Description
Objectives
The purpose of this lab is for the student to gain experience interfacing the microcontroller with
external peripherals using GPIO. In addition, the student will learn how to use delay loops and
verify their accuracy using a logic analyzer.
Overview
For this lab, you will be writing a ten bit binary counter in assembly that increments at a
frequency of 2 Hz (0.5 seconds). The value of the counter will be displayed on an LED bar graph.
Three buttons will be used to start, stop and reset the counter. The program will be run on your
microcontroller and it will interface with an LED display and switches using GPIO ports. All
timing requirements will be verified using a logic analyzer.
Preparation
1. Prior to lab, please read chapters 2.2 and 4.2.2 in volume one of your textbook
2. Read this document in entirety
3. Come with the following:
Breadboard (1)
Tiva C Series LaunchPad Evaluation Kit
10 LED bar graph (1) – Available for purchase at the ECE store
Push Button (1) – Available for purchase at the ECE store
Bar resistor 220 Ohm (1) – Available for purchase at the ECE store
Standard 220 Ohm Resistor (1) – Available for purchase at the ECE store
Leads for the voltage source
Procedure
1 LED Display
The LED display will be connected to your microcontroller via a breadboard; power for the LEDs
will be supplied by an external voltage source. Connect one ground pin from your
microcontroller and the voltage source to the breadboard (this allows the voltage source and
the microcontroller to share a common ground). Read over the datasheets for the ten LED bar
graph and bar resistor to determine how to connect the LEDs to the microcontroller (a single
LED from the display is depicted
below); you will need to decide
which ports/pins to use. Note
that this is a ‘sinking
configuration’ (i.e. the
microcontroller is sinking current
instead of supplying it). When
connecting the LED display, use an
active-low configuration (0 [low]
means active [LED on], 1 [high]
means inactive [LED off]).
Caution! In general, powering external hardware through the microcontroller runs the risk of
drawing too much current and then damaging the internal components. Please give careful
consideration to power and current requirements of external hardware when connecting it to a
microcontroller.
2 Buttons
The Tiva C Launchpad that you are using only has two internal buttons available so a third
button will need to be mounted on the breadboard. The button labeled “reset” on the Tiva
board is not usable for your project – it serves as a power reset for the board. The two internal
buttons and the one external button will serve as inputs for the counter program. Connect the
external button to a port/pin of your choosing. Look over the first schematic in Appendix A of
the board’s user guide to understand how the on-board buttons work so that you’ll know how
to configure your port/pin. A datasheet for the external button can be found on the wiki.
3 Software Requirements
1. The counter will count at 2Hz +/- 5% as verified by the logic analyzer.
2. When the stop button is pressed, the counter will pause with the current value still
visible.
3. When the start button is pressed, the counter will resume from the current count.
4. When the reset button is pressed, the counter will start counting from zero.
Show the functioning programming to the TA
Definitions
Tri-state. When the GPIO pin is
disabled, the pin is left floating, which
is also known as tri-state.
Push-pull. When the pin is writeenabled without the open drain or
other options enabled, it is in a pushpull configuration. The pin will source
current or sink current depending on
its value.
Open drain. A configuration that works
well as part of a bus because it allows
multiple devices to communicate on
the same line. The configuration will
cause the device to sink current when
active or remain in a high-impedance
state when inactive.
Pull-up vs pull-down resistor. The
difference between the two
configurations is the placement of the switching device; see the image. The pull-up
resistor is more efficient at supplying current, while the pull-down resistor is more
efficient at draining current. Read V1.Ch9.5 for more details.
Documentation
The report must include the following
Logic analyzer screen shot that shows the delay using the markers.
Description of your software design – through diagrams, flow-charts, etc.
The code of the program will form the final addendum to the report.
Schematic of your physical design, noting port/pin configuration.
Be sure that your reports meet all other criteria in the “Guidelines for ECE 3710 Lab
Reports” section of the course wiki – you can find it under lab supplementary materials.
Hints and Tips
Sections 10.3—4 and Table 10.3 of the microcontroller datasheet will be useful for help
in configuring the ports/pins (NOTE: the Advanced Peripheral Bus is used in the course
notes).
Use Sections 2.1.5 and Appendix A of the user guide to figure out how uC ports/pins are
mapped to the board’s headers.
The recommended configuration for this lab is open-drain w/pull-up resistor while input
pins should use a pull-up resistor. You must be able to explain/justify the port/pin
configuration in your lab report (that is, if you use the recommended setting then you
need to be able to tell us why it works the way it does).
Pins C0-C3 are JTAG connectors, and therefore unavailable for GPIO.
Certain pins may be locked, which will cause errors during configuration. Two of these
pins are PD7 and PF0. You can tell if your pin is locked if the corresponding GPIOCR
register value is 0. Unlocking a GPIO allows the GPIOCR to be written to, which will allow
configuration of the pin.
◦ Unlocking is performed by writing a specific 32-bit value to the GPIOLOCK register.
Look in the datasheet and Valvano V1.Ch4.2.1 (I/O ports) for more information.
◦ Writing to a GPIO when it’s locked causes a hard fault. To verify a GPIO pin is locked,
view the GPIOCR and GPIOLOCK registers in the debug mode System Viewer.
