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TMS
TRSTB
TAp Controller
State Machine
(16 States)
Instruction Decode
&
Control Logic
RDI
Mux
Instruction Register
Boundary-Scan Register
(Data Register)
Mux
TDO
Bypass
Register
Internal
Register
I/O Registers
User Defined Data Register
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Microprocessors and Application Specific Integrated Circuits (ASICs) pose many design
challenges, one problem being the accessibility of test points. The Joint Test Access Group
(JTAG) formed in response to this challenge, resulting in IEEE standard 1149.1, the
Standard Test Access Port and Boundary Scan Architecture.
The JTAG boundary scan test methodology allows complete observation and control of the
boundary pins of a JTAG-compatible device through JTAG software. A Test Access Port
(TAP) controller works in concert with the Instruction Register (IR), which allow users to run
three required tests along with several user-defined tests.
JTAG tests allow users to reduce system debug time, reuse test platforms and tools, and reuse
subsystem tests for fuller verification of higher level system elements.
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