1T-SRAM

1T-SRAM is a pseudo-static random-access memory (PSRAM) technology introduced by MoSys, Inc., which offers a high-density alternative to traditional static random access memory (SRAM) in embedded memory applications. Mosys uses a single-transistor storage cell (bit cell) like dynamic random access memory (DRAM), but surrounds the bit cell with control circuitry that makes the memory functionally equivalent to SRAM (the controller hides all DRAM-specific operations such as precharging and refresh). 1T-SRAM (and PSRAM in general) has a standard single-cycle SRAM interface and appears to the surrounding logic just as an SRAM would.

Due to its one-transistor bit cell, 1T-SRAM is smaller than conventional (six-transistor, or “6T”) SRAM, and closer in size and density to embedded DRAM (eDRAM). At the same time, 1T-SRAM has performance comparable to SRAM at multi-megabit densities, uses less power than eDRAM and is manufactured in a standard CMOS logic process like conventional SRAM.

MoSyS markets 1T-SRAM as physical IP for embedded (on-die) use in System-on-a-chip (SOC) applications. It is available on a variety of foundry processes, including Chartered, SMIC, TSMC, and UMC. Some engineers use the terms 1T-SRAM and "embedded DRAM" interchangeably, as some foundries provide Mosys's 1T-SRAM as “eDRAM”. However, other foundries provide 1T-SRAM as a distinct offering.

1T SRAM is built as an array of small banks (typically 128 rows × 256 bits/row, 32 kilobits in total) coupled to a bank-sized SRAM cache and an intelligent controller. Although space-inefficient compared to regular DRAM, the short word lines allow much higher speeds, so the array can do a full sense and precharge (RAS cycle) per access, providing high-speed random access. Each access is to one bank, allowing unused banks to be refreshed at the same time. Additionally, each row read out of the active bank is copied to the bank-sized SRAM cache. In the event of repeated accesses to one bank, which would not allow time for refresh cycles, there are two options: either the accesses are all to different rows, in which case all rows will be refreshed automatically, or some rows are accessed repeatedly. In the latter case, the cache provides the data and allows time for an unused row of the active bank to be refreshed.

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