Automated Boundary Scan Test Vectors for Multi Site Register Verification
Automated multi-site boundary scan verifies module registers by routing broadcast stimulus to parallel sockets with isolated per-site response comparators.
TDO Contention
Meaning
Electrical conflict occurring when multiple devices on a JTAG chain attempt to drive the Test Data Out line at the same time. Concurrent drive attempts lead to tdo contention when the instruction register is misprogrammed or the boundary scan path is incorrectly defined.
Logic State
Digital states become indeterminate when two drivers with different values compete for the same physical wire. In the event of tdo contention, the resulting potential often sits between the high and low thresholds of the receiving buffer. Data corruption occurs as the controller fails to decode the incoming bitstream.
Daisy Chain
Serial topologies require that only the device in the Shift-DR or Shift-IR state activates its output buffer. If the bypass logic fails, tdo contention prevents the test data from reaching the master controller. Designers use tri state buffers to ensure that inactive devices remain in a high impedance state to allow the active part to drive the bus.
Verification of the boundary scan description language file helps prevent overlapping drive conditions during the design phase.
Hardware Protection
Continuous conflict causes excessive heat and potential damage to the output drivers. Detection of tdo contention usually requires current monitoring or observing unexpected waveforms on an oscilloscope. The final assembly must be screened for shorts that might force multiple pins into an active state simultaneously.