Design Engineers Role in Boundary-Scan Test

One common misconception about boundary-scan test is that it is completely the responsibility of the test or manufacturing department. Unlike in-circuit testers (ICT), boundary-scan design problems cannot be resolved by simply adding a few more test points to the board. Boundary-scan testing utilizes a whole new set of design constraints that need to be taken into consideration before a board goes to layout.

The following is a short list of items that should be considered up front:

  1. Use fully compliant IEEE-1149.1 devices.
  2. Incorporate workarounds for boundary-scan parts having non-compliant behavior.
  3. Correctly design the scan-chain including trace layout and routing.
  4. Use fan-out buffering for TCK and TMS signals.
  5. Properly terminate the scan chain signals on the board for improved signal integrity.
  6. Verify that BSDL files are available and tested.
  7. Provide external connector access to the scan-chain.
  8. Allow control to disable non-boundary-scan parts.
  9. Allow programmable devices to be tested while they are erased or not configured.
  10. Ensure boundary-scan control of clock pins for memory tests.

Board designers should not see this list as extra work because they can actually take advantage of boundary-scan testing themselves by using it to assist in debugging of their prototype designs. When a prototype board first arrives from the assembly house, the first thing the engineer does is to visually inspect it to make sure components are installed correctly, make sure there are no obvious shorts or opens, check it for proper impedance between the voltage rails and ground, and probably apply some current-limited power to ensure nothing gets hot.

At some point the designers will try to load some executable code to see if the board boots properly. If the board doesn’t boot properly, the designer will likely spend a considerable amount of time troubleshooting which may eat up precious weeks in getting the product released. Boundary-scan testing can actually help the design engineer by ensuring the board is free from manufacturing related defects.

The following list represents some of the benefits that design engineers will receive by implementing a boundary-scan friendly design:

  1. Boundary-scan tests can be executed on boards that do not boot or are partially bootable.
  2. Boundary-scan is a fixtureless test. There is no need for the designer to add a large number of test points.
  3. Boundary-scan provides detailed diagnostics down to the pin and net level so engineers are immediately aware of where prototype problems are located.
  4. Boundary-scan helps build the design engineer’s confidence when the interconnect test passes.
  5. Boundary-scan test vectors used by the design engineers can be reused in production. The time saved can eliminate redundant work and reduce cost.

It is important to realize that although boundary-scan is a test methodology, design engineers should not isolate themselves from the process and leave it entirely up to the test engineering department to implement. By spending some time and effort up front, designers can help save time in their own board bring-up while easing effort in the manufacturing and test departments as well. This presents a win-win situation for any company utilizing boundary-scan test.