Why JTAG Exists

Simon Payne, CEO, XJTAG | Subscribe to LinkedIn newsletter

Electronic systems are extraordinary.

Today’s boards can contain:

FPGAs
processors
memories
high-speed interfaces
mixed-signal devices
complex power and communication architectures

All integrated onto increasingly dense multilayer PCBs.

But as semiconductor devices became more capable, a quiet engineering problem began to emerge: engineers could no longer physically access many of the signals they needed to test.

The Problem Before JTAG

Years ago, testing electronic boards was relatively straightforward.

Devices used larger packages, signals were accessible and engineers could probe pins directly with oscilloscopes and test equipment.

As semiconductor technology evolved, however:

pin counts increased
packages became smaller
BGAs replaced accessible leads
routing moved inside multilayer boards

Suddenly, many critical signals became physically inaccessible.

Ironically, as systems became more advanced, they also became harder to test and debug.

The Visibility Problem

This created a major challenge across the electronics industry.

How could engineers:

test connectivity?
verify assembly quality?
program devices?
diagnose faults?
debug boards?

…when they could no longer physically reach many of the connections?

Traditional probing methods were no longer enough.

A different approach was needed.

The Industry’s Answer

That answer became:

JTAG

Originally standardised as IEEE 1149.1, JTAG was developed to solve a very practical problem:

provide a standardised way to access and test increasingly complex electronic systems, without requiring physical access to every signal.

What Made JTAG So Important

Instead of probing pins externally, JTAG allowed devices to help test the board from within.

Semiconductor devices could:

observe signals
drive signals
shift data internally
communicate test information serially

This fundamentally changed what engineers could do.

Suddenly, it became possible to:

test interconnections beneath BGAs
verify board assembly
program devices
debug systems
and gain visibility into hardware behaviour

all through a small standardised interface.

Why It Became Widely Adopted

One reason JTAG became so successful is that it solved problems affecting almost every part of electronics development and manufacturing.

JTAG is commonly used for:

board bring-up
manufacturing test
FPGA configuration
processor debug
in-system programming
fault diagnosis
hardware validation

And because many semiconductor devices support it directly, JTAG quietly became one of the most widely adopted technologies in todays electronics. Even though many engineers interact with it every day without necessarily thinking about it explicitly.

More Than Just Programming

One of the common misconceptions about JTAG is that it only exists for programming devices.

In reality, programming is just one part of a much larger capability.

At its core, JTAG is really about:

visibility and access

The ability to:

interact with devices
understand board behaviour
and establish confidence in hardware systems

Especially when physical access is limited.

Why This Matters Today

As boards continue becoming:

denser
faster
more integrated
and more difficult to probe physically

…the original reasons JTAG was created are becoming even more relevant.

Not less.

The need for:

visibility
structural understanding
and reliable hardware access

continues to grow across:

telecommunications
aerospace
defence
automotive
industrial electronics
and AI hardware platforms

Closing Thought

JTAG was never really about the connector.

It was about solving a fundamental engineering problem:

how to understand, test, and trust increasingly complex electronic hardware.

And as modern systems continue to evolve, that problem remains just as important today as when JTAG was first introduced.

JTAG Tip of the Week

Always include accessible and clearly labelled JTAG headers or test access points in early prototypes.

Why it matters:

Missing or difficult-to-access JTAG connections can severely limit debug and bring-up options
Early hardware issues are much easier to diagnose when reliable access already exists
Small layout decisions early can save significant engineering time later

What to watch out for: