The TCK and
TMS signals go to all devices in the chain;
consequently, their signal quality is important. For example,
TCK should transition monotonically at all
receivers to ensure proper JTAG functionality and must be properly
terminated. The quality of TCK can limit the maximum
frequency for reliable JTAG configuration.
Additionally, if the chain is large (three
devices or more), TMS and TCK should be
buffered to ensure that they have sufficient drive strength at all
receivers, and the voltage at logic High must be compatible with all devices
in the chain.
When interfacing to devices from other
manufacturers, optional JTAG signals can be present (such as
TRST and enables) and might need to be driven.