Written by Pat Hanrahan; edited by Omar Rizwan
Due: Tuesday, January 30, 2018 at 6:00 PM
Your next assignment is to build a ‘clock’ using a 4-digit 7-segment display.
The goals of this assignment are for you to:
- Understand bare metal programming in C on the Raspberry Pi.
- Start building a simple, modular library of useful functions.
- Learn how to write and use automated test cases to validate your program’s behavior.
- Learn how to use the Raspberry Pi’s system timer peripheral.
- Understand how to refresh a 4-digit 7-segment display.
assign2 branch from your 107e assignment repo:
$ git clone -b assign2 https://github.com/cs107e/[YOUR-GITHUB-USERNAME]-assignments assign2
Familiarize yourself with the starter code and review the provided Makefile. The project also includes a few magic files (
cstart.c) that are necessary for C programs. We will be explaining their purpose in the upcoming lectures.
For this assignment, you will implement functions
across three different C files:
clock.c. The first two files implement reusable modules, the latter is the application program file.
These modules provide access to two of the Raspberry Pi peripherals. The
gpio module has routines that control the GPIO pins and the
timer module retrieves the system tick count. You will use these modules when building your clock application, but more broadly, these modules are intended to be reusable in any future application which requires similar functionality. These two modules are the first of many more to come. By the end of the quarter, you will have implemented a complete set of modules that you can package into a library of reusable Raspberry Pi functions.
A module is divided into an interface and its implementation. The module’s header file, e.g.
timer.h, defines the interface. The interface details the functionality that is exported by the module. Each public function is listed with its name, prototype, and documentation about what the function does. The corresponding
.c file will contain the implementation of the functions. We provide the module interface; it will be your job to write the module implementation.
The module interfaces are given in
timer.h. You are not to change anything in these header files. All your edits will be in the corresponding C files. You shouldn’t expose additional public
timer_ functions, for instance. You can add your own helper functions by declaring those functions at the top of the C file with the
static keyword to make them private to the implementation.
clock.c contains your application program code that configures and operates your clock. The clock code will not directly manipulate the gpio and timer peripherals, instead it will call on the functions exported from the gpio and timer modules. There is no required interface for the clock application, but you should strive for a clean and well-decomposed design that would make your CS106 section leader proud.
The subsequent assignments in CS107e will be structured on similar lines. Each week you will be implementing new module(s) to add your Raspberry Pi library in the context of an application program that uses those modules.
Now that you’re writing larger programs, we want to introduce you to more sophisticated ways to test your program than in assignment 1. We hope these ideas will serve you well throughout your programming life.
The starter project includes an additional program file
defines an alternate
main() function. Instead of running the clock, this
main() function makes a series of calls to
timer functions and uses
assert() in order to validate the operations work correctly. You can run the test program by running
Recall from lab2 how
assert() uses the red and green LEDs
on the Pi as a simple status indicator. If an assert fails, the program
halts and blinks the red LED of doom. If all asserts succeed,
the program completes normally and the green LED of happiness will turn on. Your goal is to make that little green light shine!
test.c has some very simple tests. You should extend the test program with additional tests of your own so as to thoroughly exercise the functionality of your modules. Unit-testing each module in isolation before going on to integrate its use into an application is an important strategy for taming the complexity of these larger programs.
Basic part: a simple clock
1. Wire up clock hardware
The first step is to establish your hardware.
- Complete the breadboard for the 4-digit 7-segment display you started in lab2. Be sure to test your breadboard with jumper cables so that you know the wiring is correct before you connect it to the Raspberry Pi.
- Connect the Raspberry Pi GPIO pins 20-27 to the 1K current limiting resistors that drive the segments A, B, C, D, E, F, G on your breadboard. The 8th pin controls the decimal point, which is optional.
- Connect GPIO pins 10-13 to the base of the transistors controlling digits 1-4.
- At this point if you output 3.3V on GPIO 20 and 3.3V on GPIO 10, you should turn on segment A of digit 1. Hooray!
- Take a picture of your finished hardware setup and commit to your repo. We want to see your beautiful handiwork!
2. Implement gpio module
Review the documentation in
gpio.h so you understand the module as a whole and the expected behavior of each function.
We recommend that the first operations you implement are
gpio_get_function. The Broadcom BCM2835 peripheral manual documents the GPIO function select registers and GPIO functions. For the clock, we will be using only
GPIO_FUNC_OUTPUT, but make sure
gpio_set_function works for all GPIO functions. Also take care that
gpio_set_function can be called multiple times with different pins. Each call affects only the pin passed
in as an argument; the function of all other pins should be unchanged.
In addition to properly handling any valid call, your functions should also be robust against client error. If asked to set an invalid pin or function, your function should detect and ignore the request, rather than blunder on with wrong or broken behavior. The function documentation in the module interface gives specific guidance on the expected handling for improper calls.
gpio_set_output is an easy next step, as these are just simple wrappers around
gpio_set_function to set a pin
as an input or output pin.
Before moving on, test what you’ve implemented. Edit the
main() function of
test.c to uncomment the call to
make test to see the result of executing these tests. If the green LED on the Pi turns on, all tests ran
successfully – bravo! If you see the red LED blinking, this means
you are failing a test. If neither LED lights, your program may be freezing or
crashing somewhere during the tests.
If you don’t get the green light, comment out each individual assert within
add them back one by one and re-run to determine which specific test is
failing. Use that information to narrow in on your bug, and fix
and re-test until you achieve green light nirvana.
Now review the given test cases and consider what isn’t covered. For example, what about testing alternate GPIO functions or GPIO pins across the full range of 54? Add tests of your own to validate these options. Re-run to verify that you also pass your new tests before moving on.
The remaining two functions of the gpio module to implement are
gpio_write function accesses the SET and CLR device registers to turn pins on and off. Which device register does
gpio_read access to get the current pin state? Hint: Check the
Broadcom BCM2835 peripheral manual.
Once you’ve implemented these functions, test them by
uncommenting the call to
test_gpio_read_write() in the
main() function of
test.c. Re-run the test program and make sure you get the green light. Add further test cases of your own to verify your functions work correctly in all situations. Congratulations, you’ve just completed your first Raspberry Pi module!
3. Display a digit
clock.c, create an array of 16 elements, one for each
hexadecimal value between 0 and 15. Each array element should
be a byte (8-bits). Bit 0 (the least significant) will represent
segment A, bit 1 segment B, and so on. If a bit is set, then that
segment should be lit. For example, digit
0 consists of segments A-F, so its bit pattern is
1 consists of just segments B and C, so its bit pattern is
0b is what you put in front of a binary number
literal, just as you’d put
0x in front of a hexadecimal
Bit 7 (the most significant) will be used to represent
DP, the dot. Since we won’t be
using the dot in this assignment, bit 7 should always be 0.
Create bit patterns for all the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. You won’t be displaying A-F for this assignment, but they may be useful in the future.
Test this part by adding code to the
main() function of
clock.c to display a single digit. Verify that your bit patterns are correct by displaying each digit value from
4. Implement timer module
In order to implement a clock, we’ll need some way to determine the passage of time. Thankfully, the Raspberry Pi includes a “system timer”. The system timer is an on-board peripheral that is initialized to zero when the Pi powers up and is continuously incremented once every microsecond behind the scenes.
timer_get_ticks function in
timer.c to fetch
the current system tick count from the system timer. Chapter 12 of the
Broadcom BCM2835 Peripherals Manual
contains the documentation for the system timer peripheral.
Note that, for this assignment, we only care about the lower 32-bits of the system timer. Don’t forget that we use ARM physical addresses, not bus addresses (0x7E… for peripherals), so you’ll need to change the 0x7E… prefix in any peripheral address to 0x20.
Now you can uncomment the call
should be passing all our module tests now – super! Now you’re ready to implement the rest of the clock application.
5. Write display refresh loop
GPIO pins 20 to 26 drive the seven segments A to G. Segments A to G are shared by all four digits on the display. There is another, different set of pins that control which digit is currently active. There is no way to turn on the display segments to show a
5 on the leftmost digit while simultaneously showing a
3 on the rightmost digit.
Instead, we drive the four digits on the display by continuously looping over the digits one-by-one. The inner loop will display all four digits in quick succession. It turns on the leftmost digit, waits a short length of time, and turns off that digit, then repeats the process for each of the other three digits. You might think that turning a digit on and off would cause it to flicker. The key is to sequence through the digits so fast that our eyes cannot see them changing. Good thing computers are fast!
Implement the display refresh loop in
clock.c. Use the functions from the
timer module to control the wait time. Loop though all
four digits, turning each on for 2500 microseconds. Do you see any flicker?
6. Implement clock
In the basic assignment, the clock should display the minutes and seconds that have elapsed since the clock program began executing.
The clock displays zero when the program begins. The inner loop performs one refresh cycle of the 4-digit display, and then increments the elapsed time by the length of time needed to perform the refresh. Make sure to test that the timer is calibrated correctly so the clock is running at the right rate.
Note that the clock time is counting elapsed time since the clock program started, which is not quite the same value as the system tick count. Clock time and the system tick count change at the same rate, but start at different values. Your program will need to implement this logic.
Extension: set time
Add two buttons to your clock breadboard and connect them to GPIO pins 2 and 3. Build a user interface that allows you to set the time. Try to design an interface that is easy to use and that works even after the clock starts running. It can be challenging to build an interface with just a few buttons!
Make sure to document your interface in a
README.md file so that your grader
can test it out!
The starter project contains additional code files
files provide the minimum startup to set up a C program to run in the
bare metal environment. A bit later in the course, we will discuss what this code
does and why is necessary. In the meantime, sit tight, and don’t think about it too much.
The starter project also includes a file
memmap, which is used by the linker. We will also
cover why that file is needed soon.
Submit the finished version of your assignment by making a git “pull request”, following the steps given in the Assignment 0 writeup.