Chapter 19. IEEE 1149.1 Test Access Port (JTAG)
19-5
JTAG Register Descriptions
19.4.1 JTAG Instruction Shift Register
The MCF5307 IEEE Standard 1149.1 implementation uses a 3-bit instruction-shift register
(IR) without parity. This register transfers its value to a parallel hold register and applies
one of six instructions on the falling edge of TCK when the TAP state machine is in
Update-IR state. To load instructions into the shift portion of the register, place the serial
data on TDI before each rising edge of TCK. The msb of the instruction shift register is the
bit closest to the TDI pin, and the lsb is the bit closest to TDO.
Table 19-2 describes customer-usable instructions.
Table 19-2. JTAG Instructions
Instruction
Class
IR
Description
EXTEST
(EXT)
Required
000
Selects the boundary-scan register. Forces all output pins and bidirectional pins
congured as outputs to the preloaded xed values (with the SAMPLE/PRELOAD
instruction) and held in the boundary-scan update registers. EXTEST can also
congure the direction of bidirectional pins and establish high-impedance states on
some pins. EXTEST 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.
IDCODE
(IDC)
Optional
001
Selects the IDCODE register for connection as a shift path between TDI and TDO.
Interrogates the MCF5307 for version number and other part identication. The
IDCODE register is implemented in accordance with IEEE Standard 1149.1 so the lsb
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 rst bit shifted out after selecting the IDCODE
register is always a logic 1. The remaining 31-bits are also set to xed values. See
IDCODE is the default value in the IR when a JTAG reset occurs by either asserting
TRST or holding TMS high while clocking TCK through at least ve rising edges and
the falling edge after the fth rising edge. A JTAG reset causes the TAP state machine
to enter test-logic-reset state (normal operation of the TAP state machine into the
test-logic-reset state also places 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 in Capture-IR state and a TCK rising edge occurs.
SAMPLE/
PRELOAD
(SMP)
Required
100
Provides two separate functions. It obtains a sample of the system data and control
signals at the MCF5307 input pins and before 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 in the instruction register. Sampled data is observed
by shifting it through the boundary-scan register to TDO by using shift-DR state. The
data capture and shift are transparent to system operation. The users must provide
external synchronization to achieve meaningful results because there is no internal
synchronization between TCK and CLK.
SAMPLE/PRELOAD also initializes the boundary-scan register update cells before
selecting EXTEST or CLAMP. This is done by ignoring data shifted out of TDO 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 external
outputs when an instruction listed above is applied.
HIGHZ
(HIZ)
Optional
101
Anticipates the need to backdrive outputs and protects inputs from random toggling
during board testing. Selects the bypass register, forcing all output and bidirectional
pins into high-impedance.
HIGHZ goes active on the falling edge of TCK in the Update-IR state when instruction
shift register data held is equivalent to octal 5.
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Freescale Semiconductor, Inc.
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