.. _jtag:

======
 JTAG
======

JTAG is an interface for low-level debugging of digital devices. It
gives instruction by instruction control over the microprocessor and
allows data to be read and written to arbitrary memory and register
locations. It is typically used with a debugging tool like `gdb
<http://www.gnu.org/software/gdb/>`_ when hacking low level routines
and hardware peripherals (we use it when working on :ref:`libmaple
<libmaple>`) or to flash a new bootloader.

Note that the STM32 on the Maple has a built-in low level serial
debugger which could also be used to flash bootloaders, and that the
:ref:`ASSERT <language-assert>` framework allows basic debugging over
a USART serial channel.  We expect only advanced users to use this
feature.

.. contents:: Contents
   :local:


Wiring Diagram
--------------

.. figure:: /_static/img/jtag-wiring.png
   :align: center
   :alt: JTAG wiring diagram

   JTAG wiring diagram to connect a standard 20-pin ARM JTAG device to
   the 8-pin JTAG port on the Maple.

.. TODO jtag wiring diagram (above) looks terrible; replace it

The Maple has holes for a 8-pin JTAG header but that header is not
soldered on by default. If you know ahead of time that you'll be
needing it, and you order `straight from LeafLabs
<http://leaflabs.com/store/>`_, add a comment to your order and we can
probably solder one on for no charge.  Otherwise, you can simply
attach standard 0.1" pitch male header pins (either the exact 4x2
block or two 4-pin pieces of breakaway straight header). For a one-off
usage hack, the header can be jammed in the holes and twisted to
ensure electrical contact; this is what we do to flash our bootloader.

The above schematic assumes that the header has been soldered on to
the *bottom* of the board, not the top; most ribbon cable connectors
will interfere with the power header. If you don't want a header
coming off the bottom, you can use a slim connector and invert this
diagram appropriately.


Compatible Devices
------------------

We have had good experience with the `Olimex ARM-USB-OCD
<http://www.olimex.com/dev/arm-usb-ocd.html>`_ device, which costs
about 55 euro plus shipping (as of October 2010).


Recommended Reading
-------------------

* `Wikipedia Article on Joint Test Action Group (JTAG) <http://en.wikipedia.org/wiki/Joint_Test_Action_Group>`_
* `STM32/gdb/OpenOCD HOWTO <http://fun-tech.se/stm32/OpenOCD/gdb.php>`_
* STMicro documentation for STM32F103RB microcontroller:

  * `All <http://www.st.com/mcu/devicedocs-STM32F103RB-110.html>`_
  * `Datasheet <http://www.st.com/stonline/products/literature/ds/13587.pdf>`_ (pdf)
  * `Reference Manual <http://www.st.com/stonline/products/literature/rm/13902.pdf>`_ (pdf)