SN54BCT8245A, SN74BCT8245A

SCAN TEST DEVICES

WITH OCTAL BUS TRANSCEIVERS

SCBS043E – MAY 1990 – REVISED JULY 1996

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, 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 shows 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 shown, the

device contains an 8-bit instruction register and three test-data registers: an 18-bit boundary-scan register, a

2-bit boundary-control register, and a 1-bit bypass 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