When using JTAG with a Device Under Test (DUT) connected to a bed-of-nails test fixture, often little attention is paid to the way the JTAG signals are wired. This article (published in SMT magazine last year) gives a helpful explanation of why it is important to get this right, and gives tips for best practice.
The traditional technique for debugging printed circuit boards is to “observe” the state using oscilloscopes or multi-meters and deduce a fault. This method actually suppresses a very powerful engineering instinct that would help us a lot if we could only give it a better chance. […]
Many of us have been designing electronics for years and haven’t really considered pull up and down resistor values. We just use the same old values like 10K. With some new silicon the leakage currents are higher than we have been used too. This means the pull resistors might not being doing the task we require. When there is a need to reduce power consumption, again these values need to be looked at in greater detail. XJTAG often finds resistors on boards that don’t actually perform the task intended. […]