SN54ABTH18504A, SN54ABTH182504A, SN74ABTH18504A, SN74ABTH182504A

SCAN TEST DEVICES WITH

20-BIT UNIVERSAL BUS TRANSCEIVERS

SCBS165C – AUGUST 1993 – REVISED JULY 1996

9

POST OFFICE BOX 655303

register overview

With the exception of the bypass and device-identification registers, any test register can be thought of as a

serial shift register with a shadow latch on each bit. The bypass and device-identification registers differ in that

they contain only a shift register. During the appropriate capture state (Capture-IR for instruction register,

Capture-DR for data registers), the shift register can be parallel loaded from a source specified by the current

instruction. During the appropriate shift state (Shift-IR or Shift-DR), the contents of the shift register are shifted

out from TDO while new contents are shifted in at TDI. During the appropriate update state (Update-IR or

Update-DR), the shadow latches are updated from the shift register.

instruction register description

The instruction register (IR) is eight bits long and tells the device what instruction is to be executed. Information

contained in the instruction includes the mode of operation (either normal mode, in which the device performs

its normal logic function, or test mode, in which the normal logic function is inhibited or altered), the test operation

to be performed, which of the four data registers is to be selected for inclusion in the scan path during

data-register scans, and the source of data to be captured into the selected data register during Capture-DR.

Table 3 lists the instructions supported by the ’ABTH18504A and ’ABTH182504A. The even-parity feature

specified for SCOPE

™ devices is supported in these devices. Bit 7 of the instruction opcode is the parity bit.

Any instructions that are defined for SCOPE

™ devices but are not supported by these devices default to

BYPASS.

During Capture-IR, the IR captures the binary value 10000001. As an instruction is shifted in, this value is shifted

out via TDO and can be inspected as verification that the IR is in the scan path. During Update-IR, the value

that has been shifted into the IR is loaded into shadow latches. At this time, the current instruction is updated

and any specified mode change takes effect. At power up or in the Test-Logic-Reset state, the IR is reset to the

binary value 10000001, which selects the IDCODE instruction. The IR order of scan is shown in Figure 2.

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

TDO

TDI

Bit 7

Parity

(MSB)

Bit 0

(LSB)

Figure 2. Instruction Register Order of Scan