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19
19.4 Basic Operation of JTAG
19.4.1 Outline of JTAG Operation
The instruction and data registers basically are accessed in the following three operations, which
are performed based on state transitions of the TAP controller. The TAP controller changes state
according to JTMS input, and generates control signals required for operation in each state.
• Capture operation
The result of boundary-scan test or the fixed data defined for each register is sampled. As
register operation, the input data is loaded into the shift register stage.
• Shift operation
The register is accessed from outside through the boundary-scan path. The sampled value is
output to an external device at the same time data is set from outside. As register operation, bits
are shifted right between each shift register stage.
• Update operation
The data set from outside during shift is driven. As register operation, the value set in the shift
register stage is transferred to the parallel output stage.
The JTAG interface undergoes transitions of internal state depending on JTMS input as it performs
the following two operations. In either case, the operation basically is performed in order of Capture
→ Shift → Update.
• IR path sequence
Instruction code is set in the instruction register to select the data register to be operated on in the
subsequent DR path sequence.
• DR path sequence
The selected data register is operated on to inspect or set data.
19-6
19.4 Basic Operation of JTAG
JTAG
Ver.0.10
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