FAQ – Board Bring-up & Fault Diagnosis

Discover how boundary scan helps identify manufacturing faults before firmware is running and speeds up debugging of new hardware.

For product-specific questions and answers, please visit the relevant product FAQ page using the menu on the left.

How do I test a board that won’t boot?

If a board won’t boot, first check that the power supply is working, and then that the correct voltage can be found on the power rails. If power is working, more detailed checks, such as boundary scan, can be performed. This will initially tell you whether all devices on the JTAG chain are functioning properly and then allow you to perform thorough testing on the rest of the board.

If there are still problems, your boundary scan software should show the areas it can’t cover; these can be checked with an ICT system or manually as a last resort.

What’s the best way to find manufacturing faults on a dead board?

Testing a non-functional board can be tricky but as long as the power is working and your JTAG chain is linked and responding, boundary scan can quickly check large parts of the board for broken connections, failed devices and short circuits. If the JTAG chain isn’t responding properly, you have a much smaller number of devices and nets to test manually.

How do I test a prototype board before firmware works?

During prototyping, it’s likely that some early PCB designs will be finished and built before the code is ready. This means that functional testing is not available, but the board still needs to be checked.

Boundary scan can provide much of the same depth and detail as functional test, sending signals through the board and checking that it’s working electrically.

Adding flying probe testing can also allow external signals and inaccessible nets to be included. This combination is very thorough, and only misses out on testing the board at full clock speed.

For more information, see:

Why PCBs fail and what to do about it
We found a resistive short on a prototype

People also ask:

My PCB is dead. Where do I start?
My board isn’t responding
How do I debug a new PCB?
Why won’t my FPGA boot?
How do I diagnose a faulty assembled PCB?

How do I test inaccessible nets on a board?

Many modern PCBs have nets hidden under BGAs with no accessible test points, an alternative to physical probing is required. JTAG boundary scan provides a non-intrusive way to test for these as it can read signals on nets connected to JTAG devices. More advanced JTAG systems such as XJTAG can calculate the state of nets beyond active or passive devices using the signals they have direct access to, extending test coverage further.