High speed full and empty flag generators for first-in first-out memory

Abstract

A read signal and write signal for a FIFO each has a flag generating edge and a preceding edge. The read or write counter in an empty of full flag generator responds to the preceding edge of the read or write signal so that the empty or full comparator of the generator may generate an updated empty or full flag value before the onset of the flag generating edge. The empty or full flag generator also includes a gate and a pulse generating circuit. The pulse generating circuit responds to the flag generating edge by generating an enabling signal enabling the gate to pass the comparator output to a latch. When the empty or full comparator indicates that a FIFO is empty or full, the comparator output passed by the gate will force the output high, thereby asserting an empty flag or a full flag. The empty flag generator also includes a second pulse generating circuit which updates the empty flag signal to indicate a non-empty FIFO in response to each write signal. The full flag generator also includes a second pulse generating circuit which updates the full flag signal to indicate a non-full FIFO in response to each read signal.