Lab written by Pat Hanrahan
During this lab you will:
- Learn how to cross-develop on your computer for the Raspberry Pi’s ARM processor.
- Learn how to use a breadboard with LEDs and buttons.
- Learn how to download and run bare metal programs on the Raspberry Pi.
How does lab work?
When you arrive at lab, find a partner and introduce yourself to one another. Together you will tackle the exercises outlined below. Everyone is encouraged to collaborate with other labmates to share insights and offer each other useful tips. The instructor and TA will circulate the room to offer advice and answers so as to keep everyone progressing smoothly.
Lab is a time to experiment and explore. We introduce topics in readings/lectures, and the guided exercises in lab follow up to further your understanding, put your knowledge to work, give you practice with the tools in a supported environment, and prepare you to succeed at this week’s assignment.
Bare metal programming requires precision. A trivial error or slight misunderstanding can totally block your progress; the resolution may be simple yet almost impossible to see yourself. Our goal in the lab is to get you past these sticking points. Use our support to keep your momentum up and avoid the frustration from fighting with problems that appear insurmountable!
Each lab has a set of check-in questions that you can use to self-test your understanding. Try to answer these questions as you go and please talk with us to resolve any confusion. The check-in questions are intentionally simple and your responses are not graded; we use them as a gauge of how you’re doing with the material so that we know better how to help guide you.
To get the most out of lab, you should not aim to do the minimum required of you in the shortest possible time. If you already have a good handle on the material, use the lab period to dive into further nooks and crannies or help out those peers who could benefit from your experience. You should also get to know the instructors. They are masters of the craft, and you will learn a lot by talking to them and asking them questions. Any topic is fair game.
The combination of hands-on experimentation, give and take with your peers, and the expert guidance of our staff is what makes lab time truly special. Your sincere participation can really accelerate your learning!
To prepare, please do the following before coming to lab:
- Install the necessary tools on your laptop:
- If you are using a Windows laptop, install a virtual machine running Linux following our VM install instructions. This virtual machine will have the arm cross-development tools already installed.
- If you are using a Mac laptop, follow our Mac install instructions to install the arm cross-development tools and console drivers. You do not have to install a virtual machine, since OS X is already based on UNIX.
- Make sure you have installed and learned how to use Git. (You should have completed this in Assignment 0.)
- Review our guide to the Unix command line.
You should come to lab with working versions of the cross-development
tools (all beginning with
arm-none-eabi-) and a running version of
git. Test your setup by opening a Terminal, typing the following commands, and confirm you get the responses shown below:
$ arm-none-eabi-as --version GNU assembler (GNU Tools for ARM Embedded Processors) 126.96.36.19950604 Copyright 2013 Free Software Foundation, Inc. This program is free software; you may redistribute it under the terms of the GNU General Public License version 3 or later. This program has absolutely no warranty. This assembler was configured for a target of `arm-none-eabi'. $ git --version git version 1.9.3
If this doesn’t work, don’t worry. We can help you resolve issues in lab.
Finally, ensure that you have completed the cs107e-specific setup from Assignment 0. Type
the following commands and confirm the output (note that your
may contain many other entries):
$ echo $PATH ...:/Users/student/cs107e_home/cs107e.github.io/cs107e/bin:... $ echo $CS107E /Users/student/cs107e_home/cs107e.github.io/cs107e
When start lab, pull up the check-in questions in your browser so you can refer to them as you go.
0. Clone the lab repo
All CS107e labs are distributed as git repositories. To get the lab materials, you clone the lab repo. You will have an individual repo for each lab.
To keep your work organized, we recommend that you store these lab repositories as subdirectories of the
that you created in Assignment 0. Change to that directory now:
$ cd cs107e_home
Now pull the latest courseware repo and clone the lab repo:
$ cd cs107e.github.io $ git pull $ cd .. $ git clone https://github.com/cs107e/lab1
You’ll repeat the above steps at the start of each lab to update your courseware repo and clone the lab.
1. Assemble blink
Having just cloned the lab above, change to the lab subdirectory that contains the
blink example and build the
blink program using these commands:
$ cd lab1/code/blink $ arm-none-eabi-as blink.s -o blink.o $ arm-none-eabi-objcopy blink.o -O binary blink.bin
If this works, you are good to go!
2. Inventory your kit
You will receive your CS107e Raspberry Pi kit when you arrive at lab. Take a moment to identify all your parts and compare to the kit inventory to ensure your kit is complete.
3. Power your breadboard
Next you will wire up a simple circuit on your breadboard to light an LED. Be sure you understand how breadboards work before you begin. Here is a short explanation of how to use a breadboard that may be a useful review. What holes are connected to what holes? How are the power and ground rails connected?
Note that an LED is directional. The longer lead is the anode and the shorter lead is the cathode. The voltage from anode to the cathode should be positive. If the polarity of voltages are switched, the LED will not light up. A LED also needs a 1K current limiting resistor otherwise it can literally blow up in a fiery, smoky extravaganza!
In the photo below of our circuit, we connected the cathode of the LED to the 1K resistor and then connected the other end of the resistor to the blue ground rail. Note how the LED crosses over the middle of the breadboard. To light up the LED, we need to apply power to the anode and complete the circuit by connecting the cathode to GND.
To check that the LED is working, you need to power the circuit. We will draw power from your laptop using a USB to Serial Adapter (hereafter referred to as just “USB-serial”). This is the small black breakout board with a USB-A connector on one side and a 6-pin header on the other side. The USB connector is inserted into a USB-A port on your laptop. If your laptop does not have a USB-A port, you will need an adapter.
When wiring, electronics gurus use colored wires to indicate what type of signal is being carried by that wire. This makes debugging tangled wires much easier. Generally, we will use the following conventions.
- Black (GND)
- Red (5V)
- Orange (3.3V)
- Blue (host output)
- Green (host input)
In this next step, we choose red and black jumpers because we are routing power and ground.
To provide power to your breadboard, do the following steps in precisely this order.
Pick out two female-male jumper cables, one red and one black.
Connect the female ends of the jumpers to the header pins on the USB-serial breakout board. Connect the black jumper to the header labeled GND and the red jumper to the header labeled 5V.
Connect the male ends of the jumpers to the breadboard. Plug the male end of the black jumper into the blue ground rail. Plug the male end of the red jumper to the LED anode (longer leg). Remember to include the 1k resistor in the circuit between the LED cathode (shorter leg) and GND.
Plug the USB connector of the USB-serial into your laptop. A small led on the breakout board lights up to indicate that it has power. The LED on the breadboard connected to the red jumper should also be lit.
While the LED is lit, make the following measurements with the multimeter.
- Measure and record the voltage across the resistor.
- Measure and record the voltage across the LED.
Calculate the current flowing through the LED. You should now be able to answer the first check in question.
4. Power via the Pi
Identify the 40-pin GPIO header on the Raspberry Pi A+ board and orient it to match the pinout diagram (image shown below or use the postcard from your kit or poster on lab wall). The labels in the diagram identify a pin’s purpose, such as GND, power, or its GPIO number. Inspect the diagram and identify two pins labeled as 5V power and two pins labeled GND; you’ll use these pins in this step.
You will re-wire your circuit to run power/ground from the USB-serial first to the Raspberry Pi and from there to the breadboard. Follow these steps:
- Unplug the USB-serial from your laptop so that no power is flowing. Disconnect the jumpers between the USB-serial and breadboard.
- Connect power and ground from the USB-serial to the Raspberry Pi using two female-female jumpers. Use a black jumper to connect the GND of the USB-serial to a GND GPIO on the Pi. Similarly connect a red jumper for the 5V power.
- Connect power and ground from the Raspberry Pi to the breadboard using the two female-male jumpers. The black jumper connects a GND GPIO to the blue ground rail on the breadboard. The red jumper connects a 5V GPIO to the LED anode.
Power is now flowing from the USB-serial to the Raspberry Pi and then to the breadboard.
After finishing wiring things up,
plug the USB-serial in your laptop. All three LEDs should light: the one on the USB-serial, the red power LED on the Raspberry Pi, and the
the LED on the breadboard. Your circuit is complete!
Replace your 1K resistor with a 10K resistor. How does the brightness of the LED change? (You might want to coordinate with another group so you can compare them side by side.) Why does it change?
5. Use the SD card
Your Raspberry Pi kit contains a microSDHC card (shown in the photo below left). This microSDHC card is inserted in the card holder on the bottom of the Pi (shown in the photo below right). The contents of the card determine what program the Pi runs.
Many laptops contain a SD card slot. To copy software to the microSDHC card, you need to mount it on your laptop. To do this, insert the microSDHC card into the SD card holder, and then insert the holder into your laptop’s SD card slot.
When you insert the SD card it should mount automatically. You should see it show up in your file explorer.
Another way to confirm that the card is mounted is to list the mounted Volumes. If you’re on a Mac:
$ ls /Volumes Macintosh HD NO NAME
By default, the SD card volume is named
You can change the name if you wish.
Now, we’ll obtain the Raspberry Pi firmware from the courseware repository. Right now, your current directory might be the
blink folder. Change
your shell’s current directory to the firmware folder.
$ pwd /Users/[USERNAME]/cs107e_home/lab1/code/blink $ cd /Users/[USERNAME]/cs107e_home/cs107e.github.io/firmware
(The part of that path before
cs107e_home might be different depending on
where you initially created your directory.)
There should be 4 files in the
$ ls blink-actled.bin bootloader.bin bootcode.bin start.elf
bootcode.bin is the code that boots the GPU,
start.elf is the GPU startup code. The two additional files
bootloader.bin are programs.
The SD card will need an additional file named
kernel.img is the operating system kernel you want to run
on the Pi, like Linux or Windows. But notice that we don’t give you a
kernel.img! In this course, we will write our own program to take the place of the kernel, and put our program under the name
We choose to first run the
blink-actled.bin. This program blinks the Pi’s activity LED. The green activity LED on the Raspberry Pi board next to the red power LED.
Follow these steps in order:
Copy the four files from the firmware folder onto the SD card. (You can use either the Terminal or the Finder for this.)
On the SD card, make a copy of
Confirm that your SD card has the following files:
$ ls blink-actled.bin bootloader.bin start.elf bootcode.bin kernel.img
bootcode.binto boot the GPU
start.elfto start up the GPU
kernel.imgbinary which the Pi runs once powered up
blink-actled.binthe program to blink the activity led
bootloader.binthe program you will use later in this lab
Note that only the first 3 files are required.
bootloader.binare ignored by the Pi.
Eject the SD card. If the Terminal prevents you from ejecting, type in
cd ..to move to the parent folder and try ejecting again.
Insert the microSDHC card into the slot on the bottom side of the Raspberry Pi board.
Power it up. The Pi’s on-board activity (ACT) LED should start blinking. Ta-da! 🎉
Keep this procedure in the back of your mind. If you ever think your Pi is not working because of a hardware problem, repeat these steps. If the ACT LED doesn’t blink after booting, then something is wrong and you may need to replace the Pi with a working one.
6. Blink breadboard LED
Next, we are going to use the
blink program (which pulses GPIO 20) in place of
blink-actled (which pulses the on-board ACT LED at GPIO 47). Start by re-wiring your circuit. Use the Pi pinout diagram to identify GPIO 20 and connect it to the anode of the LED on the breadboard.
Next, update the files on your SD card. Remove the microSDHC from your Pi, and mount it again on your laptop.
kernel.img file from the SD card. Copy your
blink.bin file (the one you assembled in step 1 of this lab)
to your SD card and rename it
Eject the SD card and insert it into the Raspberry Pi.
When you boot your Pi, the blink program should now run and blink the LED on your breadboard.
7. A better way: bootloader
Each time you change your code, you could repeat this process.
This would involve
powering down your Pi, ejecting the SD card,
inserting the SD card into your laptop,
copying the new version of your code to
unmounting and ejecting the SD card from your laptop,
inserting it into the Pi,
and then powering it up.
This quickly becomes tedious.
Even worse, the SD connectors are only designed to withstand
around 1000 insertions and deletions, after which they start to fail.
Instead, we will use a bootloader. The bootloader is a program that runs on the Pi and listens on the serial port for commands and data coming from a connected computer. On your laptop, you run a script to send your compiled program over the serial port to the waiting bootloader. The bootloader receives the program and writes it to the memory of the Pi, a process called “loading” the program. After the program is loaded, the bootloader jumps to the start address of the program, and the program begins to run. To stop that program and start another, you will reset the Pi and use the bootloader again. This is much more convenient way to run your newly compiled program than all that shuffling of SD cards. You will learn to love the bootloader!
First install the bootloader onto your SD card:
Mount the SD card and make a copy of
bootloader.binon the SD card. Name it
kernel.img, replacing the program you had there before.
Eject the SD card and insert it into the Raspberry Pi. The next (and every subsequent) time that you reset the Pi with that micro-SD card installed, the bootloader will run.
To use the bootloader, you must set up the communication channel between your computer and the Pi. The USB-serial that you are using to power your Pi also contains pins that can be used as a serial communication line.
The 6-pin header at the end of the USB-serial breakout board has two pins labeled for transmitting (TX) and receiving (RX). The Pi also has a TX and RX pin (GPIO pins 14 and 15, respectively). Use the Raspberry Pi pinout diagram to find these pins on the GPIO header.
Pick out two more female-female jumpers, one blue and one green. Use the blue jumper to connect the TX on the USB-serial to the RX on the Pi, and the green jumper to connect RX on the USB-serial to the TX on the Pi. As always, first unplug the USB-serial from your computer before fiddling with your wiring.
The proper connections are shown below. Note that your USB-serial may have pins in different positions. Confirm that your connections match the labels on your USB-serial. Don’t just follow the picture blindly!
In the above photo, the green wire connects the RX header pin on the USB-serial to the Pi’s TX pin (GPIO 14). The blue wire connects the TX header pin on the USB-serial to the Pi’s RX pin (GPIO 15).
Plug in your USB-serial to reset your Pi. The bootloader should run on reset. When the bootloader is running, it signals that it is waiting to receive a program by repeatedly giving two short flashes of the ACT LED (the green LED on the Pi board). This “da-dum” is the heartbeat that tells you the bootloader is ready and listening. Look at your Pi now and observe this rhythm. Breathe in sequence with it for a moment to celebrate having achieved bootloader enlightenment.
We wrote the Python script
rpi-install.py that runs on your computer and sends a program to the bootloader. Verify you have this script installed by typing the following command and confirm the expected output:
$ rpi-install.py -h usage: rpi-install.py [-h] [-v] [-q] [-t T] [-p | -s] [port] file This script sends a binary file to the Raspberry Pi bootloader. Version 1.0. ...
Note: Verify that rpi-install.py is Version 1.0, if not, that means you are not running the correct version! Please ask a staff member to help resolve. Note: If you are on a Mac, make sure you installed the CP2102 and serial drivers as described in the Mac install instructions. On Windows or Linux, you don’t need to do anything special here.
Let’s try bootloading a program. On your computer, change back to the
directory where you assembled
blink.bin in step 1.
Confirm that your Pi is showing the ACT LED heartbeat. To load and run
blink.bin, simply type:
$ rpi-install.py blink.bin Found serial port: /dev/cu.SLAB_USBtoUART Sending `blink.bin` (72 bytes): . Successfully sent!
The ACT LED turns on steady on while loading the program, then goes off. At this point, the blink program takes over the Pi and will now blink the LED on your breadboard.
If you change your program and wish to reload it onto the Pi, you must
power cycle the Pi. Why can’t you just run
after the bootloader has already loaded a program?
One way to power cycle the Pi is to unplug the USB-serial from the USB port on your laptop, and then plug it in again. The Pi will reset into the bootloader, ready to receive the new version of the program.
Retype the above
and the new version will be sent to the Pi and run.
Hoorah, hoorah, hoorah!! 👏
8. Study the blink program
.equ DELAY, 0x3F0000 // configure GPIO 20 for output ldr r0, FSEL2 mov r1, #1 str r1, [r0] mov r1, #(1<<20) loop: // set GPIO 20 high ldr r0, SET0 str r1, [r0] // delay mov r2, #DELAY wait1: subs r2, #1 bne wait1 // set GPIO 20 low ldr r0, CLR0 str r1, [r0] // delay mov r2, #DELAY wait2: subs r2, #1 bne wait2 b loop FSEL0: .word 0x20200000 FSEL1: .word 0x20200004 FSEL2: .word 0x20200008 SET0: .word 0x2020001C SET1: .word 0x20200020 CLR0: .word 0x20200028 CLR1: .word 0x2020002C
If there is anything you don’t understand about this program, ask questions of your partner and others.
Do the following exercises:
Look at the bytes in the
blink.binyou assembled earlier by running
xxd -g 1 blink.binat a shell in the
xxdis a command that prints the bytes in a file in a human-readable form. You can run
man xxdto learn more. What are the numbers at the beginning of each line
Find the first occurrence of
e3. What is the byte offset of
e3relative to the start of the file?
Change the program such that the blink rate slows down by a factor of 2.
Note that changing the program is a multi-step process. First you edit
blink.sin a text editor, then go through the commands in step 1 again to build
blink.binfrom it, and then finally unplug and replug the Pi and run
rpi-install.pyagain to run your new
blink.bin. Make sure you understand why these steps are all necessary.
Now perform experiments to determine how many instructions per second the Raspberry Pi executes.
9. Add a button! (optional)
This last part is optional. You do not need to use buttons for Assignment 1, but you will for Assignment 2. There are no check-in questions for this exercise.
Measure the resistance across the pushbutton legs using a multimeter and figure out which pins are always connected and which become connected when the button is pushed. Use your observations to determine how to position the button correctly on the breadboard. The pushbutton needs a 10K pull-up resistor to the red power rail. Verify that the resistor is 10K Ohms using the multimeter. Measure the voltage at the pin, and measure it again when you push the button.
Here is a program that reads a button and turns on or off the LED depending on whether the button is pressed.
// configure GPIO 10 for input ldr r0, FSEL1 mov r1, #0 str r1, [r0] // configure GPIO 20 for output ldr r0, FSEL2 mov r1, #1 str r1, [r0] // bit 10 mov r2, #(1<<10) // bit 20 mov r3, #(1<<20) loop: // read GPIO 10 ldr r0, LEV0 ldr r1, [r0] tst r1, r2 beq on // when the button is pressed (goes LOW), turn on LED // set GPIO 20 low off: ldr r0, CLR0 str r3, [r0] b loop // set GPIO 20 high on: ldr r0, SET0 str r3, [r0] b loop FSEL0: .word 0x20200000 FSEL1: .word 0x20200004 FSEL2: .word 0x20200008 SET0: .word 0x2020001C SET1: .word 0x20200020 CLR0: .word 0x20200028 CLR1: .word 0x2020002C LEV0: .word 0x20200034 LEV1: .word 0x20200038
To run this program, connect the button to GPIO 10. Make sure the jumper is connected to the correct pin on the Raspberry Pi. Also, make sure the pull-up resistor is properly installed on the breadboard.
Challenge yourself to understand what each line of code accomplishes and why it works as expected. Feel free to add your own code annotations if that helps.
Here are a few questions to test your knowledge. To answer these questions you will have to read the Broadcom peripheral manual, or ask someone who knows the answer.
What does the peripheral register with the address 0x20200034 return?
Why does the input value go to 0 (LOW) when the button is pressed?
How does the Pi know which branch to jump to when it reaches
Check in with TA
Before leaving lab, go over your check-in questions with a TA. The TA will verify your understanding and can answer any unresolved questions you have.
Note, again, that the goal of the lab is not to answer exactly and only these questions – it’s to work through the material. The questions are an opportunity to self-test your understanding and confirm with us.
It’s okay if you don’t completely finish all of the exercises during lab; your sincere participation for the full lab period is sufficient for credit. However, if you don’t finish, we highly encourage you to work those parts to solidify your knowledge of this material before moving on. In particular, having successfully completed this lab is a necessary step before tackling this week’s assignment.