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Debug Support
5-16
BYPASS (1111)
The BYPASS instruction connects a 1-bit shift register (the bypass register) between
TDI and TDO.
When the BYPASS instruction is loaded into the instruction register, all the scan cells
are placed in their normal (system) mode of operation. This instruction has no effect on
the system pins.
In the CAPTURE-DR state, a logic 0 is captured by the bypass register.
In the SHIFT-DR state, test data is shifted into the bypass register via TDI and out via
TDO after a delay of one TCK cycle. The first bit shifted out will be a zero.
The bypass register is not affected in the UPDATE-DR state.
Note
All unused instruction codes default to the BYPASS instruction.
CLAMP (0101)
This instruction connects a 1-bit shift register (the bypass register) between TDI and
TDO.
When the CLAMP instruction is loaded into the instruction register, the state of all the
output signals is defined by the values previously loaded into the currently-loaded scan
chain.
Note
This instruction should only be used when scan chain 0 is the currently selected scan
chain.
In the CAPTURE-DR state, a logic 0 is captured by the bypass register.
In the SHIFT-DR state, test data is shifted into the bypass register via TDI and out via
TDO after a delay of one TCK cycle. The first bit shifted out will be a zero.
The bypass register is not affected in the UPDATE-DR state.
HIGHZ (0111)
This instruction connects a 1-bit shift register (the bypass register) between TDI and
TDO.
Copyright © 1998, 1999 ARM Limited. All rights reserved.
ARM DDI0145B
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