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Part VI. Debug and Test
38.4 Instruction Register
The MPC860 TAP implementation includes the public instructions EXTEST,
SAMPLE/PRELOAD, BYPASS and CLAMP. An optional public instruction (HI-Z)
provides the capability for disabling all device output drivers. The MPC860 TAP
implements a 4-bit instruction register (no parity). The 4-bit TAP instructions are executed
during the update-IR controller state. The four instruction bits select the Þve unique
instructions listed in Table 38-1.
B3
0
0
0
0
0
Note: B0 (lsb) is shifted Þrst.
The instruction register is reset to all ones in the test-logic-reset controller state. Note that
the reset state is equivalent to the BYPASS instruction. During the capture-IR controller
state, the inputs to the instruction shift register are loaded with the CLAMP command code.
38.4.1 EXTEST
The external test (EXTEST) instruction enables the 475-bit boundary scan register.
EXTEST also asserts an internal soft reset for the MPC860 system logic to force a known
beginning internal state while performing external boundary scan operations. Through the
TAP, the user is capable of scanning user-deÞned values into the output buffers, capturing
values presented to input pins, and controlling the output drive of three-stateable output or
bidirectional pins. For more details on the function and use of EXTEST, refer to the IEEE
1149.1 standard.
38.4.2 SAMPLE/PRELOAD
The SAMPLE/PRELOAD instruction initializes the boundary scan register output cells
before the boundary scan register is enabled by the EXTEST command. This initialization
ensures that known data will appear on the outputs when entering the EXTEST instruction.
If the SAMPLE/PRELOAD command was not issued prior to the EXTEST command the
output signals will go to a random state when the boundary scan register is enabled and
takes control of the output buffer. The SAMPLE/PRELOAD command ensures that the
boundary scan register samples the current state of the output signal before it takes control
38-6
Table 38-1. Instruction Register Decoding
Code
B2
B1
0
0
0
0
X
1
1
0
1
0
MPC860 PowerQUICC UserÕs Manual
Instruction
B0
0
EXTEST
1
SAMPLE/PRELOAD
X
BYPASS
0
HIÑZ
1
CLAMP and BYPASS
MOTOROLA
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