What is boundary scan test?
And why should you use it on your next design?
In the next few minutes we will answer these questions by giving you a clear understanding
of the power of boundary scan.
You will discover why it is the preferred test method for a wide variety of applications.
This educational technical video was created by TechSharpen in collaboration with Flynn
Systems, the creators of the onTAP boundary scan automation software.
Historically, the most straight-forward way to test electronic systems has been functional
test.
As the name implies, it means exercising a system as it is expected to be used in the
target application.
Though this approach continues to have merit, it has some significant drawbacks.
First, it may be difficult to replicate a functional setup that sufficiently mimics
the end application.
Second, sufficiently testing the operational space of systems, with even moderate complexity,
can be very difficult.
An alternative to verifying functional operation is structural testing.
This method tests individual partitions instead of looking at the system as a whole.
It enables a more thorough testing that can verify corner cases which are difficult to
access using functional test only.
Structural test also has problems to overcome.
Gaining access to internal nodes is a challenge.
One traditional method for accessing internal nodes is employing a fixture with multiple
electrical contacts, touching down on exposed metallic pads.
This fixture is commonly called a "bed of nails."
Unfortunately these fixtures are costly to fabricate and difficult to make a solid connection
when there are many points to access simultaneously.
This method becomes even more problematic as circuit technology shrinks, with a higher
reliance on multiple-layered boards.
It makes internal access almost impossible as nodes are buried under packages such as
ball grid arrays.
Boundary scan is an ingenious approach to structural testing that overcomes these challenges.
Instead of relying on mechanical connections for access, it adds dedicated test circuitry
to the system's ICs, using special registers inserted at the I/O pins.
These registers can observe data on the I/O pins, and they can be used to override drive
signals, providing test stimulus to the circuit.
It is this combination of stimulating and observing internal nodes that provides a powerful
mechanism for accurate structural testing.
To gain test access to these registers, there is a separate serial data chain, independent
of the functional paths.
Test set-up information flows into the chain's input, propagating to the chain's output.
If there are multiple ICs with boundary registers, they can be concatenated to make a longer
chain.
This same data chain is also used to off-load measured data.
Typical boundary scan registers can have four different types of operations.
One operation allows data to pass through the registers "transparently," without modification.
This is the typical state of the circuit during normal functional operation.
Another operation "captures" data coming into the register, without changing its value.
This is how test data is observed.
A third operation "updates" the output of the register, disregarding the data coming
into the register.
This is how stimulus is applied to the circuit under test.
The fourth operation executes the "serial shift," enabling test access to each register.
Shifting the test response from the boundary registers into an off-board register allows
it to be scanned and compared to the expected response.
Hence the name, "boundary scan."
A critical issue with boundary scan implementation is coordination between organizations.
When the test department decides to utilize boundary scan, they need to work with the
board developers to add it to the design.
Next, IC manufacturers need to have boundary scan registers built into the devices they
provide.
Finally, someone needs to provide software tools that can gather the information from
these different sources and create a program.
To facilitate information exchange between these organizations, a standard has been developed
for boundary scan.
Established in 1990 by a team of industry representatives called the "Joint Test Access
Group," it became the IEEE 1149.1 standard.
The standard is often referred to simply as "JTAG."
JTAG is especially important to IC manufacturers because it enables them to provide a consistent
boundary scan configuration that is readily usable by many different customers in a variety
of applications.
Since the establishment of JTAG, a growing list of commercial devices includes boundary
scan.
This has expanded its usage into many different applications.
It continues to be an efficient method for verifying the electrical connections between
IC pins and printed circuit boards.
This application has since expanded to multichip modules and stacked die applications.
JTAG has moved beyond the chip boundary to access the chip's own test structures such
as built-in self-test for verifying internal memory and logic.
Enhancements to JTAG have made it easier to apply it at a system level, with the ability
to access internal subsystems.
JTAG has become a preferred method for field testing because it allows circuit access without
disassembly.
Design debugging is another application for JTAG because it enables an unobtrusive way
to access the state of the system.
In this video we have only touched on the many ways to use boundary scan for testing
your next design.
Look for other videos from TechSharpen to learn more about boundary scan and JTAG.