SN54ABT18502

SCAN TEST DEVICE WITH

18-BIT REGISTERED BUS TRANSCEIVER

SCBS109C – AUGUST 1992 – REVISED AUGUST 1994

4–5

POST OFFICE BOX 655303

test architecture

Serial-test information is conveyed by means of a 4-wire test bus or TAP, that conforms to IEEE Standard

1149.1-1990. Test instructions, test data, and test control signals all are passed along this serial-test bus. The

TAP controller monitors two signals from the test bus, namely TCK and TMS. The TAP controller extracts the

synchronization (TCK) and state control (TMS) signals from the test bus and generates the appropriate on-chip

control signals for the test structures in the device. Figure 1 shows the TAP-controller state diagram.

The TAP controller is fully synchronous to the TCK signal. Input data is captured on the rising edge of TCK, and

output data changes on the falling edge of TCK. This scheme ensures that data to be captured is valid for fully

one-half of the TCK cycle.

The functional block diagram illustrates the IEEE Standard 1149.1-1990 4-wire test bus and boundary-scan

architecture and the relationship among the test bus, the TAP controller, and the test registers. As illustrated,

the device contains an 8-bit instruction register and four test-data registers: an 84-bit boundary-scan register,

a 21-bit boundary-control register, a 1-bit bypass register, and a 32-bit device-identification register.

Test-Logic-Reset

Run-Test /Idle

Select-DR-Scan

Capture-DR

Shift-DR

Exit1-DR

Pause-DR

Update-DR

TMS = L

TMS = L

TMS = H

TMS = L

TMS = H

TMS = H

TMS = L

TMS = H

TMS = L

TMS = L

TMS = H

TMS = L

Exit2-DR

Select-IR-Scan

Capture-IR

Shift-IR

Exit1-IR

Pause-IR

Update-IR

TMS = L

TMS = L

TMS = H

TMS = L

TMS = H

TMS = H

TMS = L

TMS = H

TMS = L

Exit2-IR

TMS = L

TMS = H

TMS = H

TMS = H

TMS = L

TMS = H

TMS = L

TMS = H

TMS = H

TMS = H

TMS = L

Figure 1. TAP-Controller State Diagram