10-12
MC68341 USER’S MANUAL
MOTOROLA
1
MUX
1
G1
1 D
C1
CLOCK DR
FROM TDI
0
SHIFT DR
TO TDO
Figure 10-9. 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.
10.4.4 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.
10.5 MC68341 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
device-destructive configurations. The user must avoid situations in which the MC68341
output drivers are enabled into actively driven networks. Overdriving the TDO driver when
it is active is not recommended.
The MC68341 includes on-chip circuitry to detect the initial application of power to the
device. Power-on reset (POR), the output of this circuitry, is used to reset both the system
and IEEE 1149.1 logic. The purpose for applying POR to the IEEE 1149.1 circuitry is to
avoid the possibility of bus contention during power-on. The time required to complete
device power-on is power-supply dependent. However, the IEEE 1149.1 TAP controller
remains in the test-logic-reset state while POR is asserted. The TAP controller does not
respond to user commands until POR is negated.
The MC68341 features a low-power stop mode that uses a CPU instruction called
LPSTOP. The interaction of the IEEE 1149.1 interface with LPSTOP mode is as follows:
1. Leaving the TAP controller test-logic-reset state negates the ability to achieve
minimal power consumption, but does not otherwise affect device functionality.
2. The TCK input is not blocked in LPSTOP mode. To consume minimal power, the
TCK input should be externally connected to VCC or ground.
3. The TMS and TDI pins include on-chip pullup resistors. In LPSTOP mode, these two
pins should remain either unconnected or connected to VCC to achieve minimal
power consumption.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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