SN74LVTH18646AEP, SN74LVTH182646AEP

3.3V ABT SCAN TEST DEVICES

WITH 18BIT TRANSCEIVERS AND REGISTERS

SCAS745A − DECEMBER 2003 − REVISED APRIL 2004

2

POST OFFICE BOX 655303

description/ordering information (continued)

Transceiver function is controlled by output-enable (OE) and direction (DIR) inputs. When OE is low, the

transceiver is active and operates in the A-to-B direction when DIR is high or in the B-to-A direction when DIR

is low. When OE is high, both the A and B outputs are in the high-impedance state, effectively isolating

both buses.

Data flow is controlled by clock (CLKAB and CLKBA) and select (SAB and SBA) inputs. Data on the A bus is

clocked into the associated registers on the low-to-high transition of CLKAB. When SAB is low, real-time A data

is selected for presentation to the B bus (transparent mode). When SAB is high, stored A data is selected for

presentation to the B bus (registered mode). The function of the CLKBA and SBA inputs mirrors that of CLKAB

and SAB, respectively. Figure 1 shows the four fundamental bus-management functions that can be performed

with the SN74LVTH18646A and SN74LVTH182646A.

In the test mode, the normal operation of the SCOPE bus transceivers and registers is inhibited, and the test

circuitry is enabled to observe and control the I/O boundary of the device. When enabled, the test circuitry

performs boundary-scan test operations, according to the protocol described in IEEE Std 1149.1-1990.

Four dedicated test pins are used to observe and control the operation of the test circuitry: test data input (TDI),

test data output (TDO), test mode select (TMS), and test clock (TCK). Additionally, the test circuitry performs

other testing functions such as parallel-signature analysis (PSA) on data inputs and pseudorandom pattern

generation (PRPG) from data outputs. All testing and scan operations are synchronized to the TAP interface.

Active bus-hold circuitry is provided to hold unused or floating data inputs at a valid logic level.

The B-port outputs of SN74LVTH182646A, which are designed to source or sink up to 12 mA, include equivalent

25-

Ω series resistors to reduce overshoot and undershoot.