This repo serves as a collection of low level examples.  No operating
system, embedded or low level embedded or deeply embedded or bare metal,
whatever your term is for this.

I am in no way shape or form associated with the raspberry pi organization
nor broadcom.  I just happen to own one (some) and am sharing my
experiences.  The raspberry pi is about education, and I feel low
level education is just as important as Python programming.

From what we know so far there is a gpu on chip which:

1) boots off of an on chip rom of some sort
2) reads the sd card and looks for additional gpu specific boot files
bootcode.bin and start.elf in the root dir of the first partition
(fat32 formatted, loader.bin no longer used/required)
3) in the same dir it looks for config.txt which you can do things like
change the arm speed from the default 700MHz, change the address where
to load kernel.img, and many others
4) it reads kernel.img the arm boot binary file and copies it to memory
5) releases reset on the arm such that it runs from the address where
the kernel.img data was written

The memory is split between the GPU and the ARM, I believe the default
is to split the memory in half.  And there are ways to change that
split (to give the ARM more).  Not going to worry about that here.

From the ARMs perspective the kernel.img file is loaded, by default,
to address 0x8000.  (there are ways to change that, not going to worry
about that right now).

Hardware and programming information:

You will want to go here
http://elinux.org/RPi_Hardware
And get the datasheet for the part
http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
(might be an old link, find the one on the wiki page)
And the schematic for the board
http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspberry-Pi-Schematics-R1.0.pdf
(might be an old link, find the one on the wiki page)

Early in the BCM2835 document you see a memory map.  I am going to
operate based on the middle map, this is how the ARM comes up.  The
left side is the system which we dont have direct access to in that
form, the gpu probably, not the ARM.  The ARM comes up with a memory
space that is basically 0x40000000 bytes in size as it mentions in
the middle chart.  The bottom of this picture shows total system
sdram (memory) and somewhere between zero and the top of ram is a
split between sdram for the ARM on the bottom and a chunk of that
for the VC SDRAM, basically memory for the gpu and memory shared
between the ARM and GPU to allow the ARM to ask the GPU to draw stuff
on the video screen.  256MBytes is 0x10000000, and 512MBytes is
0x20000000.  Some models of raspberry pi have 256MB, newer models have
512MB total ram which is split between the GPU and the ARM.  Assume
the ARM gets at least half of this.  Peripherals (uart, gpio, etc)
are mapped into arm address space at 0x20000000.  When you see
0x7Exxxxxx in the manual replace that with 0x20xxxxxx as your ARM
physical address.  Experimentally I have seen the memory repeats every
0x40000000, read 0x40008000 and you see the data from 0x8000.  From the
Broadcom doc this looks to be giving us access to the memory with
different caching schemes (cached, uncached, etc) depending on which
upper address bits you use.

I do not normally zero out .bss or use .data so if you do this to my examples

int x;
fun()
{
  static int y;
}

dont assume x and y are zero when your program starts. Nor if you do this

int x=5;
fun()
{
  static int y=7;
}

will x=5 or y=7.  See the bssdata directory for more information.

Nor do I use gcc libraries nor C libraries so you can build most if not
all of my examples using a gcc cross compiler.  Basically it doesnt
matter if you use arm-none-linux-gnueabi or arm-none-eabi.  What was
formerly codesourcery.com still has a LITE version of their toolchain
which is easy to come by, easy to install and well maybe not easy to use
but you can use it.  Building your own toolchain from gnu sources (binutils
and gcc) is fairly straight forward see my build_gcc repository for a
build script.

As far as we know so far the Raspberry Pi is not "brickable".  Normally
what brickable means is the processor relies on a boot flash and with
that flash it is possible to change/erase it such that the processor will
not boot normally.  Brickable and bricked sometimes excludes things
like jtag or special programming headers.  From the customers perspective
a bricked board is...bricked.  But on return to the vendor they may
have other equipment that is able to recover the board without doing
any soldering, perhaps plugging in jtag or some other cable on pins/pads
they have declared as reserved.  Take apart a tv remote control or
calculator, etc and you may see some holes or pads on the circuit board,
for some of these devices just opening the battery case you have a view
of some of the pcboard.  This is no doubt a programming header.  Long
story short, so far as I know the Raspberry Pi is not brickable because
the rom/flash that performs the initial boot is for the gpu and we dont
have access to the gpu nor its boot rom/flash.  The gpu relies on the
sd card to complete the boot, so there is something in hardware or
perhaps there is an on chip flash for the gpu, from there on it is all
sd card.  It is very easy for the customer to remove and
replace/modify that boot flash.  So from a software perspective
unless you/we accidentally figure out how to change/erase the gpu boot
code (my guess is it is a one time programmable) you cant brick it.

To use my samples you do not need a huge sd card.  Nor do you need nor
want to download one of the linux images, takes a while to download,
takes a bigger sd card to use, and takes forever to write to the sd card.
I use the firmware from http://github.com/raspberrypi.  The minimum
configuration you need to get started at this level is:

go to http://github.com/raspberrypi, you DO NOT need to download
the repo, they have some large files in there you will not need (for
my examples).  go to the firmware directory and then the boot directory.
For each of these files, bootcode.bin and start.elf (NOT kernel.img,
dont need it, too big)(loader.bin is no longer used/required).  Click
on the file name, it will go to another page then click on View Raw and
it will let you download the file.  For reference, I do not use nor
have a config.txt file on my sd card.  I only have the three files (
bootcode.bin, start.elf, and then kernel.img from one of my examples).

My examples are basically the kernel.img file.  Not a linux kernel,
just bare metal programs.  Since the GPU bootloader is looking for
that file name, you use that file name.  The kernel.img file is just a
blob that is copied to memory, nothing more.

What I do is setup the sd card with a single partition, fat32.  And
copy the above files in the root directory.  bootcode.bin and start.elf.
From there you take .bin files from my examples and place them on the sd
card with the name kernel.img.  It wont take you very long to figure out
this is going to get painful.

1) power off raspi
2) remove sd card
3) insert sd card in reader
4) plug reader into computer
5) mount/wait
6) copy binary file to kernel.img
7) sync/wait
8) unmount
9) insert sd card in raspi
10) power raspi
11) repeat

There are ways to avoid this, one is jtag, which is not as expensive
as it used to be.  It used to be in the thousands of dollars, now it
is under $50 and the software tools are free.  Now the raspi does have
jtag on the arm, getting the jtag connected requires soldering on some
models, but not on the newer models.  I do not yet have a newer model.
How to use the jtag and how to hook it up is described later and in
the armjtag sample.

Another method is a bootloader, typically you use a serial port connected
to the target processor.  That processor boots a bootloader program that
in some way, shape, or form allows you to interact with the bootloader
and load programs into memory (that the bootloader is not using itself)
and then the bootloader branches/jumps to your program.  If your program
is still being debugged and you want to try again, you reboot the processor
the bootloader comes up and you can try again without having to move any
sd cards, etc.  The sd card dance above is now replaced with the
bootloader dance:

1) power off raspi
2) power on raspi
3) type command to load and start new program

I have working bootloader examples.  bootloader05 is the currently
recommended version.  But you need more hardware (no soldering is
required).  For those old enough to know what a serial port is, you
CANNOT connect your raspberry pi directly to this port, you will fry
the raspberry pi.  You need some sort of serial port at 3.3V either
a level shifter of some sort (transceiver like a MAX232) or a usb
serial port where the signals are 3.3V (dont need to use RS232 just
stay at the logic level).  The solution I recommend is a non-solder
solution:

A recent purchase, experimentally white is RX and green is TX, black GND
http://www.nexuscyber.com/usb-to-ttl-serial-debug-console-cable-for-raspberry-pi

http://www.sparkfun.com/products/9873
plus some male/female wire
http://www.sparkfun.com/products/9140

Solutions that may involve soldering
http://www.sparkfun.com/products/718
http://www.sparkfun.com/products/8430

Or this for level shifting to a real com port.
http://www.sparkfun.com/products/449

Or see the pitopi (pi to pi) directory.  This talks about how to take
two raspberry pi's and connect them together.  One being the
host/development platform (a raspberry pi running linux is a native
arm development platform, no need to find/get/build a cross compiler)
the other being the target that runs your bare metal programs.

Lastly and perhaps the best solution IMO, is the FT4232H or FT2232H
mini module from FTDI.  It gives you UART and JTAG for under $30.
See the armjtag directory README for more and you will want some
female/female wire from sparkfun or adafruit or elsewhere
https://www.sparkfun.com/products/8430
(I use these F/F wires for most projects, buy/bought the 100 pack)

---- connecting to the uart pins ----

On the raspberry pi, the connector with two rows of a bunch of pins is
P1.  Starting at that corner of the board, the outside corner pin is
pin 2.  From pin 2 heading toward the yellow rca connector the pins
are 2, 4, 6, 8, 10.  Pin 6 connect to gnd on the usb to serial board
pin 8 is TX out of the raspi connect that to RX on the usb to serial
board.  pin 10 is RX into the raspi, connect that to TX on the usb to
serial board.  Careful not to have metal items on the usb to serial
board touch metal items on the raspberry pi (other than the three
connections described).  On your host computer you will want to use
some sort of dumb terminal program, minicom, putty, etc.  Set the
serial port (usb to serial board) to 115200 baud, 8 data bits, no
parity, 1 stop bit.  NO flow control.  With minicom to get no flow
control you normally have to save the config, exit minicom, then
restart minicom and load the config in order for flow control
changes to take effect.  Once you have a saved config you dont have
to mess with it any more.

2  outer corner
4
6  ground
8  TX out
10 RX in

ground is not necessarily needed if both the raspberry pi and the
usb to serial board are powered by the same computer (I recommend
you do that) as they will ideally have the same ground.

Read more about the bootloaders in their local README files.  Likewise
if you interested in jtag see the armjatag README file.  Other than
chewing up a few more GPIO pins, and another thing you have to buy, the
jtag solution is the most powerful and useful.  My typical setup is the
armjtag binary as kernel.img, a usb to jtag board like the amontec
jtag-tiny and a usb to serial using minicom.

As far as these samples go I recommend starting with blinker01 then
follow the discovery of the chip into uart01, etc.  I took one path
with the first bootloader then switched gears to use xmodem, if
interested at all you may wish to just skip to that one.  It has no
features and isnt even robust, quick and dirty, and most of the time
it works just fine, if not power cycle the raspberry pi and try again.
(power cycle = unplug then plug back in)

The bssdata and baremetal directories attempt to explain a little
bit about taking control of the gnu toolchain to build bare metal
programs like these examples.  As with any bare metal programmer I have
my ways of doing things and these two directories hopefully will show
you some basics, get you thinking about how these tools really work,
take the fear away from using them, as well as some comments on why
I take the approach I take (not worrying about .bss nor .data).  Since
the raspberry pi is from our perspective RAM based (the GPU loads our
whole binary into memory), we dont have to deal with the things we
would deal with on a FLASH/PROM + RAM system.  This RAM only approach
makes life a lot easier, but leaves out some important bare metal
experiences that you will have to find elsewhere.

-----------

Sources for ARM ARMs

Google
  ARM DDI 0100E
or
  ARM DDI 0100I

https://www.scss.tcd.ie/~waldroj/3d1/arm_arm.pdf
http://morrow.ece.wisc.edu/ECE353/arm_reference/ddi0100e_arm_arm.pdf
http://reds.heig-vd.ch/share/cours/aro/ARM_Thumb_instructions.pdf
They have a rev B here...Which was the blue covered one in print.  This
was the I learned from (the print version) later I got a rev a, then
rev e all in print then it was only electronic from there out as far as
I know.  the older ones being more pure, but also some notable bugs
in the docs, instruction encodings and some other things.
http://www.home.marutan.net/arcemdocs/