JTAG Debuggers


In the beginning...

We have been collecting a bucket of JTAG “sinkers” going back to the Medi-Evil era of parallel ports and captive audiences. How did we get into this situation?

It all started with an Altera Bit Blaster on a parallel port in the 1990s, followed by the Motorola/ Freescale PowerPC family with a BDM port. Next was the COP interface for the PowerPC in Xilinx FPGAs and the Motorola 5200 family in the early 2000s.

The MIPS work at the time sorely lacked any reasonable debug ports, so we examined various options with FPGAs assisted debug. Before that came to anything, we caved in to customer demand and chose ARM with their many suppliers for debug/ ETM interfaces. We will mostly cover those in the future links from this page.

Along the way we picked up these probes hoping they would pay their way, but they have zero second-hand value, unless purchased from Segger and using their trade-in-program. In reality, it was a lot of money down an expensive pit. We doubt that we have reached the bottom pit yet, or that we won't buy another crop of probes. Tracing is improving, devices are becoming invisible behind peripheral pins, so there really is little alternative.

Incompatible

After noticing that the bucket of “sinkers” was filling up, someone casually asked — What are these and why do you need so many? The short answer is that nobody can make one that is universally adopted and still lock in customers.

Another anomaly is that the same debugger (costing in the hundreds of dollars) can be installed on a sub-$40 evaluation board. Some launchpad boards are even shipping for under $20 with on-board probes.

Links to more details below

We will be adding in a fairly large section on the probes as we get time. In the meantime, here are links to the initial offerings.

Parallel Port Era

The vintage here is from the late 1990s to about 2006. The PC parallel port was slowly fading off the desktop. We will add the collection of USB probes as we update this part of our cyber home.

JTAG probes

Bucket of JTAG dongles. Clockwise from the top left corner: Digilent's Xilinx Parallel Cable III compatible probe supplied with their S3 board, the GreenHills Slingshot (USB) supplied with the 5200 PowerPC kits, the ColdFire FlySwatter built by BetaTech, the Chameleon POD probe from Amontec, Altium probe for Xilinx and Altera with one hard port and a second soft port for instrumentation, Xilinx Parallel IV and III cables, another Slingshot, and the Wiggler for the PowerPC.

There are links to individual pages where we took them apart to show how we were part of the suckers who paid so much for a couple of gates. The semiconductor vendors could have cut the nonsense and given the diagrams with pinouts so that a standard ribbon cable could connect between the PC parallel port and a 26-pin header on the FPGA, CPLD or processor board. Xilinx gets the prize for taking the most money for the least hardware, which was followed up with outragously priced configuration memory, then the ACE closed configurator. The Wiggler was up there with the best for price opportunists, but they did provide a free GNU compatible toolchain that worked with their probe — a very cost effective offer. We bought it to work with WindRiver's VxWorks for the PPC860T.


Pricing

Most JTAG vendors have online stores on their websites. Sometimes they need your finger prints and shoes sizes, but a bit of digging can extract pricing. Some of the most popular probes are from Segger, who bravely make their pricelist publicly available.

Closing Comments

From pricing, it is obvious that the USB to JTAG interface is not a simple matter. Should the chip vendors care? We think so.

Evaluation boards with built-in probes should included an option for an external programmer. We would prefer to see more effort put into a decent low-cost serial programmer external to the board.

Would it be possible to bundle the probe separately and make it available at a reasonable price? This looks like a trend that semiconductor vendors don't seem to want to breach. Even the ARM camp repeats this exercise over and over again, often with different chips for the USB to JTAG interface (from the same vendor).