Boundary Scan Test Interface
Élan™SC520 Microcontroller User’s Manual 25-3
25.4.1 Instruction Register
The Instruction register is a 4-bit register that allows instructions to be serially shifted into
the device. The instruction determines the test to be executed and the data register to be
accessed. The least significant bit is nearest the JTAG_TDO output. When the test access
port (TAP) controller is reset, the Instruction register is loaded with the default instruction
IDCODE.
25.4.1.1 Implemented Instructions
The ÉlanSC520 microcontroller supports all three mandatory boundary-scan instructions:
BYPASS, SAMPLE/PRELOAD, and EXTEST, along with three additional instructions:
IDCODE, HIGHZ and DEBUG.
Table 25-2 shows the test access port (TAP) instructions that are supported on the
ÉlanSC520 microcontroller.
25.4.1.1.1 EXTEST Instruction
The instruction code is 0000b. The EXTEST instruction allows testing of circuitry external
to the component package, typically board interconnects. It does so by driving the values
loaded into the microcontroller’s Boundary Scan register out on to the output pins
corresponding to each boundary scan cell. It then captures the values on the
microcontroller’s input pins to be loaded into their corresponding Boundary Scan register
locations. I/O pins are selected as input or output, depending on the value loaded into their
control setting locations in the Boundary Scan register. Values shifted into input latches in
the Boundary Scan register are never used by the internal logic of the ÉlanSC520
microcontroller.
Note: After using the EXTEST instruction, the ÉlanSC520 microcontroller should be reset
before normal (non-boundary scan) use to ensure the state of the ÉlanSC520
microcontroller.
25.4.1.1.2 SAMPLE/PRELOAD Instruction
The instruction code is 0001b. The SAMPLE/PRELOAD instruction performs two functions.
■ When the TAP controller is in the Capture-DR state, the SAMPLE/PRELOAD instruction
allows a “snapshot” of the normal operation of the ÉlanSC520 microcontroller without
interfering with that normal operation. The instruction causes Boundary Scan register
cells associated with outputs to sample the value being driven by the microcontroller. It
causes the cells associated with inputs to sample the value being driven into the
microcontroller. On both outputs and inputs, the sampling occurs on the rising edge of
JTAG_TCK.
Table 25-2 Test Access Port Instruction Set
Instruction IR3–IR0
EXTEST 0000
SAMPLE/PRELOAD 0001
IDCODE 0010
HIGHZ 0011
Reserved 0100
DEBUG 0101
Reserved 0110
±
1110
BYPASS 1111
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