8.7.7. Scan chain 1

The primary use for scan chain 1 is for debugging, although it can be used for EXTEST on the data bus. Scan chain 1 is selected via the SCAN_N TAP Controller instruction. Debugging is similar to INTEST, and the procedure described above for scan chain 0 should be followed.

Scan chain length and purpose

This scan chain is 33 bits long—32 bits for the data value, plus the scan cell on the BREAKPT core input. This 33rd bit serves four purposes:

  1. Under normal INTEST test conditions, it allows a known value to be scanned into the BREAKPT input.

  2. During EXTEST test conditions, the value applied to the BREAKPT input from the system can be captured.

  3. While debugging, the value placed in the 33rd bit determines whether ARM7DMT synchronizes back to system speed before executing the instruction. See System‑speed access for further details.

  4. After ARM7DMT has entered debug state, the first time this bit is captured and scanned out, its value tells the debugger whether the core entered debug state due to a breakpoint (bit 33 LOW), or a watchpoint (bit 33 HIGH).

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