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IEEE 1149.1 Test (JTAG) and Debug Pipe Control Modes
9-6
M68060 USER’S MANUAL
MOTOROLA
through at least five rising edges and the falling edge after the fifth rising edge. A JTAG reset
will cause the TAP state machine to enter the test-logic-reset state (normal operation of the
TAP state machine into the test-logic-reset state will also result in placing the default value
of $5 into the instruction register). The shift register portion of the instruction register is
loaded with the default value of $5 when in the Capture-IR state and a rising edge of TCK
occurs.
9.1.2.6 CLAMP.
The CLAMP instruction selects the bypass register while simultaneously
forcing all output pins and bidirectional pins configured as outputs, to the fixed values that
are preloaded and held in the boundary scan update registers. This instruction enhances
test efficiency by reducing the overall shift path to a single bit (the bypass register) while con-
ducting an EXTEST type of instruction through the boundary scan register. The CLAMP
instruction becomes active on the falling edge of TCK in the update-IR state when the data
held in the instruction shift register is equivalent to $6.
It is recommended that the boundary scan register bit equivalent to the RSTI pin be pre-
loaded with the assert value for system reset prior to application of the CLAMP instruction.
This will ensure that CLAMP asserts the internal reset for the MC68060 system logic to force
a predictable benign internal state while isolating all pins from signals generated external to
the part. However, if it is desired to hold the processor in the LPSTOP state when applying
the CLAMP instruction, do not preload the boundary scan register bit equivalent to the RSTI
pin with an assert value because this action forces the processor out of the LPSTOP state.
9.1.2.7 HIGHZ.
The HIGHZ instruction is an IEEE 1149.1 option that is provided as a Motor-
ola public instruction designed to anticipate the need to backdrive the output pins and pro-
tect the input pins from random toggling during circuit board testing. The HIGHZ instruction
selects the bypass register, forces all output and bidirectional pins to the high-impedance
state, and isolates all input signal pins except for CLK, IPL, and RSTI. The HIGHZ instruction
becomes active on the falling edge of TCK in the update-IR state when the data held in the
instruction shift register is equivalent to $7.
It is recommended that the boundary scan register bit equivalent to the RSTI pin be pre-
loaded with the assert value for system reset prior to application of the HIGHZ instruction.
This will ensure that HIGHZ asserts the internal reset for the MC68060 system logic to force
a predictable benign internal state while isolating all pins from signals generated external to
the part.
9.1.2.8 BYPASS.
The BYPASS instruction selects the single-bit bypass register, creating a
single bit shift register path from the TDI pin to the bypass register to the TDO pin. This
instruction enhances test efficiency by reducing the overall shift path when a device other
than the MC68060 becomes the device under test on a board design with multiple chips on
the overall IEEE-1149.1-defined boundary scan chain. The bypass register has been imple-
mented in accordance with IEEE 1149.1 so that the shift register stage is set to logic zero
on the rising edge of TCK following entry into the capture-DR state. Therefore, the first bit
to be shifted out after selecting the bypass register is always a logic zero (this is to differen-
tiate a part that supports an idcode register from a part that supports only the bypass regis-
ter).
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