Why JTAG Testing Is More Powerful Than Most Engineers Realise
Simon Payne, CEO, XJTAG
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Over the past few weeks, we’ve explored why JTAG was created, what it enables, why it has become so important to semiconductor devices, and why visibility matters when bringing up and supporting complex hardware.
For many engineers, JTAG is already familiar.
It’s used to program FPGAs.
Debug processors.
Configure devices.
Identify parts on a board.
But there is another capability that often receives far less attention.
And it is arguably one of the most powerful.
The Assumption Most Engineers Make
When engineers first encounter JTAG testing, they often assume it can only be used to interact with JTAG devices. It’s an understandable conclusion, after all, the JTAG interface exists inside semiconductor devices. So it seems logical that its usefulness would stop there.
In reality, that is only part of the story.
Looking Beyond The Device
One of the key ideas behind boundary scan is that it provides access to signals at the pins of semiconductor devices. Those signals do not stop at the device boundary, they continue into the surrounding circuitry. This means that JTAG-enabled devices can provide visibility into much more than the devices themselves.
They can provide visibility into the board.
A Different Way Of Thinking About Testing
Consider a common engineering problem, a new board arrives from manufacturing. The FPGA can be identified, the processor appears to respond, yet something still doesn’t work. Perhaps a memory interface is failing, a peripheral isn’t responding. Or an important communication path behaves unexpectedly. At this point, many teams immediately start investigating the failing subsystem.
Experienced engineers often take a different approach.
Instead of asking:
“Why doesn’t it work?”
They first ask:
“What can we prove already works?”
Can signals be driven?
Can signals be observed?
Can connectivity be verified?
Can devices be accessed?
Each answer removes uncertainty.
And removing uncertainty is often the fastest route to finding the real problem.
Why This Matters
Modern electronic systems contain far more than processors and FPGAs.
They include:
- Memories
- Clock generators
- Power-management devices
- ADCs and DACs
- SPI peripherals
- I²C devices
- Communication interfaces
Many of these devices do not contain JTAG interfaces themselves.
Yet they are connected to devices that do.
That creates opportunities to validate and understand large portions of the board using the visibility provided by the JTAG infrastructure already present.
The Real Benefit
The true value is not simply test coverage.
The real benefit is confidence.
Confidence that:
- Devices are connected correctly
- Signals are reaching their destinations
- Critical interfaces are functioning
- The hardware foundation is sound
When that confidence exists, debugging becomes significantly more focused.
Instead of investigating every possible cause, engineers can concentrate on what remains unproven.
Why This Is Becoming More Important
The original reasons JTAG was created have not disappeared.
If anything, they have become more relevant.
Today’s boards contain:
- Higher pin counts
- More BGAs
- Faster interfaces
- Greater integration
- More layers
At the same time, physical access to signals continues to decrease. Traditional probing techniques remain important, but they are no longer sufficient on their own. Engineers increasingly need alternative ways to understand what is happening on the board.
Visibility Creates Momentum
One of the observations we’ve made repeatedly over the years is that engineering teams rarely get delayed because they lack technical capability. They get delayed because uncertainty remains for too long. The longer it takes to establish what is known to be working, the longer investigations tend to take.
Visibility changes that.
It allows engineers to establish confidence earlier, reduce assumptions, and focus effort where it matters most.
Closing Thought
Many engineers think of JTAG as a programming interface.
Others think of it as a debug interface.
Some think of it as a manufacturing test interface.
In reality, its greatest strength may be something much simpler.
It helps engineers understand their hardware.
And in increasingly complex electronic systems, that understanding is often more valuable than ever.
JTAG Tip of the Week
When designing a board, don’t think about JTAG solely as a programming interface.
Consider how it might help establish confidence in the hardware during bring-up, manufacturing, and future debug activities.
The more visibility that is preserved during the design stage, the more options engineers will have when investigating unexpected behaviour later.
Good JTAG design is often less about today’s requirements and more about tomorrow’s questions.
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:
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