TDO Output of the JTAG circuitry

Question: When Boundary Scan input signals are applied to TDI, TCK and TMS pins of the chip, no output signal can be detected from TDO pin of the chip. What is the problem?

 

Answer:

The TDO by default will be in a Hi-Z mode except during the Shift-IR and Shift-DR states. The former state puts out the data from the Instruction register and also the data from the TDI. The latter state puts out the data from the data register and the data from the TDI. In all the other states, TDO will be in a Hi-Z state. For further information regarding the complete operation of JTAG please refer to the JTAG IEEE 1149.1 specification.