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21-8
MCF5272MCF5272 User’s Manual
MOTOROLA
Restrictions
Figure 21-8 shows the structure of the bypass register.
Figure 21-8. Bypass Register
21.6 Restrictions
The control afforded by the output enable signals using the boundary scan register and the
EXTEST instruction requires a compatible circuit board test environment to avoid
congurations that could damage the device. The user must avoid situations in which the
MCF5272 output drivers are enabled into actively driven networks. Overdriving the TDO
driver when it is active is not recommended.
0010
SAMPLE/
PRELOAD
The SAMPLE/PRELOAD instruction selects the boundary scan register and provides two
separate functions. First, it provides a means to obtain a snapshot of system data and
control signals. The snapshot occurs on the rising edge of TCK in the capture-DR controller
state. The data can be observed by shifting it transparently through the boundary scan
register.
Because there is no internal synchronization between the IEEE 1149.1 clock (TCK) and the
system clock (CLKOUT), the user must provide some form of external synchronization to
achieve meaningful results.
The second function of SAMPLE/PRELOAD is to initialize the boundary scan register output
bits prior to selection of EXTEST. This initialization ensures that known data appears on the
outputs when entering the EXTEST instruction.
1001
HI-Z
The HI-Z instruction anticipates the need to backdrive the output pins and protect the input
pins from random toggling during circuit board testing. The HIGHZ instruction selects the
bypass register, forcing all output and bidirectional pins to the high-impedance state.
The HI-Z instruction goes active on the falling edge of TCK in the update-IR state when the
data held in the instruction shift register is equivalent to octal 5.
1100
CLAMP
When the CLAMP instruction is invoked, the boundary scan multiplexer control signal
EXTEST is asserted, and the BYPASS register is selected. CLAMP should be invoked after
valid data has been shifted into the boundary scan register, e.g., by SAMPLE/PRELOAD.
CLAMP allows static levels to be presented at the MCF5272 output and bidirectional pins,
like EXTEST, but without the shift latency of the boundary scan register from TDI to TDO.
1101
Reserved
1111
BYPASS
The BYPASS instruction selects the single-bit bypass register as shown in
Figure 21-8. This
creates a shift register path from TDI to the bypass register and, nally, to TDO,
circumventing the boundary scan register. This instruction is used to enhance test efciency
when a component other than the MCF5272 becomes the device under test. 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 rst bit to be
shifted out after selecting the bypass register is always a logic zero.
Table 21-2. Instructions (Continued)
B[3:0]
Instruction
Description
1
MUX
1
G1
1D
C1
CLOCK DR
FROMTDI
0
SHIFT DR
TOTDO