Low-Power CMOS SRAM By: Tony Lugo Nhan Tran Adviser: Dr. David Parent

OUTLINE 1 Introduction 2 SRAM Architecture 3 Design Strategy: Self-Timing Concept 4Design Considerations 5Conclusion

1 Introduction 1.1: More Memory, More Possibilities, More Power Consumption Memory is used widely in all electrical systems: mainframes, microcomputers and cellular phones, etc. More memory means more information, make the system run faster but more power consumption > The need for low power memory With the emerging of portable and compact devices such as smart cards, PDAs > The need for low power memory The demand for Low-Power Memory is very great.

1 Introduction 1.2: Project Goal Design and characterize an embedded Low-Power, synchronous CMOS SRAM module in 0.25um process Wide range applications in electric consumer chips, specially in ASIC This memory has a Low AC power consumption P=V 2.f.C

2 SRAM Architecture 2.1: Design Specification and Features Configuration: 64x4m4 (256 bits) Low voltage operation: 2.25V-2.75V Zero DC power consumption Self-timed to reduce AC power consumption and cycle time Access time: 5.0 ns Performance: 200 MHz for clock cycle in worst case performance Power consumption: 0.15 mW/MHz at typical power consumption

2 SRAM Architecture 2.2: Logic Block Diagram Address latch & Pre-decoder Control Circuit Memory Array Pre-charge & Equalize circuit Column Decoder Sense Amplifier Write Circuit Output Buffer/ Tristate Row Decoder q[3:0] d[3:0] clk ce we oe a[5:0]

2 SRAM Architecture 2.3: Timing Diagram READ Cycle clk a[i] we ce q[i] t AS t AH t ACC previous dataoutput valid output tristate

2 SRAM Architecture 2.3: Timing Diagram (continued) WRITE Cycle clk a[i] we ce d[i] t AS t AH t DH t DS

3 Self-Timing 3.1: SRAM Cell Operation and Short Circuit Current vdd gnd wl bl bln Bitline leakage current

3 Self-Timing 3.1: SRAM Cell Operation and Short Circuit Current (continued) Turn on word line (wl) to write to and read from a SRAM cell Bitline leakage current will appear and dissipate power Turns on wl long enough to access a SRAM cell, then turn off wl to save power

3 Self-Timing 3.2: Save Even More Power: Turning off Pre-Decoder, Row-Decoders and Column Decoder. Also, in read cycle, every Sense Amplifier can be turned off as long it finishes sensing data to output

3 Self-Timing 3.3: Self-Timing Signal Self-Timing Signal generated by memory itself like a feed back loop Pre-charges the bit lines and makes the memory get ready for the next evolution A reference cell (or dummy) is stored (hard coded) with 0 or 1 This cell is get accessed whenever the memory start an evolution (either READ or WRITE cycle)

3 Self-Timing 3.3: Self-Timing Signal Scheme Row Decoder SRAM cell Mux Sense Amplifier Column Decoder Dummy Cell Dummy Sense Amplifier

3 Self-Timing 3.3: Timing Diagram with Self-Timing Signal clk self-timing signal wl clksa

4 Design Considerations 4.1: SRAM Cell ( 6 T): Schematic

4 Design Considerations 4.1: SRAM Cell ( 6T): Layout

4 Design Considerations 4.1: SRAM Cell ( 4T): Schematic

4 Design Considerations 4.1: SRAM Cell ( 4T): Layout

4 Design Considerations 4.1: SRAM Cell : d vs. dn

4 Design Considerations 4.1: SRAM Cell : Static Noise Margin (SNM) SNM depends only on threshold voltage, VDD and the transconductance factor k ratio or cell ratio, not on the absolute value of k’s. SNM increase with cell ratio (k Wn /k Wp ) but if it is too high, it is hard to write Cell stability is controlled by the cell ratio (k Wn /k Wp ) and effected by: Bitline bias Asymmetry (Offsets) Statistical variations -Defects

4 Design Considerations 4.2 Clock-sense Amplifier: Schematic

4 Design Considerations 4.2 Clock-sense Amplifier : Layout

4 Design Considerations 4.2 Clock-sense Amplifier : Plot

4 Design Considerations 4.2 Clock-sense Amplifier : Clock Sense-Amplifier Latch is very high gain ∆V at Clock ( Φ ) rise must be sufficient to reliably set latch --- Offset voltage, cap mismatch --- Limits speed compared to static sense-amp Maintain high performance by limiting voltage swing ∆t = [C(B/L)/I read ]* ∆ V Sense Amplifier Clock often generated with self-timing signal

4 Design Considerations 4.3 Control Block: Schematic

4 Design Considerations 4.3 Control Block: Layout

4 Design Considerations 4.3 Control Block: Layout

4 Design Considerations 4.3 Top Level: Schematic

4 Design Considerations 4.3 Top Level: Layout

4 Design Considerations 4.3 Top Level: Plot

4 Design Considerations 4.3 Top Level: Plot

4 Design Considerations 4.3 Top Level: Power

5 Conclusion SRAM architecture with Self-Timing signal 1. Can save AC Power significantly 2. Uses up little area in the design Access time of SRAM 1. Limited/enhanced by the fan-out of the word line driver 2. Bit-line multiplexer incurs delay

5 Conclusion Current and future trends in SRAM design A. IBM and Motorola collaborated to build SRAM with copper interconnects Advantages: 1. A ramp up in frequency 2. Very small access times 3. Memory cells use higher threshold voltage (V t ) Future trends A. Intel built a one-square micron SRAM cell on its 90-nm process technology Mbit chips 2. SRAM chips aid building and testing