The Many Faces of JTAG

Wouldn’t it be great if there were just one standard for attaching to, programming, and debugging hardware?  If you could just plug in and everything would just work? Dream on, dreamer! But of course we hobbyists aren’t the only people to suffer from multiple standards. Industry has the same problems, writ large. In response to the proliferation of smart devices — microcontrollers, sensors, and their friends — on any given PCB makes it difficult to test them all, much less their function as a system.

The Joint Test Action Group (JTAG) got together in the mid-80s to make automated testing of circuit boards a standardized process. A JTAG port can be found on almost any piece of consumer electronics with enough brains to warrant it, and it’s also a tremendously useful entry point for debugging your own work and hacking into other’s. You’re going to need to use JTAG someday.

Implemented right, it’s a very cool system that lets you test any compliant IC on the board all from a single connector. It’s mostly used by hackers for its ability to run and halt individual processors, and put them in debugging modes, inspecting their memory states, etc. Essentially every microcontroller responds to JTAG commands, and it’s an incredibly widespread and powerful standard. A victory for rationality and standardization!

The connector pinout was, of course, left up to the manufacturer. The horror!

Five Signals

In principle, JTAG uses five signal lines. They form a chain starting at the debugger, where one device’s output is the next device’s input, until the result is returned back to the debugger.

JTAG, as imagined by Vindicator CC BY 2.5
  • Test Data In (TDI) is the input from the debugger
  • Test Data Out (TDO) is the return end of the chain
  • Test Clock (TCK) clocks this data along synchronously, similarly to SPI
  • Test Mode Select (TMS) lets the devices know that they’re being debugged — it’s a global chip select
  • Test Reset (TRST) is an optional signal that resets all devices in the chain

Continue reading “The Many Faces of JTAG”

Script makes custom pinout labels for your chips


After years of prototyping hobby electronics we’ve learned (several times actually) that when something’s not working it’s a problem with the hardware. Usually the jumper wires aren’t hooked up correctly, or we needed to throw a pull-up resistor in and forgot to. One thing that can really help sort these problems out quickly is a pinout label for each chip like the ones seen above. This is a project which [John Meacham] came up with. It uses a script to generate chip pinouts on a label maker.

The label maker he started with is a Brother PT-1230PC. It connects to a computer via USB and can use a few different widths of self adhesive label tape. [John] found that the 1/4″ wide tape is nearly a perfect fit for PDIP components.

His script takes a YAML file as the input. This formatting standard makes is quick an easy to whip up a label for a new chip using just your text editor. From there his Pearl script turns the data into a Portable Network Graphics (.png) file with the labels spaced for the 0.1″ pitch of the chip. Send this graphic to your label maker and you’ve got an adhesive reminder that will help reduce the time you spend pawing through datasheets just for the pinouts.

Automatic JTAG Pinout Detection

Figuring out the JTAG pinout on a device turns out to be the most time consuming hardware portion of many hacks. [hunz] started a project called JTAG Finder to automatically detect the JTAG pinouts on arbitrary devices using an 8bit AVR ATmega16/32L microcontroller. Check out the slides (PDF) from the talk as they break down how one finds JTAG ports on an arbitrary device, with or without a pinout detection tool. [hunz] is looking for people to pick up the project where he left off.

Once you determine the correct pinout, you will need a JTAG cable: there are two main types, buffered and unbuffered, both of which I have soldered up and tested from these circuit diagrams (image of completed buffered cable here). The software most hardware people use today are the openwince JTAG Tools. To get the JTAG Tools to compile, grab the latest source directly from their CVS repository.

The last time we featured JTAG was with regards to Linksys devices, but the tools listed above can be applied to any device with JTAG.