What Can Boundary Scan Actually Test?

Simon Payne, CEO, XJTAG | Subscribe to LinkedIn newsletter

Last week, we explored what Boundary Scan is and why it is already built into so many of today’s semiconductor devices. For many engineers, that raises the next obvious question:

What can it actually test?

The answer often surprises people. Because Boundary Scan is not really about testing a single device. It is about analysing how the devices on a board are connected together.

Every Board Is Built From Connections

Whether you’re designing an FPGA platform, an embedded processor board or a complex SoC-based system, the finished product is really a collection of thousands of electrical connections.

Processors, FPGAs, memories, power-management devices and peripherals are all interconnected as well as external connections.

If any one of those connections is incorrect, the behaviour of the entire system can change.

Looking Beyond Functionality

During bring-up, engineers often discover a symptom.

A memory test fails.

An interface doesn’t respond.

A processor refuses to boot.

Those symptoms are useful, but they don’t necessarily identify the underlying cause.

Boundary Scan approaches the problem differently.

Instead of asking whether the interface works, it asks whether the electrical connections that make up the interface are correct.

Verifying PCB Interconnections

One of the most common uses of Boundary Scan is verifying the connections between semiconductor devices.

For example, it can help identify:

Open circuits
Short circuits
Incorrectly assembled connections
Missing or incorrectly fitted devices

Rather than investigating an interface as a whole, engineers can often verify the individual signals that make up that interface. That provides a much clearer starting point for further investigation.

Looking Beyond JTAG Devices

One of the biggest misconceptions about Boundary Scan is that it can only be used with JTAG-compatible devices.

In reality, JTAG-enabled devices can often be used to interact with surrounding components that have no JTAG capability at all.

Depending on the design, this may include:

DDR memories
Flash memories
SPI peripherals
I²C devices
Ethernet PHYs
DACs and ADCs
Displays, indicators and buttons
General-purpose logic

By using the Boundary Scan logic already built into neighbouring devices, engineers can establish confidence in much larger sections of the hardware than they might initially expect.

Looking Beyond Non-JTAG Devices

When interacting with surrounding components, Boundary Scan is not limited to simply checking the connections between a JTAG device and a non-JTAG device.

Those same connections can often be used to exercise the functionality of the non-JTAG device itself.

For example, engineers can:

Drive DAC outputs and observe the resulting analogue signals
Read voltages using ADC inputs
Check whether an Ethernet link successfully comes up
Interact with peripherals to confirm expected behaviour

By using the functionality of these devices, Boundary Scan can extend testing far beyond simple connectivity checks.

This means that testing can reach into many parts of the design that are physically and logically distant from the original JTAG device.

In practice, this allows engineers to move far beyond the immediate JTAG chain, reaching into analogue domains, peripheral subsystems and external interfaces, to validate behaviour across large portions of the board that could otherwise be extremely difficult to access or control.

More Than Finding Faults

Although Boundary Scan is often associated with fault finding, its value starts much earlier.

It can help engineers:

Build confidence during board bring-up
Verify hardware before software is complete
Support manufacturing test
Diagnose returned products
Validate new board revisions

In many cases, the greatest benefit is not simply finding faults.

It is proving that large parts of the hardware are already working correctly.

That confidence allows engineers to focus their investigation on the areas that remain uncertain.

The Value of Knowing What You Don’t Need to Investigate

One of the themes we’ve returned to throughout this series is reducing uncertainty.

Every verified connection removes another possible cause.

Every proven interface narrows the investigation.

Every confirmed device increases confidence in the hardware.

Good engineering is often less about finding the answer immediately and more about systematically eliminating the possibilities that are no longer relevant.

Boundary Scan is particularly effective at helping engineers do exactly that.

Closing Thought

Boundary Scan doesn’t replace application software.

It doesn’t replace oscilloscopes.

It doesn’t replace logic analysers.

Instead, it provides another source of information that helps engineers understand the hardware itself.

And the better engineers understand their hardware, the faster they can move from uncertainty to confidence.

Board Design Tip of the Week

Think about Boundary Scan while you’re still designing the board, not after it’s been built.

Preserving JTAG access, planning the scan chain and ensuring critical devices remain accessible can dramatically increase the value that Boundary Scan provides throughout development, manufacturing and long-term support.

The decisions made during schematic capture often determine how much visibility engineers will have when the first prototypes arrive.