Holds the currently active scan chain number.

5 bits.

After SCAN_N has been selected as the current instruction, when in Shift-DR state, the shift section of the Scan Chain Select Register is selected as the serial path between DBGTDI and DBGTDO. At the Capture-DR state, the binary value b10000 is loaded into this shift section. This is shifted out during Shift-DR (least significant bit first), while a new value is shifted in (least significant bit first). At the Update-DR state, the value in the shift section is loaded into the Scan Chain Select Register to become the current active scan chain. All further instructions such as INTEST then apply to that scan chain. The currently selected scan chain only changes when a SCAN_N or ITRsel instruction is executed, or a DBGTAP reset occurs. On DBGTAP reset, scan chain 3 is selected as the active scan chain.

Figure 14.6 shows the bit order in the Scan Chain Select Register.