ITP700 Debug Port Design Guide

Quickswitches can be used to replace bypass jumpers/headers if the circuit is well designed. Many

quickswitch packages contain a low resistance (~7- Ω), low capacitance (~5 pF) analog bypass

switch (FET). Both the jumpers and the quickswitches designs add capacitance and resistance to

the TDI/TDO line for each device bypassed. RC delay can slow down the rise time significantly

when multiple devices are being bypassed. Careless design in this area can easily cause severe

scan frequency degradation or, even worse, scan data corruption. Using quickswitch designs allow

the system designer to actively remove agents from the scan chain by using processor present

signals as enable lines for the quickswitch. The designer must be careful to ensure that there are no

power-up conditions that will corrupt the configuration of the scan chain and therefore render this

primary debug tool inoperative.