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C- 12 M68040 USER’S MANUAL MOTOROLA
their related encodings. Note that the least significant bit of the instruction (bit 0) is the first
bit to be shifted into the instruction shift register.
Table C-3. IEEE Standard 1149.1A Instructions
Bit 2 Bit 1 Bit 0 Instruction Selected Test Data Register Accessed
0 0 0 EXTEST Boundary Scan
0 0 1 HIGHZ Bypass
0 1 0 SAMPLE/PRELOAD Boundary Scan
1 0 0 CLAMP Bypass
1 1 0 PRIVATE
1 1 1 BYPASS Bypass
C.6.1.1 EXTEST. The external test instruction (EXTEST) selects the boundary scan
register. This instruction also activates one internal function that is intended to protect the
device from potential damage while performing boundary scan operations. EXTEST
asserts internal reset for the MC68040V and MC68EC040V system logic to force a
predictable benign internal state.
C.6.1.2 HIGHZ. The HIGHZ instruction is an optional instruction provided as a Motorola
public instruction to anticipate the need to backdrive output pins during circuit board
testing. The HIGHZ instruction asserts internal system reset, selects the bypass register,
and forces all output and bidirectional pins to the high-impedance state.
Holding TMS high and clocking TCK for at least five rising edges causes the TAP
controller to enter the test-logic-reset state. Using only the TMS and TCK pins and the
capture-IR and update-IR states invokes the HIGHZ instruction. This scheme works
because the value captured by the instruction shift register during the capture-IR state is
identical to the HIGHZ opcode.
C.6.1.3 SAMPLE/PRELOAD. The SAMPLE/PRELOAD instruction provides two separate
functions. First, it provides a means to obtain a sample system data and control signal.
Sampling occurs on the rising edge of TCK in the capture-DR state. The user can observe
the data by shifting it through the boundary scan register to output TDO using the shift-DR
state. Both the data capture and the shift operations are transparent to system operation.
The user must provide some form of external synchronization to achieve meaningful
results since there is no internal synchronization between TCK and BCLK.
The second function of the SAMPLE/PRELOAD instruction is to initialize the boundary
scan register output cells before selecting EXTEST or CLAMP, which is accomplished by
ignoring data being shifted out of TDO while shifting in initialization data. The update-DR
state can then be used to initialize the boundary scan register and ensure that known data
and output state will occur on the outputs after entering the EXTEST or CLAMP
instruction.
C.6.1.4 CLAMP. The CLAMP instruction allows the state of the signals driven from the
MC68040V and MC68EC040V pins to be determined from the boundary scan register,
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