FIFO flag timing: marching to two different drummers

A FIFO (First-in, First-Out memory device) can be thought of as a "passthrough window" between two independently-clocked digital subsystems-each marching to its own drummer, as it were. To adequately perform this role in practical digital systems, FIFOs are "schizoid": they are memory devices, and they also are logic-synchronization circuits. Increasingly, newer FIFOs have been made capable of recognizing commands from the overall digital system of which they are part, and also of providing that system with timely status information in the form of "flag" values. This paper includes a short review of FIFO basics, followed by a more detailed discussion of flag-synchronization and metastability issues. Finally, the flag-synchronization features of the new Sharp LH540215/25 18-bit synchronous FIFOs are described, as a case in point.