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+# JTAG access
+## Overview
+As detailed on the Schematic (released in the FCC documents), the Nabaztag:tag board breaks out the JTAG port. Let's see how to use it to access the CPU to be able to flash and debug some code !
+## Hardware
+### JTAG probe: BusBlaster
+{{:dev:nab:v2:jtag:bbv3.jpg?200 |}}I think you can use any JTAG probe that's 3.3v compatible, if you have one. If you don't, I recommend the [BusBlaster v3](http://dangerousprototypes.com/docs/Bus_Blaster_v3_design_overview). It's a great board, versatile and cheap; oh ! and it's also open hardware and open source ! :-)
+On of the best features of the BusBlaster is you can reconfigure the CPLD to emulate any FT2232 JTAG probe. I think it comes pre-programmed with a JTAGkey buffer logic, which should work fine for this application.
+_Note_: My BusBlaster (a v4 !) uses a KT-link buffer logic, because I needed the SWD capabilities for another application. If the following steps don't work, try to reprogram the CPLD with a KT-link buffer and try again !
+You also need some jumper wires (also called _Dupont wire_ sometimes) F/F to connect the BusBlaster to the Nabaztag
+### Nabaztag:tag
+Well, of course you need a Nabaztag:tag ( a version 2 ). You also must be ready to open it.
+Grab a small flat screwdriver and remove the four screws on the base. You can also use a triangle screwdriver if you happen to have one, I don't.
+And this is about it. the JTAG header is on the top left corner of the board.
+Pinout is the following (top to bottom):
+^ Nabaztag Pin ^ Function ^ BusBlaster pin ^
+|  1           |  3.3v    |  VTG (or NC)   |
+|  2           |  Gnd     |  GND           |
+|  3           |  nTRST   |  TRST          |
+|  4           |  TDI     |  TDI           |
+|  5           |  TMS     |  TMS           |
+|  6           |  TCK     |  TCK           |
+|  7           |  TDO     |  TDO           |
+|  8           |  RESETN  |  TSRST         |
+## Software: OpenOCD
+### Patch and compile OpenOCD
+The Nabaztag:tag uses a ML67Q4051 micro-controller which is not on the supported hardware list of OpenOCD. It's an ARM7TDMI variant, so I thought it should be possible to use it. I spent some time hacking code and I eventually ended up with a patch good enough to:
+- access the CPU registers, which means debug is possible !
+- read and write the Internal flash
+  - First version was slow as hell: 5 to 10 minutes to read/write the whole flash
+  - Second version is much better: under 5 seconds. I had to write ARM assembler code to be run from RAM to write the Flash, which was a rather painful but interesting experience...
+So ! Download OpenOCD **0.8.0** from their website, or here (mirror): {{:dev:nab:v2:jtag:openocd-0.8.0.tar.gz|openocd 0.8.0}}
+Download the patch: {{:dev:nab:v2:jtag:openocd_0.8.0_oki.patch.gz|openocd_0.8.0_oki}}
+#### Prepare
+    sudo aptitude install libtool autoconf automake libusb-1.0-0-dev
+    gzip -d openocd_0.8.0_oki.patch.gz
+    tar xzvf ./openocd-0.8.0.tar.gz
+#### Patch
+    cd openocd-0.8.0/
+    patch -p1 < ../openocd_0.8.0_oki.patch
+#### Compile
+    autoreconf -fi
+    ./configure
+    make
+When it's done, you should have a binary named **openocd** in the `src/` folder
+### Usage
+Now that you have a patched version of OpenOCD capable of debugging the OKI chip, you need some configuration files. Here are mine: {{:dev:nab:v2:jtag:openocd_nab.tar.gz|openocd_nab}}
+_Note_: You might need to tweak them a little bit, especially the `interface/ftdi/dp_busblaster.cfg` to get you BusBlaster recognized by OpenOCD. I think I renamed my BusBlaster, and OpenOCD checks the name... Shoot me an email if ou need some help.
+Once you have the config files, just copy the openocd binary in the same folder and then it's as easy as:
+    ./openocd -f nabaztagv2.cfg
+## GDB
+I think you can use telnet to interact with OpenOCD, but I use GDB... Here are a few notes:
+### Launch GDB and connect
+    arm-none-eabi-gdb
+Once in GDB
+    tar ext :3333
+The Nabaztag should have "frozen". you can (always) restart it with
+    mon reset run
+### Dump the flash
+    mon reset init
+    dump ihex memory dump.hex 0x08000000 0x08020000 # Get an hex file, useful for analyzing it
+    dump binary memory dump.bin 0x08000000 0x08020000 # Get a bin file, useful for backup and restore
+_Note_: The files end up in the same folder as the openocd binary (or the folder from where you run it)
+### Write an image
+    mon reset init
+    mon flash write_image erase ./dump.bin 0x08000000
+_Note_: The files should be in the same folder as the openocd binary (or the folder from where you run it)
+## Pictures
+{{.:jtag:nabjtag.jpg?250| BusBlaster connected to the Nabaztag, side view}} {{.:jtag:nabjtag_bottom.jpg?250| BusBlaster connected to the Nabaztag, bottom view}}
+## Links