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1T-SRAM

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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

Contents

  • 1 Technology
  • 2 Comparison with other embedded memory technologies
  • 3 See also
  • 4 References

Technologyedit

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

There have been four generations of 1T-SRAM:

Original 1T-SRAM  About half the size of 6T-SRAM, less than half the power 1T-SRAM-M  Variant with lower standby power consumption, for applications such as cell phones 1T-SRAM-R  Incorporates ECC for lower soft error rates To avoid an area penalty, it uses smaller bit cells, which have an inherently higher error rate, but the ECC more than makes up for that 1T-SRAM-Q  This "quad-density" version uses a slightly non-standard fabrication process to produce a smaller folded capacitor, allowing the memory size to be halved again over 1T-SRAM-R This does add slightly to wafer production costs, but does not interfere with logic transistor fabrication the way conventional DRAM capacitor construction does

Comparison with other embedded memory technologiesedit

1T-SRAM has speed comparable to 6T-SRAM at multi-megabit densities It is significantly faster speed than eDRAM, and the "quad-density" variant is only slightly larger 10–15% is claimed On most foundry processes, designs with eDRAM require additional and costly masks and processing steps, offsetting the cost of a larger 1T-SRAM die Also, some of those steps require very high temperatures and must take place after the logic transistors are formed, possibly damaging them

1T-SRAM is also available in device IC form The Nintendo GameCube was the first video game system to use 1T-SRAM as a primary main memory storage; the GameCube possesses several dedicated 1T-SRAM devices 1T-SRAM is also used in the successor to the GameCube, Nintendo's Wii console

Note that this is not the same as 1T DRAM, which is a "capacitorless" DRAM cell built using the parasitic channel capacitor of SOI transistors rather than a discrete capacitor

MoSys claims the following sizes for 1T-SRAM arrays:

1T-SRAM Cell sizes μm²/bit or mm²/Mbit
Process node 250 nm 180 nm 130 nm 90 nm 65 nm 45 nm
6T-SRAM bit cell 756 465 243 136 071 034
with overhead 1128 718 373 209 109 052
1T-SRAM bit cell 351 197 110 061 032 015
with overhead 70  36  19  11  057 028
1T-SRAM-Q bit cell 050 028 015 007
with overhead 105 055 029 014

See alsoedit

US Patent 7,146,454 "Hiding refresh in 1T-SRAM Architecture" by Cypress Semiconductor describes a similar system for hiding DRAM refresh using an SRAM cache

Referencesedit

  • Glaskowsky, Peter N 1999-09-13 "MoSys Explains 1T-SRAM Technology: Unique Architecture Hides Refresh, Makes DRAM Work Like SRAM" PDF Microprocessor Report 13 12 Retrieved 2007-10-06 
  • Jones, Mark-Eric 2003-10-14 "1T-SRAM-Q: Quad-Density Technology Reins in Spiraling Memory Requirements" PDF MoSys, Inc Retrieved 2007-10-06 
  • MoSys homepage
  • US Patent 6,256,248 shows the DRAM array at the heart of 1T-SRAM
  • US Patent 6,487,135 uses the term "1T DRAM" to describe the innards of 1T-SRAM
  • Scouras, Ismini 2005-06-15 "1T-SRAM macros are preconfigured for fast integration in SoC designs" eeProductCenter Retrieved 2007-10-06 
  • Cataldo, Anthony 2002-12-16 "NEC, Mosys push bounds of embedded DRAM" EE Times ISSN 0192-1541 Retrieved 2007-10-06 

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    1T-SRAM beatiful post thanks!

    29.10.2014


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