This document summarizes a student project to design a 32KB SRAM memory array using Cadence Virtuoso and a 45nm technology. It includes the design of a 6 transistor SRAM cell, row and column decoders, critical path components like the precharge circuit and sense amplifiers. Two types of sense amplifiers - differential and latched-based were modeled and their read delay, power consumption and sizing were compared through simulation. The results showed that the latched-based sense amplifier consumed 14x less power, had less read delay and size making it a better choice compared to the differential sense amplifier.

1. Under The Supervision of
Prof. Krishanu Datta
Department of Electronics and
Communication
Heritage Institute of Technology
VLSI SRAM READ, WRITE OPERATION
AND SENSE AMPLIFIER STUDY

2. •ABRARUL HAQUE (125243)
•DEBASMITA SIKDER (125222)
•JAYEESHA CHAKRABORTY (125241)
•SOUMYAJIT LANGAL (125239
Presented By:

3. LITERATURE SURVEY
• In the initial phase of our project we had gone through several articles from
the web resources.
• For better understanding we have gone through “CMOS VLSI Design
A Circuits and Systems Perspective” by Weste Harris Banerjee.
• And after all we are highly grateful to our mentor
Prof. Krishanu Datta for the detailed and complete understanding of our
project.

4. • Focus to design : 32 KB SRAM memory array
• Memory Size : 512*512 (512 rows & 512 columns)
• Tool used : Cadence Virtuoso
• Technology used : Cadence gpdk45nm
• Supply Voltage applied : 1.1V
• Comparative Study : a) Differential Sense Amplifier
VS.
b) Latch Based Sense Amplifier
• Key Parameters to analyze performance :
a) Access time
b) Speed
c) Power consumption
PROJECT OVERVIEW

5. 6 TRANSISTOR SRAM CELL

6. SRAM MEMORY ARCHITECTURE

7. ROW DECODER AND COLUMN DECODER

8. CRITICAL PATH MODELLING
Schematics to be designed on the critical path :
 Precharge circuit
 RC model for bitline and bitline_bar
 Column multiplexer for write operation
 Write enable circuit
 Column multiplexer for read operation
 Sense Amplifier
For write operation
For read operation

9. PRECHARGE CIRCUIT

10. RC MODELLING OF BIT LINES
PROBLEM
How to model 512 bit line
segments on a single
column ??
OPTIMUM SOLUTION
Grouping 64 such bit line
segment to form a segment and
8 such segments
SOLUTION 2
Grouping 512 bit line segments
to an equivalent one
Equivalent bit line
represented using Pi
model and source to
drain capacitance of
NMOS
SOLUTION 1
Modeling for each of 512 bit line
segments

11. SPECIFICATIONS GIVEN
Parameter Value
Channel length(L) of Memory
cell MOS
45nm
Channel width(W) of Memory
cell MOS
120nm
Area Formula L=√A/12
Resistance for mid-level wire 1.01 Ω/ µm for 50nm
Capacitance for mid-level wire 0.294 fF/ µm for 50nm

12. CALCULATION FOR MODELING
Sl.
No.
Name of the parameter Specification
1. Cell Area (A) A=X2=144*L2 =144* 45nm2
2. Length of each side of the cell (X=√A) X=0.54 µm
3. Using 20% guard band(X´=1.2*X) X´=0.648 µm
4. Bitline length per segment (grouped by 64 cells) 64*0.648 µm = 41.472 µm
5. Resistance of Bitline per segment (grouped by 64 cells) 41.472*1.01 Ω=42.3 Ω
6. Capacitance of Bitline per segment: 41.472*0.294 fF=12.1927 fF
7. Source to drain capacitance of pass transistor w=120n*64=7.68u, l=45n

13. RC MODELLING
8 such
segments

14. COLUMN MULTIPLEXER FOR WRITE
Input/Output
Buffer

15. WRITE ENABLE CIRCUIT
wrt =1 to
enable AND
gate

16. WRITE STABILITY
For successful write operation PMOS PM5 must be weaker than the access transistor NM8.
So size of PM5 ≤ size of NM8

17. COLUMN MULTIPLEXER FOR READ

18. READ STABILITY
To read successfully from the cell the driver NM5 must be stronger than the access transistor NM7.
So size of NM4 ≥ size of NM7

19. SRAM without Sense
Amp
SRAM with Sense Amp
SENSE AMPLIFIER

20. TYPES OF SENSE AMPLIFIER
SENSE
AMPLIFIER
DIFFERENTIAL LATCHED

21. DIFFERENTIAL SENSE AMPLIFIER
Current source
senb
bit bit_b
rout
rout_b

22. LATCH BASED SENSE AMPLIFIER

23. CRITICAL PATH SCHEMATIC

24. SIMULATION & RESULTS
• The constraints are as follows
a) Timing of control signals
b) Sizing of NMOS & PMOS in critical path
• Results include 2 measurements in case of both the sense amplifiers during read operation.
a) Delay (the time lag between the wordline activation and
the Output from sense amplifier)
b)Power (during read operation the amount of power that is
consumed by the sense amplifier)

25. TIMING DIAGRAM
Parameter Logic State
Precharge (pre) Active Low
Wordline (wl) Active High
Multiplexer enable for
write (colselw) Active High
Write circuit enable
(wrenb) Active High
Multiplexer enable for
read (colselr) Active Low
Sense Amplifier enable
(senb) Active High
For Latched Sense Amplifier
For Differential Sense Amplifier
Write ReadInitialize
ReadWriteInitialize

26. SIZING
Pi circuit
Col. Mux.
Write
Diff. Sense
Amp

27. READ SIMULATION
For Latch Based Sense Amplifier
bit
bit_b
rout
Precharge+Mem
cell+pi model
Sense Amplifier

28. FOR DIFFERENTIAL SENSE AMPLIFIER
r_out
bit_b
bit
Precharge+Mem
cell+pi model
Sense Amplifier

29. POWER AND DELAY MEASUREMENT
FOR READ
TYPE OF
AMPLIFIER
Average
Current
(µA)
POWER
(µWatt)
DELAY
(pSec)
WIDTH
(µm)
Latched Sense
Amplifier
0.8564 0.94 120.2 0.84
Differential
Sense Amplifier
12.82 14.1 211.0 7.2

30. CONCLUSION
Latched base consume 14X less power.
Latched based has less READ delay.
Latched based has less size.
LATCHED DIFFERENTIALVS.
LATCHED BASE IS
THE BEST