Lecture 19:
SRAM
Outline
 Memory Arrays
 SRAM Architecture
– SRAM Cell
– Decoders
– Column Circuitry
– Multiple Ports
 Serial Access Memories
19: SRAM
CMOS VLSI Design 4th Ed.
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Memory Arrays
Memory Arrays
Random Access Memory
Read/Write Memory
(RAM)
(Volatile)
Static RAM
(SRAM)
Dynamic RAM
(DRAM)
Mask ROM
Programmable
ROM
(PROM)
19: SRAM
Content Addressable Memory
(CAM)
Serial Access Memory
Read Only Memory
(ROM)
(Nonvolatile)
Shift Registers
Serial In
Parallel Out
(SIPO)
Erasable
Programmable
ROM
(EPROM)
Queues
Parallel In
Serial Out
(PISO)
Electrically
Erasable
Programmable
ROM
(EEPROM)
CMOS VLSI Design 4th Ed.
First In
First Out
(FIFO)
Last In
First Out
(LIFO)
Flash ROM
3
Array Architecture
 2n words of 2m bits each
 If n >> m, fold by 2k into fewer rows of more columns
 Good regularity – easy to design
 Very high density if good cells are used
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12T SRAM Cell
 Basic building block: SRAM Cell
– Holds one bit of information, like a latch
– Must be read and written
 12-transistor (12T) SRAM cell
– Use a simple latch connected to bitline
– 46 x 75 l unit cell
bit
write
write_b
read
read_b
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6T SRAM Cell
 Cell size accounts for most of array size
– Reduce cell size at expense of complexity
 6T SRAM Cell
– Used in most commercial chips
– Data stored in cross-coupled inverters
 Read:
bit
– Precharge bit, bit_b
word
– Raise wordline
 Write:
– Drive data onto bit, bit_b
– Raise wordline
19: SRAM
CMOS VLSI Design 4th Ed.
bit_b
6
SRAM Read




Precharge both bitlines high
Then turn on wordline
One of the two bitlines will be pulled down by the cell
Ex: A = 0, A_b = 1
– bit discharges, bit_b stays high
– But A bumps up slightly
 Read stability
– A must not flip
– N1 >> N2
bit_b
bit
word
P1 P2
N2
N4
A
A_b
N1 N3
A_b
bit_b
1.5
1.0
bit
word
0.5
A
0.0
0
100
200
300
400
500
600
time (ps)
19: SRAM
CMOS VLSI Design 4th Ed.
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SRAM Write




Drive one bitline high, the other low
Then turn on wordline
Bitlines overpower cell with new value
Ex: A = 0, A_b = 1, bit = 1, bit_b = 0
– Force A_b low, then A rises high
 Writability
– Must overpower feedback inverter
– N2 >> P1
bit_b
bit
word
P1 P2
N2
A
N4
A_b
N1 N3
A_b
A
1.5
bit_b
1.0
0.5
word
0.0
0
100
200
300
400
500
600
700
time (ps)
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CMOS VLSI Design 4th Ed.
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SRAM Sizing
 High bitlines must not overpower inverters during
reads
 But low bitlines must write new value into cell
bit_b
bit
word
weak
med
med
A
A_b
strong
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CMOS VLSI Design 4th Ed.
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SRAM Column Example
Read
Write
Bitline Conditioning
Bitline Conditioning
2
2
More
Cells
More
Cells
word_q1
word_q1
SRAM Cell
bit_b_v1f
out_b_v1r
H
bit_v1f
H
bit_b_v1f
bit_v1f
SRAM Cell
write_q1
out_v1r
data_s1
1
2
word_q1
bit_v1f
out_v1r
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SRAM Layout
 Cell size is critical: 26 x 45 l (even smaller in industry)
 Tile cells sharing VDD, GND, bitline contacts
GND
BIT BIT_B GND
VDD
WORD
Cell boundary
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CMOS VLSI Design 4th Ed.
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Thin Cell
 In nanometer CMOS
– Avoid bends in polysilicon and diffusion
– Orient all transistors in one direction
 Lithographically friendly or thin cell layout fixes this
– Also reduces length and capacitance of bitlines
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CMOS VLSI Design 4th Ed.
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Commercial SRAMs
 Five generations of Intel SRAM cell micrographs
– Transition to thin cell at 65 nm
– Steady scaling of cell area
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CMOS VLSI Design 4th Ed.
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Decoders
 n:2n decoder consists of 2n n-input AND gates
– One needed for each row of memory
– Build AND from NAND or NOR gates
Static CMOS
A1
Pseudo-nMOS
A0
A1
word0
word1
19: SRAM
1
1
8
A1
1
4
A0
1
A0
word0
word
word1
word2
word2
word3
word3
CMOS VLSI Design 4th Ed.
A0
1/2
4
16
A1
1
1
2
8
word
14
Decoder Layout
 Decoders must be pitch-matched to SRAM cell
– Requires very skinny gates
A3
A3
A2
A2
A1
A1
A0
A0
VDD
word
GND
buffer inverter
NAND gate
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CMOS VLSI Design 4th Ed.
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Large Decoders
 For n > 4, NAND gates become slow
– Break large gates into multiple smaller gates
A3
A2
A1
A0
word0
word1
word2
word3
word15
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CMOS VLSI Design 4th Ed.
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Predecoding
 Many of these gates are redundant
– Factor out common
gates into predecoder
– Saves area
– Same path effort
A3
A2
A1
A0
predecoders
1 of 4 hot
predecoded lines
word0
word1
word2
word3
word15
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Column Circuitry
 Some circuitry is required for each column
– Bitline conditioning
– Sense amplifiers
– Column multiplexing
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CMOS VLSI Design 4th Ed.
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Bitline Conditioning
 Precharge bitlines high before reads

bit
bit_b
 Equalize bitlines to minimize voltage difference
when using sense amplifiers

bit
19: SRAM
bit_b
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Sense Amplifiers
 Bitlines have many cells attached
– Ex: 32-kbit SRAM has 128 rows x 256 cols
– 128 cells on each bitline
 tpd  (C/I) DV
– Even with shared diffusion contacts, 64C of
diffusion capacitance (big C)
– Discharged slowly through small transistors
(small I)
 Sense amplifiers are triggered on small voltage
swing (reduce DV)
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CMOS VLSI Design 4th Ed.
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Differential Pair Amp
 Differential pair requires no clock
 But always dissipates static power
sense_b
bit
P1
P2
N1
N2
sense
bit_b
N3
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CMOS VLSI Design 4th Ed.
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Clocked Sense Amp
 Clocked sense amp saves power
 Requires sense_clk after enough bitline swing
 Isolation transistors cut off large bitline capacitance
bit
bit_b
isolation
transistors
sense_clk
regenerative
feedback
sense
19: SRAM
sense_b
CMOS VLSI Design 4th Ed.
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Twisted Bitlines
 Sense amplifiers also amplify noise
– Coupling noise is severe in modern processes
– Try to couple equally onto bit and bit_b
– Done by twisting bitlines
b0 b0_b b1 b1_b b2 b2_b b3 b3_b
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Column Multiplexing
 Recall that array may be folded for good aspect ratio
 Ex: 2 kword x 16 folded into 256 rows x 128 columns
– Must select 16 output bits from the 128 columns
– Requires 16 8:1 column multiplexers
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CMOS VLSI Design 4th Ed.
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Tree Decoder Mux
 Column mux can use pass transistors
– Use nMOS only, precharge outputs
 One design is to use k series transistors for 2k:1 mux
– No external decoder logic needed
B0 B1
B2 B3
B4 B5
B6 B7
B0 B1
B2 B3
B4 B5
B6 B7
A0
A0
A1
A1
A2
A2
Y
19: SRAM
to sense amps and write circuits
CMOS VLSI Design 4th Ed.
Y
25
Single Pass-Gate Mux
 Or eliminate series transistors with separate decoder
A1
A0
B0 B1
B2 B3
Y
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Ex: 2-way Muxed SRAM
2
More
Cells
More
Cells
word_q1
A0
A0
write0_q1
2
write1_q1
data_v1
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Multiple Ports
 We have considered single-ported SRAM
– One read or one write on each cycle
 Multiported SRAM are needed for register files
 Examples:
– Multicycle MIPS must read two sources or write a
result on some cycles
– Pipelined MIPS must read two sources and write
a third result each cycle
– Superscalar MIPS must read and write many
sources and results each cycle
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Dual-Ported SRAM
 Simple dual-ported SRAM
– Two independent single-ended reads
– Or one differential write
bit
bit_b
wordA
wordB
 Do two reads and one write by time multiplexing
– Read during ph1, write during ph2
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Multi-Ported SRAM
 Adding more access transistors hurts read stability
 Multiported SRAM isolates reads from state node
 Single-ended bitlines save area
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Large SRAMs
 Large SRAMs are split into subarrays for speed
 Ex: UltraSparc 512KB cache
–
–
–
–
–
4 128 KB subarrays
Each have 16 8KB banks
256 rows x 256 cols / bank
60% subarray area efficiency
Also space for tags & control
[Shin05]
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CMOS VLSI Design 4th Ed.
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Serial Access Memories
 Serial access memories do not use an address
– Shift Registers
– Tapped Delay Lines
– Serial In Parallel Out (SIPO)
– Parallel In Serial Out (PISO)
– Queues (FIFO, LIFO)
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Shift Register
 Shift registers store and delay data
 Simple design: cascade of registers
– Watch your hold times!
clk
Din
Dout
8
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Denser Shift Registers
 Flip-flops aren’t very area-efficient
 For large shift registers, keep data in SRAM instead
 Move read/write pointers to RAM rather than data
– Initialize read address to first entry, write to last
– Increment address on each cycle
Din
clk
11...11
reset
19: SRAM
counter
counter
00...00
readaddr
writeaddr
dual-ported
SRAM
Dout
CMOS VLSI Design 4th Ed.
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Tapped Delay Line
 A tapped delay line is a shift register with a
programmable number of stages
 Set number of stages with delay controls to mux
– Ex: 0 – 63 stages of delay
clk
delay2
CMOS VLSI Design 4th Ed.
SR1
delay3
SR2
19: SRAM
delay4
SR4
delay5
SR8
SR16
SR32
Din
delay1
Dout
delay0
35
Serial In Parallel Out
 1-bit shift register reads in serial data
– After N steps, presents N-bit parallel output
clk
Sin
P0
19: SRAM
P1
P2
P3
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Parallel In Serial Out
 Load all N bits in parallel when shift = 0
– Then shift one bit out per cycle
P0
P1
P2
P3
shift/load
clk
Sout
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Queues
 Queues allow data to be read and written at different
rates.
 Read and write each use their own clock, data
 Queue indicates whether it is full or empty
 Build with SRAM and read/write counters (pointers)
WriteClk
WriteData
FULL
19: SRAM
ReadClk
Queue
ReadData
EMPTY
CMOS VLSI Design 4th Ed.
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FIFO, LIFO Queues
 First In First Out (FIFO)
– Initialize read and write pointers to first element
– Queue is EMPTY
– On write, increment write pointer
– If write almost catches read, Queue is FULL
– On read, increment read pointer
 Last In First Out (LIFO)
– Also called a stack
– Use a single stack pointer for read and write
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