Chapter 2. General

2.1. JTAG

JTAG basics can be found all over the Internet. This section should go into some more details about working with JTAG. What hardware do you need, what is the usage of JTAG, where do I get files. What file formats are available…

2.1.1. Introduction

JTAG (IEEE 1149.1) is a serial interface for testing devices with integrated circuits. The problem that the JTAG interface was designed to solve is checking if connections between ICs are OK. Therefore you can set and check in- and outputs of ICs. In order to save pins and logic a very simple serial design was invented.

  • One pin serial input

  • One pin serial output

  • One pin clock

  • One pin control

The control pin (together with clock) allows to switch device states. A state machine inside each chip can be controlled, e.g. to reset the device. This control machine also allows to have two internal shift registers in each device (although we only have on in- and one output-pin). The registers are called instruction register (IR) and data register (DR). The current UrJTAG tool allows you to set the IR and set and get the DR. It doesn't allow you to directly control the state machine (yet).

2.1.2. Interfaces

The simplest interface that you can build is like the Xilinx parallel cable (also called DLC5). If your device works with a 5V or 3.3V supply voltage then this device can even be built just with passive parts. (picture missing here) UrJTAG also supports a number of other interface adapters.

2.1.3. Additions

In the meantime the JTAG specification was used as a basis for programming flash files and debugging processors. UrJTAG supports programming a couple of different flash devices. It also supports programming of non-flash devices via SVF files. UrJTAG does not support debugging yet. Other open source solutions such as OpenOCD allow you to debug ARM processors with gdb.

2.1.4. BSDL and UrJTAG data files

The BSDL file format describes the JTAG interface for one IC. It is a VHDL syntax with the needed information (like pin-names, register lengths and commands) that is usually created by the supplier. e.g. Xilinx BSDL files are all included in their free web-pack (using file extension ".bsd").

UrJTAG uses a different file format internally. So in order to add a new device to UrJTAG you need to convert those files and produce a directory structure. Currently there are at least three tools available to do that; included with UrJTAG is "bsdl2jtag". Please ask on the mailing list in case of problems with that. Please also send proven working files back to this project.

Starting with post-0.7 releases, UrJTAG contains a BSDL subsystem that retrieves the descriptions for chips in the chain from BSDL files on the fly. "bsdl2jtag" is in fact a wrapper that uses the BSDL subsystem to convert the BSDL file.

2.1.5. SVF files

The SVF file format contains a number of high level commands to drive the JTAG bus. For example you can shift the IR or DR and even check for the results. The Xilinx Impact and Altera QuartusII tools allow you to write this file to program devices.

The player has been developed according to the "Serial Vector Format Specification", Revision E, 8 March 1999 issued by ASSET InterTech, Inc. The full specification can be found at http://www.asset-intertech.com/support/svf.pdf.

UrJTAG features an "SVF player" that can read SVF files and perform the described actions on the bus.

SVF parser and lexer are also copyright 2002, CDS at http://www-csd.ijs.si/. They have been reused from the "Experimental Boundary Scan" project at http://ebsp.sourceforge.net/.

2.1.6. JAM/STAPL files

Another format for describing actions over JTAG interfaces is STAPL, actually standardized as JEDEC "JESD-71A". Compared to SVF, it looks more like an actual programming language and features looping, conditional execution, and more. STAPL is not yet supported by UrJTAG.