Scan Chain Signal Termination

Termination of the scan chain signals, including both data and clock lines, is
often an afterthought in board design but can have a strong effect on signal
integrity. We like to stress in our
DFT
Guidelines, documentation, and discussions that signal termination is
essential to maintaining proper signal integrity. The diagram below illustrates
the JTAG signal connections between JTAG controller and UUT.

Figure 1: JTAG signal connections between JTAG controller & UUT

For any series resistor on a TDO net, we recommend selecting a value that is
suitable for series termination to improve the signal quality. While our DFT
guidelines specify rule of thumb resistor values of 33 ohms—a value that
frequently works—the optimum value will of course be PCB dependent. Signal
quality is also improved if the PCB route length between the series resistor and
the driving pin is minimized.

Figure 2: TDO to TDI connection between boundary-scan devices
Resistor pullups and pulldowns are recommended on TMS, TRST, TCK and TDI being
driven from the boundary scan controller into the board. A common value is 1K
ohm. Pullups are preferred for electrical reasons, but pulldowns will usually
also work—it’s a good idea to check the boundary-scan device documentation for
any specific requirements or recommendations. If there is some doubt, we
recommend incorporating both the pullup and pulldown in the design and
installing the one that works best. If the design has several devices with
conflicting pullup/pulldown requirements, the design will need to accommodate
them. Some methods include using tri-stating buffers, multiplexers, multiple TAP
connectors, and jumpers. These methods may also be necessary if the pullup/pulldown
requirements are different between normal operation and boundary scan operation.

Finally, note that if the TCK into the board is buffered (which is often the
case for multi-device systems), it is best if the buffered TCK signals are also
appropriately terminated.