Scan Chain Test Access Port
8-10
MC68360 USER’S MANUAL
MOTOROLA
8.4 INSTRUCTION REGISTER
The QUICC JTAG implementation includes the public instructions (EXTEST, SAMPLE/
PRELOAD, and BYPASS), and also supports the CLAMP instruction. One additional public
instruction (HI-Z) provides the capability for disabling all device output drivers. The QUICC
includes a 3-bit instruction register without parity consisting of a shift register with three par-
allel outputs. Data is transferred from the shift register to the parallel outputs during the
update-IR controller state. The three bits are used to decode the five unique instructions
listed in Table 8-3.
The parallel output of the instruction register is reset to all ones in the test-logic-reset con-
troller state. Note that this preset state is equivalent to the BYPASS instruction.
During the capture-IR controller state, the parallel inputs to the instruction shift register are
loaded with the CLAMP command code.
8.4.1 EXTEST
The external test (EXTEST) instruction selects the 196-bit boundary scan register. EXTEST
also asserts internal reset for the QUICC system logic to force a predictable benign internal
state while performing external boundary scan operations.
By using the TAP, the register is capable of a) scanning user-defined values into the output
buffers, b) capturing values presented to input pins, c) controlling the direction of bidirec-
tional pins, and d) controlling the output drive of three-stateable output pins. For more details
on the function and use of EXTEST, refer to the scan chaindocument.
8.4.2 SAMPLE/PRELOAD
The SAMPLE/PRELOAD instruction initializes the boundary scan register output cells prior
to selection of EXTEST. This initialization ensures that known data will appear on the out-
puts when entering the EXTEST instruction. The SAMPLE/PRELOAD instruction also pro-
vides a means to obtain a snapshot of system data and control signals. In the case of the
QUICC, this functionality is not supported.
NOTE
Since there is no internal synchronization between the scan
chain clock (TCK) and the system clock (CLKO1), the user must
Table 8-3. Instruction Decoding
Code
B2
B1
B0
Instruction
0
0
0
EXTEST
0
0
1
SAMPLE/PRELOAD
X
1
X
BYPASS
1
0
0
HI-Z
1
0
1
CLAMP and BYPASS