
17-8
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
JTAG Port Architecture
17
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 DSP 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,
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 DSP
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, as required in IEEE
1149.1-1993a.
17.5.1.3 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, as specified by IEEE 1149.1-1993a. This instruction provides
two separate functions. 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
functionality. Data latched into the BSR during the Capture-DR controller state may not
match the drive state of the package signal if the system requiring pull-ups are not present
within the test environment.
Figure 17-3. Bypass Register Diagram
DQ
SHIFT_DR
CLOCK_DR
CK
TDI
To TDO MUX