8.3.4. Public instructions

Table 8.1 lists the public instructions that are supported.

Table 8.1. Public instructions

Instruction

Binary code

EXTEST

0000

SCAN_N

0010

INTEST

1100

IDCODE

1110

BYPASS

1111

SAMPLE/PRELOAD

0011

RESTART

0100

In this section it is assumed that TDI and TMS are sampled on the rising edge of TCK and all output transitions on TDO occur as a result of the falling edge of TCK.

EXTEST (0000)

The selected scan chain is placed in test mode by the EXTEST instruction. The EXTEST instruction connects the selected scan chain between TDI and TDO.

When the instruction register is loaded with the EXTEST instruction, all the scan cells are placed in their test mode of operation.

In the CAPTURE-DR state, inputs from the system logic and outputs from the output scan cells to the system are captured by the scan cells.

In the SHIFT-DR state, the previously captured test data is shifted out of the scan chain on TDO, while new test data is shifted in on the TDI input. This data is applied immediately to the system logic and system pins.

SCAN_N (0010)

This instruction connects the scan path select register between TDI and TDO.

During the CAPTURE-DR state, the fixed value 10000 is loaded into the register.

During the SHIFT-DR state, the ID number of the desired scan path is shifted into the scan path select register.

In the UPDATE-DR state, the scan register of the selected scan chain is connected between TDI and TDO, and remains connected until a subsequent SCAN_N instruction is issued. On reset, scan chain 3 is selected by default. The scan path select register is five bits long in this implementation, although no finite length is specified.

INTEST (1100)

The selected scan chain is placed in test mode by the INTEST instruction. The INTEST instruction connects the selected scan chain between TDI and TDO.

When the instruction register is loaded with the INTEST instruction, all the scan cells are placed in their test mode of operation.

In the CAPTURE-DR state, the value of the data applied from the core logic to the output scan cells, and the value of the data applied from the system logic to the input scan cells is captured.

In the SHIFT-DR state, the previously captured test data is shifted out of the scan chain on the TDO pin, while new test data is shifted in on the TDI pin.

IDCODE (1110)

The IDCODE instruction connects the device identification register (or ID register) between TDI and TDO. The ID register is a 32-bit register that allows the manufacturer, part number, and version of a component to be determined through the TAP. The ID register is loaded from the TAPID[31:0] input bus. This must be tied to a constant value that represents the unique JTAG IDCODE for the device.

When the instruction register is loaded with the IDCODE instruction, all the scan cells are placed in their normal (system) mode of operation.

In the CAPTURE-DR state, the device identification code is captured by the ID register.

In the SHIFT-DR state, the previously captured device identification code is shifted out of the ID register on the TDO pin, while data is shifted in on the TDI pin into the ID register.

In the UPDATE-DR state, the ID register is unaffected.

BYPASS (1111)

The BYPASS instruction connects a 1‑bit shift register (the bypass register) between TDI and TDO.

When the BYPASS instruction is loaded into the instruction register, all the scan cells are placed in their normal (system) mode of operation. This instruction has no effect on the system pins.

In the CAPTURE-DR state, a logic 0 is captured by the bypass register.

In the SHIFT‑DR state, test data is shifted into the bypass register on TDI and out on TDO after a delay of one TCK cycle. The first bit shifted out is a 0.

The bypass register is not affected in the UPDATE-DR state.

Note

All unused instruction codes default to the BYPASS instruction.

SAMPLE/PRELOAD (0011)

When the instruction register is loaded with the SAMPLE/PRELOAD instruction, all the scan cells of the selected scan chain are placed in the normal mode of operation.

In the CAPTURE-DR state, a snapshot of the signals of the boundary scan is taken on the rising edge of TCK. Normal system operation is unaffected.

In the SHIFT-DR state, the sampled test data is shifted out of the boundary scan on the TDO pin, while new data is shifted in on the TDI pin to preload the boundary scan parallel input latch. This data is not applied to the system logic or system pins while the SAMPLE/PRELOAD instruction is active.

You must use this instruction to preload the boundary scan register with known data prior to selecting INTEST or EXTEST instructions.

RESTART (0100)

This instruction restarts the processor on exit from debug state. The RESTART instruction connects the bypass register between TDI and TDO and the TAP controller behaves as if the BYPASS instruction is loaded. The processor resynchronizes back to the memory system when the RUN‑TEST/IDLE state is entered.

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