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JTAG Reverse Engineering


JTAG is a rather ubiquitous technology for debugging and testing circuit boards, integrated circuits, and embeded software. JTAG stands for Joint Test Action Group and the standard was initially developed to enable testing of assembled circuit boards. With the invention of surface mount components and the continual increase in complexity and desnity of circuit boards, the JTAG interface allowed for non-intrusive testing of all of a board's connections at a cost of 4 pins per chip and a single JTAG header on the board.

This application of JTAG is called 'boundary scan' and it allows the JTAG TAP (test access port) on the chip to take control of all of its IO pins. When multiple chips with JTAG TAPs are installed on a board, their JTAG interfaces can be daisy-chained into a JTAG scan chain. This enables a single test connector on the board to access all of the JTAG controllable I/O pins at he same time, allowing boundary scan software to check all of the connections in the board's netlist for any faults.

Later on, JTAG was extended to also access the interior of the chip as well. JTAG can be used on many chips for loading firmware and configuration information as well as debugging embedded software. This makes JTAG a very powerful tool for not only manufacture but also development of complex circuitry. With JTAG, a board can be tested and programmed in a sigle step during manufacture. The same JTAG port can also be used for development and debugging of device firmware.

The capabilities of the JTAG TAP also make it a very good tool for reverse-engineering PCBs. The ability to control the I/O pins of chips on a JTAG scan chain can be used not only to check a netlist, but also to reconstruct one. The JTAG TAP can also be used to download firmware and configuration information for disassembly and further reverse-engineering.

Netlist Extraction

Once a JTAG access port is located on a board, extracting a netlist is relatively straightforward. Well, with the right tools, anyway. The utility of having the netlist depends on the purpose of the board. For some boards, the netlist is of little use. However, some very good deals on boards with very large FPGAs on them pop up from time to time on eBay and other surplus and auction sites. These boards can be used as very powerful FPGA development boards, but without information about the interconnections between the FPGAs, they are quite difficult to use. The folks over at NSA@home used repurposed HD video transform boards loaded with FPGAs to build an MD5 hash cracking machine, and they developed a JTAG reverse-engineering tool called jrev to extract the netlist.

The process of netlist extraction is very simple. Turn all the I/O pins to inputs, turn one on, look at all the pins, turn it off, look at all the pins again, and see what changed. Repeat this for every JTAG accessible pin on the board. The jrev tool does this for a parallel port JTAG cable, and the ftjrev tool does this for FTDI based USB jtag cables. However, these tools can only find connections between JTAG enabled pins.