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Low-Power SRAM
ECE 4332 Fall 2010
Team 2:
Yanran Chen
Cary Converse
Chenqian Gan
David Moore
Metric
Metric = (Active Energy per Access)2*Delay*Area*IdlePower
• Active Energy per Access = 9.704 fJ
• Delay = 7.186 ns
• Area = ~1.2 mm2
• Idle Power = 57.78 uW
• Our Metric = 4.692e-41 J2*s*mm2*W
Metric Breakdown Values
• 1 Bitcell Area = 0.649 um2
• Read Energy = 9.678 fJ
• Write Energy = 9.834 fJ
• Read Delay = 7.186 ns
• Write Delay = 6.491 ns
• Idle Power = 57.71 uW
o With 0.3 V VDDsleep => 0.483 uW
Full SRAM Diagram
Memory Block Diagram
Special Features Overview
• Latching Voltage Sense Amplifier
• Low Voltage (w/ sleep mode)
• Single Bit Error Correcting Code
Latching Voltage Sense Amplifier
• Minimizes BL
sagging to reduce
the energy/read
• Allows faster read
Modified from Ryan & Calhoun, 2008
Low Voltage
Sleep
VDD
Data Retention Voltage:
• 0.6 V for active operation
• 0.3 V for sleep mode
0.35v
HOLD butterfly plots at lower
voltages indicate data can
be retained when sleep voltage 0.30v
is as low as 0.3 V.
0.25v
Single Bit Error Correcting Code
• Importance:
o Compensate for smaller SNMs due to lower voltage
o Maintaining important data
• Hamming Code:
o 6 Parity Bits
o 32 Data Bits
o Allows correction of 1 error per word, detection of 2 errors
per word
• Process:
o Determine parity bits at Write
o Correct word at Read
Single Bit ECC, continued
• Requires significant overhead:
o Area
o Delay
o Power
• Additional components required:
o Parity generation/check circuits: XORs
o Decoder
o Correction circuit: Inverter, 2:1 Multiplexer
ECC Diagram
Parity Bit Generation
(at write)
Parity Checking & Correction
(at read)
Design Considerations
• Ensure Voltage is high enough to protect data
• Avoid extreme delay due to low voltage
• Minimize impact of ECC on area, delay
• Memory block division
Block Size
• Tradeoff between block complexity and top level complexity
• Smaller blocks have lower access energy as shown using
data from an early model
• Additional blocks require wider output muxes, more
complicated distribution of Input Data
• Chose to use 16 256x256 blocks - later extended for ECC
Layout/ Notes on topology
• High Vt bitcells to reduce leakage, and require less cell ratio,
pull down ratio  reduced area
• Blocks of memory to decrease WL capacitance
4-Bitcell Array
Block Layout (64kb)
• All Row Periphery, Column Periphery Complete
• Array consumes majority of area
• Uses metal4 and lower
Sources
• ECE 4332 2009 Group Projects Pages. UVa ECE Wiki.
• Kaxiras, S., Zhigang, H., & Martonosi, M. Cache Decay: Exploiting
Generational Behavior to Reduce Cache Leakage Power. 9th
International Symposium on Computer Architecture, 2001.
• Ling, S., Kim, Y. B., & Lombardi, F. A Low-Leakage 9T SRAM Cell
for Ultra-Low Power Operation, 2008.
• Rabaey, J. Digital Integrated Circuits: A Design Perspective. Prentice
Hall, 2003.
• Ryan, J. F., & Calhoun, B. H. Minimizing Offset for Latching Voltage
Mode Sense Amplifiers for Sub-Threshold Operation. 9th
International Symposium on Quality Electronic Design, 2008.
• Wang, A., Calhoun, B. H., & Chandrakasan, A. P. Sub-Threshold
Design for Ultra Low-Power Systems. Springer, 2006.