SCBS125D - JULY 1992 - REVISED SEPTEMBER 1995
Widebus is a trademark of Texas Instruments Incorporated.
A FIFO memory is a storage device that allows data to be read from its array in the same order it is written. The SN74ABT7819 is a high-speed, low-power BiCMOS bidirectional clocked FIFO memory. Two independent 512 × 18 dual-port SRAM FIFOs on board 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 SN74ABT7819 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 
and W/R\A. When both and W/R\A are low, the outputs are
active. The A0-A17 outputs are in the high-impedance state when
either or W/R\A is
high. Data is written to FIFOA-B from port A on the low-to-high
transition of CLKA when is
low, W/R\A is high, WENA is high, and the IRA flag is high. Data is
read from FIFOB-A to the A0-A17 outputs on the low-to-high transition
of CLKA when is low, W/R\A
is low, RENA is high, and the ORA flag is high.
The state of the B0-B17 outputs is controlled by 
and 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 or
W/R\B is high. Data is written to FIFOB-A from port B on the
low-to-high transition of CLKB when is low, W/R\B is high, WENB is high, and the IRB flag is
high. Data is read from FIFOA-B to the B0-B17 outputs on the
low-to-high transition of CLKB when is low, W/R\B is low, RENB is high, and the ORB flag is
high.
The setup- and hold-time constraints for the chip selects (, ) 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 and output-ready 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 input-ready flag of a port is low, the FIFO receiving input from the port is full and writes are disabled to its array. When the output-ready 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 output-ready flag is asserted (high). When the memory is read empty and the output-ready flag is forced low, the last valid data remains on the FIFO outputs until the output-ready flag is asserted (high) again. In this way, a high on the output-ready flag indicates new data is present on the FIFO outputs.
The SN74ABT7819 is characterized for operation from 0°C to 70°C.