B.5.4. Scan path select register

The Scan path select register purpose, bit length, and operating mode description is given below:


Changes the current active scan chain.


5 bits.

Operating mode

SCAN_N as the current instruction in the SHIFT-DR state selects the scan path select register as the serial path between DBGTDI and DBGTDO.

During the CAPTURE-DR state, b10000 is loaded into this register. This value is shifted out during SHIFT-DR (least significant bit first), while a new value is shifted in (least significant bit first). During the UPDATE-DR state, the value in the scan path select register selects a scan chain to become the currently active scan chain. All further instructions such as INTEST then apply to that scan chain.

The currently selected scan chain changes only when a SCAN_N instruction is executed, or when a reset occurs. On reset, scan chain 3 is selected as the active scan chain.


Scan chain 3 is not implemented in the ARM7EJ-S (Rev 1) processor, but all the control signals are provided at the macrocell boundary. This enables you to design your own boundary scan chain wrapper if required.

The number of the currently-selected scan chain is reflected on the DBGSCREG[4:0] output bus. You can use the TAP controller to drive external chains in addition to those within the ARM7EJ-S processor. The external scan chain is connected between DBGSDIN and DBGSDOUT, and must be assigned a number. The control signals are derived from DBGSCREG[4:0], DBGIR[4:0], DBGTAPSM[3:0] and the clock, CLK, and clock enable, DBGTCKEN.

Table B.2 shows the scan chain numbers allocated by ARM Limited.

Table B.2. Scan chain number allocation

Scan chain number







EmbeddedICE-RT programming


External boundary scan signals for user-implemented scan chain





The scan chain present between DBGSDIN and DBGSDOUT is connected between DBGTDI and DBGTDO whenever scan chain 3 is selected, or when any unassigned scan chain number is selected. If there is more than one external scan chain, a multiplexor must be built externally to apply the desired scan chain output to DBGSDOUT. The multiplexor can be controlled by decoding DBGSCREG[4:0].

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