MOTOROLA
MC68349 USER’S MANUAL
9- 11
9.4.2 SAMPLE/PRELOAD (001)
The SAMPLE/PRELOAD instruction selects the 141-bit 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.
NOTE
Since 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 will
appear on the outputs when entering the EXTEST instruction.
9.4.3 BYPASS (X1X, 101)
The BYPASS instruction selects the single-bit bypass register as shown in Figure 9-9.
This creates a shift-register path from TDI to the bypass register and, finally, to TDO,
circumventing the 141-bit boundary scan register. This instruction is used to enhance test
efficiency when a component other than the MC68349 becomes the device under test.
1
MUX
1
G1
1 D
C1
CLOCK DR
FROM TDI
0
SHIFT DR
TO TDO
Figure 9-9. 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.
9.4.4 HI-Z (100)
The HI-Z instruction is not included in the IEEE 1149.1 standard. It is provided as a
manufacturer’s optional public instruction to prevent having to backdrive the output pins
during circuit-board testing. When HI-Z is invoked, all output drivers, including the two-
state drivers, are turned off (i.e., high impedance). The instruction selects the bypass
register.