A FIFO memory is a storage device that allows data to be written into and read from its array at independent data rates. The SN74ABT7819A is a high-speed, low-power, BiCMOS, bidirectional, clocked FIFO memory. Two independent 512 × 18 dual-port SRAM FIFOs (FIFOA, FIFOB) on the chip buffer data in opposite directions. Each FIFO has flags to indicate empty and full conditions, a half-full flag, and a programmable almost-full/almost-empty flag.

The SN74ABT7819A is a clocked FIFO, which means each port employs a synchronous interface. All data transfers through a port are gated to the low-to-high transition of a continuous (free-running) port clock by enable signals. The continuous clocks for each port are independent of one another and can be asynchronous or coincident. The enables for each port are arranged to provide a simple bidirectional interface between microprocessors and/or buses with synchronous control.

The state of the A0–A17 outputs is controlled by the port-A chip select (CSA)\ and the port-A write/read select (W/R\A). When both CSA\ and W/R\A are low, the outputs are active. The A0–A17 outputs are in the high-impedance state when either CSA\ or W/R\A is high. Data is written to FIFOA–B from port A on the low-to-high transition of the port-A clock (CLKA) input when CSA\ is low, W/R\A is high, the port-A write enable (WENA) is high, and the port-A input-ready (IRA) flag is high. Data is read from FIFOB–A to the A0–A17 outputs on the low-to-high transition of CLKA when CSA\ is low, W/R\A is low, the port-A read enable (RENA) is high, and the port-A output-ready (ORA) flag is high.

The state of the B0–B17 outputs is controlled by the port-B chip select (CSB)\ and the port-B write/read select (W/R\B). When both CSB\ and W/R\B are low, the outputs are active. The B0–B17 outputs are in the high-impedance state when either CSB\ or W/R\B is high. Data is written to FIFOB–A from port B on the low-to-high transition of the port-B clock (CLKB) when CSB\ is low, W/R\B is high, the port-B write enable (WENB) is high, and the port-B input-ready (IRB) flag is high. Data is read from FIFOA–B to the B0–B17 outputs on the low-to-high transition of CLKB when CSB\ is low, W/R\B is low, the port-B read enable (RENB) is high, and the port-B output-ready (ORB) flag is high.

The setup- and hold-time constraints for the chip selects (CSA\, CSB)\ and write/read selects (W/R\A, W/R\B) enable write and read operations on memory and are not related to the high-impedance control of the data outputs. If a port read enable (RENA or RENB) and write enable (WENA or WENB) are set low during a clock cycle, the chip select and write/read select can switch at any time during the cycle to change the state of the data outputs.

The input-ready (IR) and output-ready (OR) flags of a FIFO are two-stage synchronized to the port clocks for use as reliable control signals. CLKA synchronizes the status of the input-ready flag of FIFOA–B (IRA) and the output-ready flag of FIFOB–A (ORA). CLKB synchronizes the status of the input-ready flag of FIFOB–A (IRB) and the output-ready flag of FIFOA–B (ORB). When the IR flag of a port is low, the FIFO receiving input from the port is full and writes are disabled to its array. When the OR flag of a port is low, the FIFO that outputs data to the port is empty and reads from its memory are disabled. The first word loaded to an empty memory is sent to the FIFO output register at the same time its OR flag is asserted (high). When the memory is read empty and the OR flag is forced low, the last valid data remains on the FIFO outputs until the OR flag is asserted (high) again. In this way, a high on the OR flag indicates new data is present on the FIFO outputs.

The SN74ABT7819A is characterized for operation from 0°C to 70°C.