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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
4 Design Considerations
5 Conclusion
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=V2.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 weoe
a[5:0]
2 SRAM Architecture
2.3: Timing Diagram
• READ Cycle
clk
a[i]
we
ce
q[i]
tAS tAH
tACC
previous data output valid output validoutput tristate
2 SRAM Architecture
2.3: Timing Diagram (continued)
• WRITE Cycle
clk
a[i]
we
ce
d[i]
tAS tAH
tDHtDS
3 Self-Timing
3.1: SRAM Cell Operation and Short Circuit Current
vdd
gnd
wl
bl blnBitline 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 (kWn/kWp) but if it is too high, it is hard to write
• Cell stability is controlled by the cell ratio (kWn/kWp) 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)/Iread]* ∆ 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 (Vt)Future trends
A. Intel built a one-square micron SRAM cell on its 90-nm process technology
1. 52-Mbit chips2. SRAM chips aid building and testing