12-Bit To 24-Bit Registered Bus Exchanger With 3-State Outputs
This 12-bit to 24-bit registered Bus Exchanger is designed for 1.65-V to 3.6-V VCC operation.
The SN74ALVCH16269 is used in applications in which two separate ports must be multiplexed onto, or demultiplexed from, a single port. The device is particularly suitable as an Interface between synchronous DRAMs and high-speed Microprocessors
Data is stored in the internal B-port Registers on the low-to-high transition of the Clock (CLK) input when the appropriate clock-enable (CLKENA) inputs are low. Proper control of these inputs allows two sequential 12-bit words to be presented as a 24-bit word on the B port. For data transfer in the B-to-A direction, a single storage Register is provided. The select (SEL) line selects 1B or 2B data for the A outputs. The Register on the A output permits the fastest possible data transfer, extending the period during which the data is valid on the bus. The control terminals are registered so that all transactions are synchronous with CLK. Data flow is controlled by the active-low output enables (OEA, OEB1, OEB2).
To ensure the high-impedance state during power up or power down, a Clock pulse should be applied as soon as possible, and OE should be tied to VCC through a pullup resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver. Due to OE being routed through a Register the active state of the outputs cannot be determined before the arrival of the first Clock pulse.
Active bus-hold circuitry holds unused or undriven inputs at a valid Logic state. Use of pullup or pulldown resistors with the bus-hold circuitry is not recommended.
By Texas Instruments
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