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MOTOROLA M68040 USER’S MANUAL 6- 3
6.2 INSTRUCTION SHIFT REGISTER
The M68040 IEEE standard 1149.1A implementation includes a 3-bit instruction shift
register without parity. The register shifts one of eight instructions, which can either select
the test to be performed or access a test data register, or both. Data is transferred from
the instruction shift register to latched decoded outputs during the update-IR state. The
instruction shift register is reset to all ones in the TAP controller test-logic-reset state,
which is equivalent to selecting the BYPASS instruction. During the capture-IR state, the
binary value 001 is loaded into the parallel inputs of the instruction shift register.
The M68040 IEEE standard 1149.1A implementation includes three mandatory public
instructions (BYPASS, SAMPLE/PRELOAD, and EXTEST) and four manufacturer's public
instructions. The four manufacturer’s public instructions provide the capability to disable all
device output drivers, operate the device in a BYPASS configuration without a system
clocking requirement, and select one of two output drive capabilities on a pin-by-pin basis.
The M68040 implementation does not support the optional standard public instructions.
Table 6-1 lists the three bits used in the instruction shift register to decode the instructions
and 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 6-1. 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
0 1 1 DRVCTL.T Boundary Scan
1 0 0 SHUTDOWN Bypass
1 0 1 PRIVATE Bypass
1 1 0 DRVCTL.S Boundary Scan
1 1 1 BYPASS Bypass
EXTEST, HIGHZ, DRVCTL.T, SHUTDOWN, and PRIVATE have a PCLK and BCLK
restriction. Failure to comply with this restriction results in potential internal damage to the
device (see 6.4 Restrictions). Once the restriction is complied with, SHUTDOWN,
EXTEST, HIGHZ, and DRVCTL.T can be entered regardless of order. The system clocks
(PCLK and BCLK) must be kept running while in the SAMPLE/PRELOAD, DRVCLT.S,
and BYPASS instructions. Failure to do so could result in potential internal damage to the
device.
6.2.1 EXTEST
The external test instruction (EXTEST) selects the 184-bit boundary scan register. This
instruction also activates two internal functions that are intended to protect the device from
potential damage while performing boundary scan operations.
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