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Why Good JTAG Design Matters

Image of CEO of XJTAG Simon Payne Simon Payne, CEO, XJTAG |  Subscribe to LinkedIn newsletter

Over the past few weeks, we’ve explored how JTAG helps engineers gain visibility into increasingly complex electronic systems.

We’ve discussed why visibility matters, how JTAG enables access to devices, and why JTAG testing can provide insight into much more than just the devices that contain the JTAG logic itself.

But there is another aspect that is often overlooked.

None of these benefits happen by accident.

They depend on the board being designed in a way that preserves access and visibility from the very beginning.

The Design Phase Is Usually Optimistic

When a new board is being developed, teams are naturally focused on functionality.

The design is expected to work.

The FPGA will configure.

The processor will boot.

The interfaces will communicate.

The software will run.

And most of the time, that optimism is justified.

The challenge is that engineers rarely know in advance which assumptions will eventually need to be investigated.

Future Engineers Inherit Today’s Decisions

One of the interesting things about board design is that the people who benefit from good design decisions are often not the people who make them.

A JTAG connection added during schematic capture may later help:

  • A bring-up engineer investigating first prototypes
  • A manufacturing engineer diagnosing assembly faults
  • A software engineer debugging a processor boot issue
  • A service technician investigating a returned product

The design decision may take minutes.

The benefit may last for years.

Visibility Is Easier To Preserve Than Recreate

Once a PCB has been manufactured, options become limited. If access to a signal was not provided, adding it later may require:

  • Board modifications
  • Additional fixtures
  • Rework
  • New PCB revisions

By contrast, preserving visibility during design often costs very little.

A few extra pads.

A sensible connector location.

Access to key interfaces.

The earlier visibility is considered, the easier it becomes to support future investigation.

When Good Design Pays Off

Designing for testability is not about expecting problems. It’s about preparing for the questions engineers will inevitably need to answer throughout the life of the product.

It can support:

  • Manufacturing test
  • Device programming
  • Production diagnostics
  • Design verification
  • Future revisions
  • Long-term support

Good JTAG design is therefore not about expecting problems. It is about preparing for questions.

The Cost Of Assumptions

Many engineering delays are caused by uncertainty. Engineers know something is wrong, they simply don’t know where. When visibility has been designed into the board, uncertainty can be reduced much more quickly.

Questions become easier to answer.

Assumptions become easier to verify.

And investigations become significantly more efficient.

Designing For The Entire Lifecycle

One of the reasons JTAG has remained relevant for so long is that it supports far more than initial development.

A well-designed JTAG infrastructure can continue providing value through:

  • Bring-up
  • Manufacturing
  • Production
  • Service
  • Repair
  • Future product revisions

That makes it one of the few interfaces that remains useful throughout the life of a product.

Closing Thought

Good JTAG design is rarely noticed when everything works. Its value becomes apparent when engineers need answers. The best time to think about visibility, access, and future debug capability is not when a problem occurs.

It’s during the design stage, when those options are still easy to preserve.

Because once a board has been built, engineers can only work with the visibility they were given.

JTAG Tip of the Week

Where practical, provide footprints for components that may help optimise JTAG operation in the future, even if they are not initially fitted.

Examples include:

  • JTAG termination options
  • Pull-up and pull-down resistors
  • Configuration mode selection resistors
  • Chain isolation options

These footprints consume very little PCB area but can provide valuable flexibility during bring-up, debug, and future revisions.

It is often better to have the option and not need it than to need it and discover it cannot be added.

Explore your board design with the Free XJTAG Layout Viewer

To get started find out how the Free XJTAG Layout Viewer can help you see your board connections:

Download your Free XJTAG Layout Viewer today

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Structural tests developed during prototype bring-up don't necessarily need to be discarded once the design is working. Tests that proved connectivity, memories, peripherals or interfaces during development may also provide useful coverage in production. That means the effort invested in understanding the first prototypes can potentially become part of the manufacturing test strategy. Instead of starting again, knowledge moves forward with the product. Closing Thought A string of passing boards doesn't really test your diagnostic strategy. The first failure does. That is when you discover whether your test simply detects a problem or actually helps the engineer understand it. And the difference between knowing: “This board failed.” and: “This is where you should start looking.” can determine whether the problem is diagnosed in minutes, investigated for hours or placed on a growing bone pile marked: Investigate later. The best production tests don't just catch faults. They help engineers understand them. Board Design Tip of the Week Before releasing your next design, choose three realistic manufacturing faults. For example: An open connection beneath a BGA A short between two adjacent signals An incorrectly fitted peripheral Then ask: “If this happened on the production line, what would our test actually tell us?” If the answer is simply: FAIL there may still be work to do. Discover more about your board design with the Free XJTAG Layout Viewer. To get started find out how the Free XJTAG Layout Viewer can help you see on your board: Download your Free XJTAG Layout Viewer today