Intel 386 - page 520

18-7

JTAG TEST-LOGIC UNIT

18.2.3 Instruction Register (IR)An instruction opcode is clocked serially through the TDI pin into the four-bit instruction register(Figure 18-3). The instruction determines which data register is affected. Table 18-4 lists the in-structions with their binary opcodes, descriptions, and associated registers.

Figure 18-3. Instruction Register (IR)

Instruction Register

IR Reset State

(Using TRST#): 02H

INST3 INST2 INST1 INST0

Bit

Number Bit

Mnemonic Function

3–0 INST3:0 Instruction opcode. At reset (using TRST# , or after 5 TCK cycles with

TMS held low), this field is loaded with 00 10, the opcode for th e IDCODE

instruction. Instructions are shifted into this field serially through the TDI

pin. (Table 18-4 lists the valid instruction opcodes.)

Table 18-4. Test-logic Unit Instructions

Mnemonic Opcode† Description Affected

BYPASS 1111 Bypass on-chip system logic (mandatory in struction).

Used for those components that ar e not being tested. BYPASS

EXTEST 0000 Off-chip circuitry test (mandatory instruction).

Used for testing device interconnections on a board. BOUND

SAMPRE 0001 Sample pins/preload data (mandatory instruction).

Used for controlling (preload) or observing (sample) the signals at

device pins. This test has no effect on system operation. BOUND

IDCODE 0010 ID code test (optional instruction).

Used to identify devices on a board. IDCODE

HIGHZ

1000 High-impedance/On-Circuit Emulation (ONCE) mode test

(optional instruction).

Used to place device pins into their i nactive drive states. Allows

external components to drive signals o nto connections that the

processor normally drives.

BYPASS

†The opcode is the sequence of data bits shifted seria lly into the instruction register (IR) from the TDI input.

The opcodes for EXTEST and BYPASS are mandated by IEEE 1149.1, so they should b e the same for all

JTAG-compliant devices. The remaining opcodes are designer-defined, so they may vary amon g de vice s.

All unlisted opcodes are reserved. Use of reserved opcodes could cause the device to enter reserved

factory-test modes.