PE0
Button for passenger PA3 PC4 PE1
Red LED (Stop) PA7 PB5 PE5
Yellow LED (Warn) PA6 PB4 PE4
Green LED (Go) PA5 PB3 PE3
Finite State Machine Design
Design a finite state machine that implements a traffic light system. It should include two push buttons as
an input, and three LEDs are an output. You may need to purchase one push button from EE store if you
can only find one button in your lab kie.
1) Button for start/stop
The user would press this button to start and stop the system. The system will always start from
the go stage (where the green LED is on).
2) Button for passenger
Once this button is pressed, it indicates there is a passenger that would like to cross the street. If
the current stage is stopped (where the red LED is on), nothing will happen. If the current stage is
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going (where the green LED is on), the current stage will change to warn stage (where the yellow
LED is on), then the current stage will change to stop stage (where the red LED is on) to allow the
passenger safely across the street.
1) Red LED (stop)
When the current stage is stop, the red LED is on, and all other LEDs are off.
2) Yellow LED (warn)
When the current stage is warn, the yellow LED is on, and all other LEDs are off. It only
happens when the button for the passenger is pressed.
3) Green LED (go)
When the current stage is go, the green LED is on, and all other LEDs are off.
Section B Required Tasks:
1. Try the program to interface the button.
2. Try the program for the button debouncing.
3. Try the program for the LED.
4. Design the FSM for the traffic light system.
5. Draw the finite state machine system diagram.
6. Draw the schematics for the traffic light control system.
Good Coding Practices (Courtesy of Y.Zhu, author of “Embedded systems with ARM Cortex-M”)
Program comments are used to improve code readability and to assist in debugging and maintenance. A
general principle is “Structure and documents your program the way you wish other programmers
would” (McCann, 1997).
The book titled “The Elements of Programming Style” by Brian Kernighan and P. J. Plauger gives good
advice for beginners.
1. Format your code well. Make sure it’s easy to read and understand. Comment where needed but
don’t comment obvious things it makes the code harder to read. If editing someone else’s code,
format consistent with the original author.
2. Every program you write that you intend to keep around for more than a couple of hours ought to
have documentation in it. Don’t talk yourself into putting off the documentation. A program that is
perfectly clear today is clear only because you just wrote it. Put it away for a few months, and it
will most likely take you a while to figure out what it does and how it does it. If it takes you a
while to figure it out, how long would it take someone else to figure it out?
EE 474 Introduction to Embedded Systems____________________________________ ____University of Washington
3. Write Clearly – don’t be too clever – don’t sacrifice clarity for efficiency.
4. Don’t over a comment. Use comments only when necessary.
5. Format a program to help the reader understand it. Always Beautify Code.
6. Say what you mean, simply and directly.
7. Don’t patch bad code – rewrite it.
8. Make sure comments and code agree.
9. Don’t just echo code in comments – make every comment meaningful.
10. Don’t comment bad code – rewrite it.
11. The single most important factor in style is consistency. The eye is drawn to something that
“doesn’t fit,” and these should be reserved for things that are actually different.
1. A lab report that includes: abstract, introduction, Procedures, results, and conclusion sections. The
report should address the required section A and B tasks and should include screenshots of your
programs as well as pictures of oscilloscope output. Including the traffic light controller system
schematic and finite state machine diagram in the report. Please check the lab write up
specifications on the Canvas
2. All source files with proper comments.
3. Demonstrate the output to your TA.
4. Upload the report and source files to the canvas. One submission is expected per team.