MPC555
/
MPC556
IEEE 1149.1-COMPLIANT INTERFACE (JTAG)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
22-6
Figure 22-4 Bypass Register
When the bypass register is selected by the current instruction, the shift register stage
is set to a logic zero on the rising edge of TCK in the capture-DR controller state.
Therefore, the first bit to be shifted out after selecting the bypass register will always
be a logic zero.
22.5.4 CLAMP
The CLAMP instruction selects the single-bit bypass register as shown in
, and the state of all signals driven from system output pins is completely defined by
the data previously shifted into the boundary scan register (e.g., using the SAMPLE/
PRELOAD instruction).
22.5.5 HI-Z
The HI-Z instruction is provided as a manufacturer’s optional public instruction to pre-
vent having to backdrive the output pins during circuit-board testing. When HI-Z is in-
voked, all output drivers, including the two-state drivers, are turned off (i.e., high
impedance). The instruction selects the bypass register.
22.6 Restrictions
The MPC555 / MPC556 provides flexible control of external signals using the bound-
ary scan register and EXTEST or CLAMP instructions. As a result, the circuit board
test environment must be designed to avoid signal contention which may result in de-
vice destruction.
22.7 Low-Power Stop Mode
The MPC555 / MPC556 features a low-power stop mode. The interaction of the scan
chain interface with low-power stop mode is as follows:
1. The TAP controller must be in the test-logic-reset state to either enter or remain
in the low-power stop mode. Leaving the TAP controller in the test-logic-reset
state negates the ability to achieve low-power, but does not otherwise affect de-
vice functionality.
1
1
MUX
G1
C
D
TO TDO
FROM TDI
0
SHIFT DR
CLOCK DR
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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