A sensing circuit for single-ended read-ports of SRAM cells with bit-line power reduction and access-time enhancement

摘要

The conventional sensing scheme of single-ended read-only-ports as integrated in 8T-SRAM cells suffers from low performance compared to double-ended complementary sensing schemes. In the proposed sensing scheme the pre-charge voltage of the single-ended read-bit-line is set to a level above the threshold voltage of the sensing device with an adjustable margin. This margin is minimized to speed up the read access on the one hand and kept large enough to provide a sufficient bit-line noise margin on the other hand. The pre-charge voltage level of the proposed sensing circuit tracks the threshold voltage of the sensing device under process variations in order to maintain a minimum required bit-line noise margin. To avoid unnecessary bit-line discharging, the proposed sensing scheme employs a modified 8T-SRAM cell. Compared to the conventional 8T-SRAM cell, the read port of the proposed cell provides a virtual ground line running in parallel to the bit-lines. An internal driver of the sensing circuit releases the virtual ground line during the evaluation period to prevent the charge dissipation resulting in a raised voltage level. The reduced pre-charge level and the increased virtual ground lead to a reduced bit-line voltage swing and thus a bit-line power reduction. Access time, energy dissipation, and noise margin of the proposed sensing circuit are compared with conventional sensing circuits from the literature for different numbers of memory cells connected to the bit-line. It is shown, that for a specific number of memory cells per bit-line the proposed circuit achieves fastest access time at low power operation.