74HC40105D

The 74HC40105 is a first-in/first-out (FIFO) "elastic" storage register that can store 16 4-bit words. It can handle input and output data at different shifting rates. This feature makes it particularly useful as a buffer between asynchronous systems. Each word position in the register is clocked by a control flip-flop, which stores a marker bit. A logic 1 signifies that the data at that position is filled and a logic 0 denotes a vacancy in that position. The control flip-flop detects the state of the preceding flip-flop and communicates its own status to the succeeding flip-flop. When a control flip-flop is in the logic 0 state and sees a logic 1 in the preceding flip-flop, it generates a clock pulse. The clock pulse transfers data from the preceding four data latches into its own four data latches and resets the preceding flip-flop to logic 0. The first and last control flip-flops have buffered outputs. All empty locations "bubble" automatically to the input end, and all valid data ripples through to the output end. As a result, the status of the first control flip-flop (data-in ready output - DIR) indicates if the FIFO is full. The status of the last flip-flop (data-out ready output - DOR) indicates whether the FIFO contains data. As the earliest data is removed from the bottom of the data stack (output end), all data entered later will automatically ripple toward the output. Inputs include clamp diodes that enable the use of current limiting resistors to interface inputs to voltages in excess of V_CC.