IEEE 1149.1 Test Access Port
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
54-7
a component other than the MPC885 becomes the device under test. The BYPASS instruction selects the
single-bit bypass register as shown in
Figure 54-7
.
Figure 54-7. 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 is always a logic zero.
54.4.4
CLAMP
The CLAMP instruction selects the single-bit bypass register as shown in
Figure 54-7
above, and the state
of all signals driven from the system output pins is defined by the data currently contained in the boundary
scan register.
54.4.5
HI–Z
The HI-Z instruction is provided as a manufacturer’s optional public instruction to avoid back driving the
output pins during circuit-board testing. When the HI-Z instruction is invoked all output drivers, including
the two-state drivers, are placed in a high impedance state. The HI-Z instruction also selects the bypass
register.
54.5
TAP Usage Considerations
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 MPC885 output drivers are enabled into
actively driven networks.
54.6
Recommended TAP Configuration
To ensure that the scan chain test logic is kept transparent to the system logic during normal operation, the
TAP should be forced into the test-logic-reset controller state by keeping TRST or TMS continuously
asserted.
If TRST is not properly terminated, it may be mistaken as set (during slow power ramp) and the chip will
appear to be locked. The TRST signal must be configured as follows to reset the scan chain logic:
•
If the TAP is never used, connect TRST to ground.
•
If the TAP is used, connect TRST to PORESET.
1
1
MUX
G1
C
D
To TDO
From TDI
0
Shift Dr
Clock Dr
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