The C67401AN is a 64-word by 4-bit asynchronous FIFO. It buffers data between two clock domains at up to 15 MHz, with a 40 ns access time.
40 ns access time — what it means for bus timing
The 40 ns access time is the delay from the read-enable edge to data appearing on the outputs. The 15 MHz data rate sets the maximum toggle rate on the read and write clocks.
