aug 2013

The Arduino as a simple JTAG adapter

A good while ago I won one of the free PCBs regularly given away by DIY hardware shop Dangerous Prototypes. My board of choice was a CPLD breakout board, for the Xilinx XC9572XL. CPLDs are the smaller brother of FPGAs: programmable logic chips that can be made to act as any integrated circuit within the device's limits. The XC9572XL is programmed via a standard JTAG interface. I did not have anything that speaks JTAG so went looking if my Arduino can be turned into an appropriate programmer. The solution that I found, however, did not work; so I built my own.

Normally, to program a CPLD, or FPGA, one buys an expensive interface cable and uses it with the software development suite supplied by the particular chip's vendor. Of course there are plenty of DIY alternatives; in fact, Dangerous Prototypes sell one or two. One of my goals with this project was, however, to spend next to no money on it. I got the circuit board for free, the parts cost around 3EUR, and I had already done a similar job with my Arduino Atmel programmer.

So after soldering the board I flashed the abovementioned JTAG code onto the Arduino. This was my second time SMD-soldering so I was not expecting the board to work on first try. But even after checking every connection with a multimeter, JTAGWhisperer would do apparently nothing after receiving the first chunk of data. I eventually gave up searching for the cause.

Instead I decided to write a very simple Arduino program that allows direct interaction with the JTAG interface from a serial terminal. It is called jtagbang because it is essentially bit-banging on the JTAG pins. By pure coincidence, it also requires frequent use of the exclamation mark (bang) when talking to it.

I didn't know anything about JTAG until three days ago. Now I know that it is awesome. The point of JTAG is to connect to any number of chips in some circuit design, taking up next to no space on the board, requiring only very simple support from the chip, and allowing the user to inspect and manipulate virtually every pin and connection at any time without touching anything. I call it fucking magic.

These LEDs are lit because I told the chip I needed those outputs on for testing purposes.

Unfortunately I cannot explain the magic in the space of this post, however, here is a link to the IEEE specification. While IEEE doesn't want you to read their standards, someone has helpfully put the 2001 version on slideshare… Reading that spec is still not much fun, but I made a drawing of the important part.

So, long story short: Upload the attached sketch to an Arduino, take a peek at the top of the file maybe, and connect to it with a terminal emulator (read minicom) or the Arduino IDE's serial monitor (set to line-ending Newline). Enter a capital X and it will interrogate the JTAG interface to find all the connected devices (chips). It lists their built-in identification codes which take the form of 32 bits in four groups:

59604093  [0101 1001011000000100 00001001001 1]

The groups are, from most to least significant bit: 4-bit product version (5), 16-bit product code (9604 is the XC9572XL), 11-bit manufacturer code (00001001001 is Xilinx), and one bit that is always 1 for thaumaturgic reasons.

I should find a PC mainboard to try this with.

Next, I need to get the CPLD programmed. Xilinx uses (X)SVF files for this, a file format that describes what to do on a JTAG interface in a more high-level fashion than my bit-banging. I need a player for this format that translates standard SVF commands into bang language and vice-versa. The good thing is that I can now do this in a high-level programming language of my choice entirely on the host instead of cramming it into the Arduino.

The adventure after that will be learning VHDL and designing an actual integrated circuit.

Attachment: jtagbang.ino (v0.1)

I am releasing the code under the terms of the quite permissive ISC license.