ARM Beagleboard
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TODO: VERIFY THE TUTORIAL(S)!!!
Intro
This is a tutorial on bare-metal [OS] development on the Texas Instruments BeagleBoard. This tutorial is written specifically for the BeagleBoard-xM Rev C because the author has no other hardware to test on.
Experience in Linux/nix (very important) and at least one assembly language (incredibly important, or at least extensive C knowledge) is assumed and required. Please make sure you can figure out how to get a basic x86 OS (no copypaste!) to the booting stage before attempting this guide. No need to make much more than a "Hello World", since what you know about x86 won't really apply here.
Materials
You will need a:
- BeagleBoard-xM Rev C, or perhaps an older board, with a serial port.
- An RS-232 serial port. This is the kind of serial port on the back of your x86 PC, and on the BeagleBoard-xM. using TTL (5v) or lower voltage serial will damage something.
- A serial cable that comes to DB-9 Male on at least one side (to connect to the BeagleBoard)
- Debian, or other *nix with arm-binutils [guide will document installation for Debian]
- Power supply for your BeagleBoard (5v, at least 1A but I recommend/use 3A)
- A copy of the ARM ARM (ARM Architecture Reference Manual) (download a PDF, make sure it's the new one, with ARMv7 stuff)
- A copy of the OMAP35x/DM37x TRM (Technical Reference Manual) <-- ^ THESE ARE INCREDIBLY IMPORTANT!
- Optional: Various peripherals for the BeagleBoard. Be aware that you will need to write drivers for them. Welcome to OS Development ^_^.
Getting Started
Testing your serial port
First things first, you're going to want to make sure all your hardware works. Set up your serial port, however yours works, and open up minicom. Make sure you have flow control turned off. Now jumper PIN 2 (RX) to PIN 3 (TX) on the DB-9 side. Just type some characters into minicom, and they should be echoed back. Ensure you can run at 115200 baud, 8N1, which is what you will use to connect to the BeagleBoard.
If you get 'Permission Denied' do NOT become root! This is unnecessary. Instead do:
sudo adduser <user> dialout
This will let your user use serial ports without needing root.
Or do ls -l /dev/ttyS* to find out the group that own the device, then add you into that group under /etc/group (normally the group is uucp)
Testing the BeagleBoard
Follow the guide here http://elinux.org/BeagleBoardDebian#Debian_armhf_port to install the Debian armhf image to a microSD card. We don't really need to boot Debian from it, but it's a nice simple ghetto/lazy way to get u-boot set up and working.
Connect the BeagleBoard up via serial, and make sure Minicom is running (on your PC). Put the microSD card in the BeagleBoard. Connect the BeagleBoard to its power supply. You should see some output on the serial port. If not, Google around to figure out why not. If a few bytes of garbage appear in Minicom only, make sure the microSD card is inserted and VALID. Also make sure you didn't press the 'user button'.
You should get some meaningful output. Unplug your BeagleBoard when you are done with it.
Getting an assembler
We will be using ARM development tools from the Emdebian project.
Append:
deb http://www.emdebian.org/debian/ squeeze main
to your /etc/apt/sources.list, changing 'squeeze' for your version of Debian - I'm running wheezy, for example.
now run:
sudo aptitude update
sudo aptitude install gcc-4.4-arm-linux-gnueabi # check this! it might be an older version for non-wheezy...
That will install a C compiler, and also install Binutils, which for this tutorial is really all we need, but you may later wish to write your code in C.
Writing some CODE!
Now that we have all the stuff we need, let's get to writing some code. Before we write an ASM source file, we should make a linker script and a Makefile.
Linker Script
We'll basically take the one from the stackoverflow.com link, No.1 in the ref list, with one change. The linker script there uses 0x80300000 as the start address. This will work just fine, however the default load address of u-boot's 'loady' command loads things into 0x80200000. So there's one less thing we have to specify ^_^.
linker.ld
Take note of what it does. It's pretty simple. Refer to docs if necessary.
/* rammap */
MEMORY
{
ram : ORIGIN = 0x80200000, LENGTH = 0x10000
}
SECTIONS
{
.text : { *(.text*) } > ram
}
Makefile
This is the basic makefile I use. It's similar to the one on the stackoverflow link.
ARMGNU = arm-linux-gnueabi
AOPS = --warn --fatal-warnings
COPS = -Wall -Werror -O2 -nostdlib -nostartfiles -ffreestanding
boot.bin: boot.asm
$(ARMGNU)-as boot.asm -o boot.o
$(ARMGNU)-ld -T linker.ld boot.o -o boot.elf
$(ARMGNU)-objdump -D boot.elf > boot.list
$(ARMGNU)-objcopy boot.elf -O srec boot.srec
$(ARMGNU)-objcopy boot.elf -O binary boot.bin
Assembly!
YAY! We can write some code now!
You are free to branch to C code after following this tutorial, but remember, you will need to write some things in ASM, just like on x86.
boot.asm
#UART base locations from the TRM
.equ UART1.BASE, 0x4806A000
.equ UART2.BASE, 0x4806C000
#According to the Beagleboard-xM System Reference Manual, UART3 is connected to the serial port.
.equ UART3.BASE, 0x49020000
.equ UART4.BASE, 0x49042000
#We need to be in ARM mode - we may branch to Thumb mode later if desired.
.arm
_start:
# Thankfully, U-BOOT already sets up serial for us, so we don't need to do all the reset crap.
# Just print a '!' symbol to the screen, and hang.
# Load the location of the transmit register (THR_REG) into r0. It's at the UARTx.BASE, since it is offset 0x000.
# (load value pointed by the assembler (immediate pointer) into r0)
ldr r0,=UART3.BASE
# Move a '!' character into r1
# (move a character, '!', which is immediate, into r1)
mov r1,#'!'
# According to the TRM, we may only write bytes into THR_REG, else we'll corrupt stuff.
# (store the least-significant-byte of r1 into the address pointed by r0)
strb r1,[r0]
# If we kept writing to the serial port, we'd eventually overflow the 64-byte FIFO, and since we don't handle interrupts yet, we'll hang (?)
# In ASM, labels are like case: statements in C. Code flows into them as if they don't exist - because they don't.
_hang:
# (branch (jump, JMP) to _hang)
# b _hang
'Compiling' it
Since it's ASM we technically don't 'compile' it, we 'assemble' (and 'link' it). But anyway, go to where you saved your files and type 'make'. This should progress without any errors.
Executing code on the device
Remember that Minicom window we almost forgot about? It's time to use it ;).
Introduction to u-boot
Reset your BeagleBoard (hit the reset button).
You should see:
U-Boot 2011.03-rc1-00000-g9a3cc57-dirty (Apr 04 2011 - 12:36:16)
OMAP36XX/37XX-GP ES2.1, CPU-OPP2, L3-165MHz, Max CPU Clock 1 Ghz
OMAP3 Beagle board + LPDDR/NAND
I2C: ready
DRAM: 512 MiB
NAND: 0 MiB
MMC: OMAP SD/MMC: 0
*** Warning - readenv() failed, using default environment
In: serial
Out: serial
Err: serial
Beagle xM Rev C
No EEPROM on expansion board
Die ID #2e7000029ff80000016842c813020023
Hit any key to stop autoboot: 3
Hit a key. Before Linux loads.
OMAP3 beagleboard.org #
Whee, a u-boot prompt.
Loading code
Now it's time to load our code into RAM. To do this, we will use 'loady' and the y-modem protocol. We can do all this from Minicom - that's why we needed Minicom and not GNU Screen. Hopefully you did what I asked ;).
Just type 'loady'. Don't worry about being fast on your first go, you'll miss it anyway.
OMAP3 beagleboard.org # loady
## Ready for binary (ymodem) download to 0x80200000 at 115200 bps...
Now hit Ctrl+A. A bar will appear at the bottom.
Press 'S'.
Select 'ymodem'.
Move across to '[Goto]'.
Type in the directory where you saved your code.
Move down to 'boot.bin' and hit 'Space', and then 'Enter'.
Ctrl+C out of the ymodem transfer window if nothing happens.
Reset your BeagleBoard and try again - now that the file is already selected, you can just hit enter twice in the file prompt window, preventing timeouts.
Be patient, ymodem can take a few seconds to 'sync'.
After the file is loaded into RAM,
READY: Press any key.
and
Cm - CRC mode, 9(SOH)/0(STX)/0(CAN) packets, 3 retries
## Total Size = 0x00000350 = 848 Bytes
OMAP3 beagleboard.org #
is printed (my file is bigger... don't worry about that), we just have to execute the code in memory!
Executing Code
Execution is started from an address with 'go'. Our _start address is 0x80200000. So, we type:
OMAP3 beagleboard.org # go 0x80200000
and we should get:
## Starting application at 0x80200000 ...
!
If you get that, congratulations! You've just written an 'Operating' System (;)) for the BeagleBoard! The process will seem long winded at first, but it's not really at all.
What to do now
Read the ARM ARM.
Read the TRM.
Find out how to do push/pop and how to return from functions (and how to return using a pop) from example code and the ARM.
Write routines to print integers and strings.
Set up virtual memory.
Enable interrupts
Write some device drivers.
Write a scheduler
etc. Do all the stuff you would in an x86 OS.
Branching to C
TODO: Add this
- Dependencies of TODO: Care enough to want to branch to C.
It's probably just a matter of linking in your C program and doing
bx _main
But I quite honestly haven't tried ^_^.
Help
For help, you can try joining #Beagle (note the capital B!) on irc.freenode.net.