Debug in Depth
ARM DDI 0210C Copyright © 2001, 2004 ARM Limited. All rights reserved. B-17
Scan chain 1: 33 bits.
Each scan chain cell is fairly simple and consists of a serial register and a multiplexor
as shown in Figure B-4. The scan cells perform two basic functions:
•CAPTURE
•SHIFT.
For input cells, the capture stage involves copying the value of the input to the core into
the serial shift register. During shift, this value is output serially. The value applied to
the core from an input cell is either the system input or the contents of the serial register,
and this is controlled by the multiplexor.
For output cells (see Figure B-4), the capture stage involves placing the output value of
a core into the serial shift register. During shift, this value is output serially. The value
applied to the system from an output cell is either the core output, or the contents of the
serial register.
Figure B-4 Output scan cell
All the control signals for the scan cells are generated internally by the TAP controller.
The action of the TAP controller is determined by the current instruction and the state
of the TAP state machine.
Serial data in
Shift
register
Data from core
CAPTURE clock
SHIFT clock
Serial data out
To system pin
Latch
Update clock
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