PCB Design Data to Working Boundary Scan Tests

Last week, we looked at how Boundary Scan software builds an understanding of your PCB.

The netlist describes how the board is connected. BSDL files describe the Boundary Scan devices, while models or reusable library files describe the other components.

Put that information together and the software can build a detailed model of the board.

But that raises another question:

How does that information become an actual test?

Understanding the design is useful but being able to automatically turn that understanding into tests that can be run on real hardware is where things become much more interesting.

Imagine Doing It Manually

Consider a complex PCB containing an FPGA, processor, several memories and hundreds, or potentially thousands, of interconnected signals.

If engineers had to create a test for every accessible connection individually, they would need to determine which device could drive each signal, which device could observe it, what values should be applied and what result should be expected.

Then they would have to repeat that process across the rest of the board.

It would be possible.

It would also require a considerable amount of engineering effort.

This is where Boundary Scan software becomes important.

The Board Model Changes Everything

Once the software understands the connectivity of the board and the capabilities of the devices, it can analyse the design systematically.

For each connection, it can determine whether there is Boundary Scan access and how that access can be used.

For example:

Can this signal be driven?

Can it be observed?

Which devices are connected to it?

What other components could affect the test?

Instead of treating the PCB as thousands of unrelated nets, the software can begin to understand which parts of the design are accessible and how they can be tested.

Creating Structural Tests

For connections between suitable Boundary Scan devices, the principle is relatively straightforward. One device can drive a known value onto a connection while another observes what arrives. The value can then be changed and the result observed again.

When this process is applied intelligently across groups of signals, the resulting test can identify structural problems such as opens and shorts.

The important distinction is that we’re not asking whether the application works.

We’re checking whether the physical connections on which that application depends behave as the design says they should.

What About Devices Without Boundary Scan?

This is where things become more interesting.

As we’ve discussed previously, the usefulness of Boundary Scan doesn’t necessarily stop at the pins of another JTAG device.

If a Boundary Scan-enabled device is connected to a memory, peripheral, DAC, ADC or other non-JTAG component, those connections may still provide opportunities for testing.

The software needs to understand how that component behaves and how the accessible signals can be used to interact with it.

This is where device models and reusable test libraries become particularly valuable.

Instead of starting again every time a familiar component appears on another design, knowledge about how that type of device can be tested can be reused.

Automation Doesn’t Remove Engineering

Automatic test generation doesn’t mean that an engineer presses a button and every possible part of every PCB is magically tested.

Different designs provide different levels of Boundary Scan access, and some circuitry requires additional test development or other test techniques.

What automation does is remove a considerable amount of repetitive work.

The software can analyse what is accessible, generate appropriate structural tests and identify areas where additional engineering input may be required.

That leaves engineers free to concentrate on the parts of the design that actually need their expertise.

From a Test Result to Something Useful

Generating a test is only half the job.

When something fails, the engineer needs useful information.

Knowing that a test has reported an error is very different from knowing which signals and components are associated with that failure.

Because the Boundary Scan software understands the board design, the test result can be related back to the hardware.

Instead of simply:

TEST FAILED

the objective is to help answer:

Where should I investigate?

That can make a substantial difference during both prototype bring-up and manufacturing.

Reuse Changes the Economics

There’s another important benefit.

Once structural tests have been developed for a design, they don’t become irrelevant when development finishes.

They can move with the product through its lifecycle.

The same underlying test capability can support:

• Prototype bring-up
• Hardware validation
• Manufacturing
• Diagnosis
• Repair and service
• Subsequent board revisions

Compare that with some of the bespoke diagnostic software created during bring-up, which may have been written to investigate one particular problem and never used again.

The ability to create a test once and reuse that investment is an important part of the value of structural testing.

Closing Thought

The Boundary Scan hardware inside a semiconductor device provides access.

The BSDL files describe that access.

The PCB design data explains how everything is connected.

Device models describe how surrounding components can be interacted with.

Boundary Scan software brings all of that information together and turns it into something engineers can actually use: tests.

And once those tests exist, their value can extend far beyond the first prototype.

Board Design Tip of the Week

Don’t wait until manufacturing to ask how much of your board can be tested.

Consider testability while the schematic is still being developed.

The placement of Boundary Scan-capable devices, the way signals are connected and the accessibility of important interfaces can all influence the structural test coverage available later.

A small design decision made before layout can sometimes provide significantly greater test access throughout the life of the product.

Could boundary scan make your next board easier to test?

Discover how boundary scan can help you improve test coverage, diagnose faults faster and simplify board testing:

See what you can test with XJTAG.