Efficient register for additive latency in DDR2 mode of operation

摘要

An additive latency circuit for a DDR2 standard compliant integrated circuit memory includes a half flip-flop register assigned for each case of additive latency. A unique clock is generated to control each bit in the register chain. Sufficient register bits are required in the chain to support the highest additive latency specified. For latency settings less than the maximum, those clocks assigned to the bits above the chosen latency are enabled so the data passes through un-clocked. For the additive latency zero case, a separate bypass path is provided. Both address and command information is delayed by the additive latency delay chain. Once delayed by the proper number of cycles, the address information remains in that state until the time when a new state is required. Command information remains valid for one cycle upon reaching the proper delay point. A reset circuit is provided to reset command signals.