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EE4800 CMOS Digital IC Design & Analysis
Lecture 12 SRAM
Zhuo Feng
12.1
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Outline
■ Memory Arrays
■ SRAM Architecture
► SRAM Cell
► Decoders
► Column Circuitry
► Multiple Ports
■ Serial Access Memories
12.2
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Memory Arrays
Memory Arrays
Random Access Memory
Read/Write Memory
(RAM)
(Volatile)
Static RAM
(SRAM)
Dynamic RAM
(DRAM)
Mask ROM
Programmable
ROM
(PROM)
12.3
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)
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
First In
First Out
(FIFO)
Flash ROM
Last In
First Out
(LIFO)
Array Architecture
■ 2n words of 2m bits each
■ If n >> m, fold by 2k into fewer rows of more columns
wordlines
bitline conditioning
bitlines
row decoder
memory cells:
2n-k rows x
2m+k columns
n-k
column
circuitry
k
n
column
decoder
2m bits
■ Good regularity – easy to design
■ Very high density if good cells are used
12.4
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
12.5
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
► Raise wordline
word
■ Write:
► Drive data onto bit, bit_b
► Raise wordline
12.6
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
bit_b
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_b
► A must not flip
bit_b
bit_b
bit
1.5
word
1.0
P1 P2
N2
A
N4
A_b
bit
word
0.5
A
N1 N3
0.0
0
100
200
300
time (ps)
12.7
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
400
500
600
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
N1 >> N2
N3 >> N4
► bit discharges, bit_b stays high
► But A bumps up slightly
■ Read stability
A_b
► A must not flip
bit_b
bit_b
bit
1.5
word
1.0
P1 P2
N2
A
N4
A_b
bit
word
0.5
A
N1 N3
0.0
0
100
200
300
time (ps)
12.8
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
400
500
600
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
A_b
bit_b
bit
A
1.5
word
bit_b
P1 P2
N2
A
1.0
N4
A_b
0.5
word
N1 N3
0.0
0
100
200
300
400
time (ps)
12.9
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
500
600
700
SRAM Write
■ Drive one bitline high, the other low
■ Then turn on wordline
■ Bitlines overpower cell with new value
N2 >> P1
N4 >> P2
■ Ex: A = 0, A_b = 1, bit = 1, bit_b = 0
► Force A_b low, then A rises high
■ Writability
► Must overpower feedback inverter
A_b
bit_b
bit
A
1.5
word
bit_b
P1 P2
N2
A
1.0
N4
A_b
0.5
word
N1 N3
0.0
0
100
200
300
400
time (ps)
12.10
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
500
600
700
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
12.11
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
SRAM Column Example
Bitline Conditioning
Bitline Conditioning
2
2
More
Cells
More
Cells
word_q1
out_b_v1r
H
out_v1r
SRAM Cell
write_q1
1
2
word_q1
bit_v1f
out_v1r
12.12
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
data_s1
bit_b_v1f
H
Write
bit_v1f
bit_v1f
SRAM Cell
bit_b_v1f
Read
word_q1
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
12.13
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
12.14
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Commercial SRAMs
■ Five generations of Intel SRAM cell micrographs
► Transition to thin cell at 65 nm
► Steady scaling of cell area
12.15
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
A1
A0
1
1
8
A1
1
4
A0
1
1/2
4
16
A1
1
1
2
8
word
A0
word0
word0
word1
word1
word2
word2
word3
12.16
A0
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
word3
word
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
NAND gate
12.17
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
buffer inverter
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
12.18
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Column Circuitry
■ Some circuitry is required for each column
► Bitline conditioning
► Sense amplifiers
► Column multiplexing
12.19
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Bitline Conditioning
■ Precharge bitlines high before reads

bit
bit_b
■ Equalize bitlines to minimize voltage difference
when using sense amplifiers

bit
12.20
bit_b
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Sense Amplifiers
■ Bitlines have many cells attached
► Ex: 32-kbit SRAM has 256 rows x 128 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)
12.21
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Differential Pair Amp
■ Differential pair requires no clock
■ But always dissipates static power
sense_b
bit
P1
N1
P2
N2
sense
bit_b
N3
12.22
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
12.23
sense_b
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
12.24
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Column Multiplexing
■ Recall that array may be folded for good aspect ratio
■ Ex: 2k word x 16 folded into 256 rows x 128 columns
► Must select 16 output bits from the 128 columns
► Requires 16 8:1 column multiplexers
12.25
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
B4 B5
B2 B3
B6 B7
B0 B1
B4 B5
B2 B3
A0
A0
A1
A1
A2
A2
Y
12.26
to sense amps and write circuits
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Y
B6 B7
Single Pass-Gate Mux
■ Or eliminate series transistors with separate
decoder
A1
A0
B0 B1
B2 B3
Y
12.27
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
12.28
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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]
12.29
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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)
12.30
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Shift Register
■ Shift registers store and delay data
■ Simple design: cascade of registers
► Watch your hold times!
clk
Din
Dout
8
12.31
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
12.32
counter
counter
00...00
readaddr
writeaddr
dual-ported
SRAM
Dout
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
SR1
delay3
SR2
12.33
delay4
SR4
delay5
SR8
SR16
SR32
Din
delay1
Dout
delay0
Serial In Parallel Out
■ 1-bit shift register reads in serial data
► After N steps, presents N-bit parallel output
clk
Sin
P0
12.34
P1
P2
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
P3
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
12.35
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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
ReadClk
Queue
FULL
12.36
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
ReadData
EMPTY
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
12.37
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis