US 5,426,756 A · Memory controller and method determining empty/full status of a FIFO memory using gray code counters

Abstract

A controller for asynchronous configurable FIFO (first-in-first-out) memory includes, in addition to two binary counters for the read pointer and write pointer, two Gray code counters for determining whether the FIFO is full or empty by a comparison of the read pointer and write pointer values expressed in Gray code. The Gray code counters avoid the problem of asynchronicity of read and write signals. The Gray code counters determine if the FIFO is full or empty depending on whether the pointer values match (indicating empty) or differ in accordance with particular Gray code patterns (indicating full). The Gray code counters each have an extra bit which allows determination of the full or empty condition from a straightforward comparison of the read pointer and write pointer values, while the Gray code eliminates the problem of multiple bit transition providing an incorrect indication of the pointer location. The controller is implemented in logic circuitry which is configurable depending on the FIFO configuration and system configuration considerations.

Claims11

Dependent claims

A memory controller for controlling a reading of data from and a writing of data to a memory having a plurality of locations, where the reading of data is asynchronous to the writing of data, comprising:

two counters for indicating respectively which of the locations is currently being read from and written to;
first and second Gray code counters, each having at least one more bit than needed for respectively indicating, in Gray code, which of the locations is currently being read from and written to the memory controller including a first synchronizer for synchronizing an output signal of the first Gray code counter to a system clock signal;
comparator circuitry for comparing contents of the second Gray code counter to the synchronized output signal, and if the contents of the second Gray code counter and the synchronized output signal of the first Gray code counter are identical, indicating that all the locations of the memory are empty, and if the contents of tile second Gray code counter and the synchronized output signal of the first Gray code counter differ by a quantity equal to the number of locations in the memory as determined solely by the comparison indicating that all the locations of the memory are full.

Dependencies are inferred from the claim text.

Memory controller and method determining empty/full status of a FIFO memory using gray code counters

Life of this patent

Memory controller and method determining empty/full status of a FIFO memory using gray code counters

Life of this patent

Abstract

Claims11