Although a generic ATSAMD21 directory, starting specifically with
the

SparkFun SAMD21 Dev Breakout
https://www.sparkfun.com/products/13672

Which is based on the Atmel ATSAMD21G18, a 32-bit ARM Cortex-M0+
processor with 256KB flash, 32KB SRAM

Because I use jtag and because of how Atmel is doing things with this
family, you can use any SAMD21 board, doesnt have to be this sparkfun
board.

Historically the ARM based microcontrollers from Atmel contained a
factory bootloader that supported a protocol called SAM-BA (not to
be confused with the samba package for linux/unix).  A simple
serial bootloader.  Not completely giving that up, their current
solution is to use the main flash and provide source for a SAM-BA
compatible bootloader that you can if you choose place at the
begining of the flash (where the ARM boots) with some hardware support
to help protect that region from being erased.  The docs say this is
not factory programmed.

This board from sparkfun came programmed with something that blinked
the D13 led.  And implies the PA15 pin (D5 on the board) is used as
the bootloader strap.  Ground or pull up and it is supposed to go into
the bootloader which appears to be 115200.  But strapping it either
way I didnt see the bootloader respond, so I dont know what was really
on that board.  And really dont care at this point.  If there had been
a SAM-BA or other documented serial protocol, factory programmed and
protected, then that would be interesting.  So I am going to start with
jtag.

I have a number of ST boards that have an stlink frontend on the board
that you can use as a generic ARM SWD debugger.  There are a
couple of jumpers you remove to disconnect the stlink from the ST
mcu.  Now it depends on which stlink you have, or perhaps you have a
CMSIS-DAP compatible, or some other.  Adafruit has a standalone stlink
thumb drive looking thing that you can use as well, although actually
a number of the st mcu boards (nucleos and discovery boards) are cheaper
than this dongle thing and have a ton more stuff on them (a target
microcontroller for example).

I also prefer to use openocd as the jtag software, there are other
solutions, just like there are compilers other than gcc.  But openocd
is priced right and works well and a lot of folks know how to use it.

git clone git://repo.or.cz/openocd.git
cd openocd
./bootstrap
./configure --enable-stlink
(keep adding packages until this works, perhaps want the packages to
get CMSIS-DAP to work in case that is what you have or might get
some day)
make
sudo make install

The particular stm32f4 based NUCLEO board I am using has a connector
with these pins

VDD_TARGET
SWCLK
GND
SWDIO
NRST
SWO

I am only using

VDD_TARGET
SWCLK
GND
SWDIO

No matter what ARM SWD solution you use those are the four minimum
you will see (techically possible to not need VDD_TARGET, but usually
they design it to sense rather than assume 3.3V).  VDD_TARGET simply
means connect to VDD on the target or 3.3V on the sparkfun/samd21 board.
GND likewise to GND on the samd31 boards ground.  And swclk to swclk
and swdio to swdio.

In my case lsusb shows

Bus 013 Device 002: ID 0483:374b STMicroelectronics ST-LINK/V2.1 (Nucleo-F103RB)

which matches the stlink-v2-1.cfg file

hla_vid_pid 0x0483 0x374b

these cfg files are taken from openocd.  historically openocd periodically
changes the config files it uses so they can be a moving target, so I
prefer to copy/make my own and have less churn.

I have three different stlink interface config files, check the usb
vid/pid to match with whatever stlink you have if you have an stlink.

So while in the atsamd_samples/ATSAMD21 directory I start openocd with

openocd -f stlink-v2-1.cfg -f atsamd21.cfg

if it works it will for now end with this line

Info : at91samd.cpu: hardware has 4 breakpoints, 2 watchpoints

in another terminal window

telnet localhost 4444

and this ends with

Open On-Chip Debugger
>

if it works, the openocd server window shows there was a connection

Info : accepting 'telnet' connection on tcp/4444

(some folks like to use gdb or other software to connect to openocd
I dont do that, no interest, so dont have instructions for that,
look elsewhere)

In the telnet window you can halt the processor

Open On-Chip Debugger
> halt
at91samd.cpu: target state: halted
target halted due to debug-request, current mode: Handler HardFault
xPSR: 0x51000003 pc: 0xfffffffe msp: 0xffffffd8
>

the current mode state was because I had already erased this board
and the chip boots into an empty flash and faults.

And here is where you are fully committed.  Now if you got this far
you do have a jtag debugger you can use so you only lose the firmware
put on your SAMD21 by sparkfun or whomever.

> at91samd chip-erase
chip erased

And to confirm

> mdw 0 20
0x00000000: ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
0x00000020: ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
0x00000040: ffffffff ffffffff ffffffff ffffffff

erased.

You can power cycle/reset the board at this point and if needed restart
the openocd server and telnet into it.  The chip will now be in a known
state, if you dont then the chip is in the state left by whatever
firmware you had on it and running these examples may not work the
same (the clock may be faster for example changing the blink rate
of the blinker examples).

At this moment I have not solved how to reprogram the flash, you can
use openocd to download and run programs in flash.  There will be an
sram version of the examples.  Once I have a flash programming solution
then there will also be a flash version of the example.

A simple place to get pre-built gnu tools for building these examples

https://launchpad.net/gcc-arm-embedded

or I have my own build scripts that I use to compile a toolchain from
the gnu sources

https://github.com/dwelch67/build_gcc

To run the blinker01 example using openocd (other examples work the
same way just a different directory).  In yet another window navigate
to and build the blinker01.gcc.thumb.sram.elf binary.

I intentionally started openocd in that directory not just because
the cfg files were there but so that loading can use a path relative
to that directory

In the telnet window

> halt
> load_image blinker01/blinker01.gcc.thumb.sram.elf
144 bytes written at address 0x20000000
downloaded 144 bytes in 0.015099s (9.314 KiB/s)
> resume 0x20000000

Once you have typed those three commands you can up arrow three
times and press enter, repeat three times for each time you recompile
the binary to retry it.

Start with the blinker examples.



Schematic and Datasheet

I am specifically using a sparkfun board linked above, at the time of
this writing that page has both a schematic and the Atmel datasheet
you need.  The board you are using you would need to get the schematic
elsewhere.  For the datasheet for the part  go to atmel.com and then
for this specific part search for it ATSAMD21G18 and get the complete
datasheet.  These examples are created directly from the datasheet for
that part.  I have not done the research to see how similar the SAMD21
family is from a programming perspective.

The purpose of these examples is to roll your own, not to use a
vendor supplied generic library.  I am sure there are plenty of examples
out ther for those libraries.

http://infocenter.arm.com is the starting point for ARM documentation.
ARM doesnt make chips they make source code for processors that other
folks put in their chips, like Atmel for this product line.  As far as
the ARM world goes you want to determine what processor core is used
hopefully the chip vendors documentation indicates this.  For this chip
it is a cortex-m0+.  So under Cortex-M series processors on the
ARM link above.  Expand the cortex-M+ and go to the most recent revision
login (free to login just have to give up an email address) and get
the TRM (Technical Reference Manual).  Somewhere in there and/or on the
web page it tells you the Cortex-M0+ uses the arm architecture ARMv6-M.
So then you expand the ARM architecture tab and find the ARM ARM (ARM
Architectural Reference Manual) for the cortex-6m.  In here you will
find ARM core specific stuff like the systick timer for example.