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JTAG Port Architecture
5685X Digital Signal Controller User Manual, Rev. 4
17-8
Freescale Semiconductor
17.5.1.1 External Test Instruction (EXTEST)
The External Test (EXTEST) instruction enables the BSR between TDI and TDO, including cells
for all digital device signals and associated control signals. The EXTAL and RESET pins, and
any codec pins associated with analog signals, are not included in the BSR path.
In EXTEST, the BSR is capable of scanning user-defined values onto output pins, capturing
values presented to input signals, and controlling the direction and value of bidirectional pins.
EXTEST instruction asserts internal system reset for the DSC system logic during its run in order
to force a predictable internal state while performing external boundary scan operations.
17.5.1.2 Bypass Instruction (BYPASS)
The BYPASS instruction enables the single-bit bypass register between TDI and TDO, illustrated
in
Figure 17-3
.
This creates a Shift register path from TDI to the bypass register and finally to
TDO, circumventing the BSR. This instruction is used to enhance test efficiency by shortening
the overall path between TDI and TDO when no test operation of a component is required. In this
instruction, the system logic is independent of the TAP. When this instruction is selected, the test
logic has no effect on the operation of the on-chip system logic, required in IEEE 1149.1-1993a.
17.5.2 Sample and Preload Instructions (SAMPLE/PRELOAD)
The SAMPLE/PRELOAD instruction enables the BSR between TDI and TDO. When this
instruction is selected, the test logic operation has no effect on the operation of the on-chip
system logic. Nor does it have an effect on the flow of a signal between the system pin and the
on-chip system logic, specified by IEEE 1149.1-1993a. This instruction provides two separate
functions.
1. First, it provides a means to obtain a snapshot of system data and control signals
(SAMPLE). 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 BSR.
In a normal system configuration, many signals require external pull-ups assuring proper
system operation. Consequently, the same is true for the SAMPLE/PRELOAD
Figure 17-3. Bypass Register Diagram
D
Q
SHIFT_DR
CLOCK_DR
CK
TDI
To TDO MUX