IEEE 1149.1 Test Access Port
MPC885 PowerQUICC Family Reference Manual, Rev. 2
54-6
Freescale Semiconductor
instructions are executed during the update-IR controller state. The four instruction bits select the five
unique instructions listed in
Table 54-1
.
The instruction register is reset to all ones in the test-logic-reset controller state. During the capture-IR
controller state, the inputs to the instruction shift register are loaded with the CLAMP command code.
54.4.1
EXTEST
The external test (EXTEST) instruction enables the 475-bit boundary scan register. EXTEST also asserts
an internal soft reset for the MPC885 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-defined values into the output buffers, capturing values presented to input pins, and controlling the
output drive of three-statable output or bidirectional pins. For more details on the function and use of
EXTEST, refer to the IEEE 1149.1 standard.
54.4.2
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 of the associated output buffer. The SAMPLE/PRELOAD instruction also provides an
opportunity to obtain a snapshot of system data and control signals.
Note that there is no internal synchronization between the TCK and CLKOUT; the user must provide some
form of external synchronization between the JTAG operation at TCK frequency and the system operation
CLKOUT frequency to achieve meaningful results.
54.4.3
BYPASS
The BYPASS instruction creates a shift register path from TDI through the bypass register to TDO,
circumventing the 475-bit boundary scan register. This instruction is used to enhance test efficiency when
Table 54-1. Instruction Register Decoding
Code
Instruction
B3
B2
B1
B0
0
0
0
0
EXTEST
0
0
0
1
SAMPLE/PRELOAD
0
X
1
X
BYPASS
0
1
0
0
HI—Z
0
1
0
1
CLAMP and BYPASS
Note:
B0 (lsb) is shifted first.
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