
IEEE 1149.1 Test Access Port (JTAG)
15-4
MCF5206 USERS MANUAL Rev 1.0
MOTOROLA
also configure the direction of bidirectional pins and establish high-impedance states on
some pins. The EXTEST 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 octal 0.
15.3.1.2 IDCODE. The IDCODE instruction selects the 32-bit IDcode register for
connection as a shift path between the TDI pin and the TDO pin. This instruction lets you
interrogate the MCF5206 to determine its version number and other part identification
data. The IDcode register has been implemented in accordance with IEEE 1149.1 so that
the least significant bit of the shift register stage is set to logic 1 on the rising edge of TCK
following entry into the capture-DR state. Therefore, the first bit to be shifted out after
selecting the IDcode register is always a logic 1. The remaining 31-bits are also set to
fixed values (see 15.3.2 IDcode Register) on the rising edge of TCK following entry into
the capture-DR state.
The IDCODE instruction is the default value placed in the instruction register when a
JTAG reset is accomplished by either asserting TRST or holding TMS high while clocking
TCK through at least five rising edges and the falling edge after the fifth rising edge. A
JTAG reset causes the TAP state machine to enter the test-logic-reset state (normal
operation of the TAP state machine into the test-logic-reset state also results in placing
the default value of octal 1 into the instruction register). The shift register portion of the
instruction register is loaded with the default value of octal 1 when in the Capture-IR state
and a rising edge of TCK occurs.
15.3.1.3 SAMPLE/PRELOAD INSTRUCTION. The SAMPLE/PRELOAD instruction
provides two separate functions. First, it obtains a sample of the system data and control
signals present at the MCF5206 input pins and just prior to the boundary scan cell at the
output pins. This sampling occurs on the rising edge of TCK in the capture-DR state when
an instruction encoding of octal 4 is resident in the instruction register. You can observe
this sampled data by shifting it through the boundary-scan register to the output TDO by
using the shift-DR state. Both the data capture and the shift operation are transparent to
system operation. You are responsible for providing some form of external
synchronization to achieve meaningful results because there is no internal
synchronization between TCK and the system clock, CLK.
The second function of the SAMPLE/PRELOAD instruction is to initialize the boundary
scan register update cells before selecting EXTEST or CLAMP. This is achieved by
ignoring the data being shifted out of the TDO pin while shifting in initialization data. The
update-DR state in conjunction with the falling edge of TCK can then transfer this data to
the update cells. This data is applied to the external output pins when one of the
instructions listed above is applied.
15.3.1.4 HIGHZ INSTRUCTION. The HIGHZ instruction anticipates the need to
backdrive the output pins and protect the input pins from random toggling during circuit
board testing. The HIGHZ instruction selects the bypass register, forcing all output and
bidirectional pins to the high-impedance state.
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