18-5
JTAG TEST-LOGIC UNIT
For example, assume that the TAP controller is in its test-logic-reset state and you want it to startshifting the contents of the instruction register from TDI toward TDO (Shi ft-IR state). This statechange requires a zero, two ones, then two zeros on TMS at the next five rising edges of TCK(see Table 18-3). By supplying the proper values in the correct sequence, you can move the TAPcontroller from any state to any other state.
Select-IR-Scan Test-logic is idle and the instruction register retains
its previous state. Capture-IR Test-Logic-Reset
Capture-IR Loads the SAMPLE/PRELOAD instru ction
instruction (0001) into the instruction register. Shift-IR Exit1-IR
Shift-IR Shifts the SAMPLE/PRELOAD instructi on one
stage toward TDO while shifting the new instruction
in from TDI on each rising edge of TCK. Shift-IR Exit1-IR
Exit1-IR The instruction register retains its pre vious state. Pause-IR Update-IR
Pause-IR The instruction register tempor arily stops shifting
and retains its previous st ate. Pause-IR Exit2-IR
Exit2-IR The instruction register retains i ts previous state. Shift-IR Update-IR
Update-IR Latches the current i nstruction onto the instruction
register’s parallel output on the falling edge of TCK. Run-Test/Idle Select-DR-Scan
Table 18-3. Example TAP Controller State Selections
Initial State TMS Value at TCK Rising Edge Resulting State
Test-Logic-
Reset 0Run-Test/Idle
Run-Test/Idle 1Select-DR-Scan
Select-DR-
Scan 1Select-IR-Scan
Select-IR-
Scan 0Capture-IR
Capture-IR 0Shift-IR
Table 18-2. TAP Controller State Descriptions (Sheet 2 of 2)
State Description
Next State
(on TCK Rising Edge)
TMS = 0 TMS = 1
NOTE: By convention, the abbreviation
DR
stands for
data register
, and
IR
stands for
instruction register
.
The
active register
is the register that the current instruction has placed in the serial path between
TDI and TDO.