MPC555
/
MPC556
IEEE 1149.1-COMPLIANT INTERFACE (JTAG)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
22-5
The parallel output of the instruction register is reset to all ones in the test-logic-reset
controller state. Note that this preset state is equivalent to the BYPASS instruction.
During the capture-IR controller state, the parallel inputs to the instruction shift register
are loaded with the CLAMP command code.
22.5.1 EXTEST
The external test (EXTEST) instruction selects the 346-bit boundary scan register. EX-
TEST also asserts internal reset for the MPC555 / MPC556 system logic to force a pre-
dictable beginning internal state while performing external boundary scan operations.
By using the TAP, the register is capable of scanning user-defined values into the out-
put buffers, capturing values presented to input pins and controlling the output drive of
three-state output or bi-directional pins. For more details on the function and use of
EXTEST, refer to the scan chain document.
22.5.2 SAMPLE/PRELOAD
The SAMPLE/PRELOAD instruction initializes the boundary scan register output cells
prior to selection of EXTEST. This initialization ensures that known data will appear on
the outputs when entering the EXTEST instruction. The SAMPLE/PRELOAD instruc-
tion also provides a means to obtain a snapshot of system data and control signals.
NOTE
Since there is no internal synchronization between the scan chain
clock (TCK) and the system clock (CLKOUT), the user must provide
some form of external synchronization to achieve meaningful results.
22.5.3 BYPASS
The BYPASS instruction selects the single-bit bypass register as shown in
. This creates a shift register path from TDI to the bypass register and, finally, to TDO,
circumventing the 463-bit boundary scan register. This instruction is used to enhance
test efficiency when a component other than the MPC555 / MPC556 becomes the de-
vice under test.
Table 22-2 Instruction Decoding
Code
B3
B2
B1
B0
Instruction
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
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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