SN74LVTH18652A

The 'LVTH18652A and 'LVTH182652A scan test devices with 18-bit bus transceivers and registers are members of the Texas Instruments (TI) SCOPE testability integrated-circuit family. This family of devices supports IEEE Std 1149.1-1990 boundary scan to facilitate testing of complex circuit board assemblies. Scan access to the test circuitry is accomplished via the 4-wire test access port (TAP) interface.

Additionally, these devices are designed specifically for low-voltage (3.3-V) V_CC operation, but with the capability to provide a TTL interface to a 5-V system environment.

In the normal mode, these devices are 18-bit bus transceivers and registers that allow for multiplexed transmission of data directly from the input bus or from the internal registers. They can be used either as two 9-bit transceivers or one 18-bit transceiver. The test circuitry can be activated by the TAP to take snapshot samples of the data appearing at the device pins or to perform a self test on the boundary-test cells. Activating the TAP in the normal mode does not affect the functional operation of the SCOPE bus transceivers and registers.

Data flow in each direction is controlled by clock (CLKAB and CLKBA), select (SAB and SBA), and output-enable (OEAB and OEBA\) inputs. For A-to-B data flow, 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). When OEAB is high, the B outputs are active. When OEAB is low, the B outputs are in the high-impedance state.

Control for B-to-A data flow is similar to that for A-to-B data flow but uses CLKBA, SBA, and OEBA\ inputs. Since the OEBA\ input is active-low, the A outputs are active when OEBA\ is low and are in the high-impedance state when OEBA\ is high. Figure 1 illustrates the four fundamental bus-management functions that can be performed with the 'LVTH18652A and 'LVTH182652A.

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 pseudo-random 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 'LVTH182652A, which are designed to source or sink up to 12 mA, include equivalent 25- series resistors to reduce overshoot and undershoot.

The SN54LVTH18652A and SN54LVTH182652A are characterized for operation over the full military temperature range of -55°C to 125°C. The SN74LVTH18652A and SN74LVTH182652A are characterized for operation from -40°C to 85°C.