SN54ACT8999, SN74ACT8999

SCANPATH SELECTORS WITH 8BIT BIDIRECTIONAL DATA BUSES

SCANCONTROLLED IEEE STD 1149.1 (JTAG) TAP MULTIPLEXERS

SCAS158D − JUNE 1990 − REVISED DECEMBER 1996

7

POST OFFICE BOX 655303

POST OFFICE BOX 1443

Exit1-IR, Exit2-IR

These are temporary states used to end the shifting process. It is possible to return to the Shift-IR state from

either Exit1-IR or Exit2-IR without recapturing the instruction register. The last shift occurs on the TCK cycle in

which the TAP state changes from Shift-IR to Exit1-IR. TDO changes from the active state to the

high-impedance state on the falling edge of TCK in Exit1-IR.

Pause-IR

The TAP can remain in this state indefinitely. The Pause-IR state suspends and resumes shift operations without

loss of data.

Update-IR

In this state, the latches shadowing the instruction register are updated with the new instruction.

instruction register description

The instruction register (IR) is an 8-bit serial register that outputs control signals to the device. Table 2 lists the

instructions implemented in the ’ACT8999 and the data register selected by each instruction. The MSB of the

IR is an even-parity bit. If the value scanned into the IR during Shift-IR does not contain even parity, an error

signal (IRERR) is generated internally as shown in Table 3. The ’ACT8999 can be configured to output IRERR

via DCO if the TAP enters the Pause-IR state.

During the Capture-IR state, the IR status word is loaded. The IR status word contains information about the

most recently loaded values of the instruction and select registers and the logic level present at the DCI input.

The IR status word is encoded as shown in Table 4. Figure 2 shows the order of scan for the IR.

TDO

Bit 7

TDI or DTDI

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

(LSB)

(MSB)

Figure 2. Instruction-Register Bits and Order of Scan