US Patent 7646668 Maintaining dynamic count of FIFO contents in multiple clock domains

Disclosed is a system that generates a write count value for indicating whether data can be read from a FIFO and a system that generates a read count value for indicating whether data can be written to a FIFO. Each of these systems operates in two separate clock domains. In the system that generates a write count value, write strobes are stored in parallel in a register in a first clock domain. The plurality of synchronizers trigger on a rising edge of the write strobe that is stored in the parallel register and generate an increment pulse in a second clock domain. An up/down counter reads the increment pulse in parallel and increments the up/down counter in parallel. A decrement signal from a read strobe decrements the up/down counter. The output of the counter is fed to a register that provides a write counter value to handshake logic that indicates whether data can be read from a FIFO without underflowing the FIFO.