IEEE 1149.1 Test Access Port Controller (JTAGC)
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 35-11
35.4.4.4 EXTEST—External Test Instruction
EXTEST selects the boundary scan register as the shift path between TDI and TDO. It allows testing of
off-chip circuitry and board-level interconnections by driving preloaded data contained in the boundary
scan register onto the system output pins. Typically, the preloaded data is loaded into the boundary scan
register using the SAMPLE/PRELOAD instruction before the selection of EXTEST. EXTEST asserts the
internal system reset for the MCU to force a predictable internal state while performing external boundary
scan operations.
35.4.4.5 HIGHZ Instruction
HIGHZ selects the bypass register as the shift path between TDI and TDO. While HIGHZ is active, all
output drivers are placed in an inactive drive state (for example, high impedance). HIGHZ also asserts the
internal system reset for the MCU to force a predictable internal state.
35.4.4.6 IDCODE Instruction
IDCODE selects the 32-bit device identification register as the shift path between TDI and TDO. This
instruction allows interrogation of the MCU to determine its version number and other part identification
data. IDCODE is the instruction placed into the instruction register when the JTAGC is reset.
35.4.4.7 SAMPLE Instruction
The SAMPLE instruction obtains a sample of the system data and control signals present at the MCU input
pins and immediately before the boundary scan register cells at the output pins. This sampling occurs on
the rising edge of TCK in the capture-DR state when the SAMPLE instruction is active. The sampled data
is viewed by shifting it through the boundary scan register to the TDO output during the Shift-DR state.
There is no defined action in the update-DR state. Both the data capture and the shift operation are
transparent to system operation.
35.4.4.8 SAMPLE/PRELOAD Instruction
The SAMPLE/PRELOAD instruction has two functions:
• First, the SAMPLE portion of the instruction obtains a sample of the system data and control
signals present at the MCU input pins and immediately before the boundary scan register cells at
the output pins. This sampling occurs on the rising edge of TCK in the capture-DR state when the
SAMPLE/PRELOAD instruction is active. The sampled data is viewed by shifting it through the
boundary scan register to the TDO output during the shift-DR state. The data capture and the shift
operation are transparent to system operation.
• Secondly, the PRELOAD portion of the instruction initializes the boundary scan register cells
before selecting the EXTEST or CLAMP instructions to perform boundary scan tests. This is
achieved by shifting in initialization data to the boundary scan register during the shift-DR state.
The initialization data is transferred to the parallel outputs of the boundary scan register cells on
the falling edge of TCK in the update-DR state. The data is applied to the external output pins by
the EXTEST or CLAMP instruction. System operation is not affected.
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