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4 shunt tips for JTAG boundary-scan testing

Shorting jumper shunts are frequently used to configure a unit under test (UUT) a particular way for boundary-scan testing–perhaps a compliance enable signal must be met or FPGA configuration needs to be inhibited. Because these shunts are meant to configure the system for test mode and could cause functional failures, it is important to ensure that test-only shunts are removed before products head out to the field. Here’s some advice to help eliminate the possibility of shunts remaining in place after a UUT is tested.

Use highly visible shunts

Shunts are available in a variety of colors, so make use of them! Whenever possible, use test shunts that contrast with shunts that that remain on the UUT after testing to ensure that none are missed during inspection. Additionally, shunts with a pull-tabs such as the TE Connectivity AMP Connector model 2-881245-2 shown below can be easier for test operators to see and remove.

Red Jumper
TE Connectivity AMP Connector 2-881545-2 

Attach a flag or string

Some shunts, such as the Harwin M7685-05 shunts shown below, have a slotted tab suitable for attaching a flag or a string. Appropriately large labels are ideal, but in a pinch a long strip of anti-static bag attached to the shunt is a simple way to increase its visibility.

Harwin M7685-05
Harwin M7685-05

Tether your shunts

Shunts with a loop for attachment can be attached to the JTAG TAP cable using a string. When the operator completes testing and disconnects the TAP cable from the UUT TAP connector, they also need to remove tethered shunts before the UUT can leave the test station.

Go shuntless!

In some cases, boards can be designed to avoid having to individually attach shunts in the first place. For example, an extra pair of pins can be added to the TAP header; a special TAP cable can short those pins when connected, allowing the TAP cable to act as a shunt between those two pins.