EE 5324 – VLSI Design II
Part V: Memory Design
Kia Bazargan
University of Minnesota
Spring 2006
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199
References and Copyright
• Textbooks referenced
 [Rab96] J. M. Rabaey
“Digital Integrated Circuits: A Design Perspective”
Prentice Hall, 1996.
• Slides used(Modified by Kia when necessary)
 [©Hauck] © Scott A. Hauck, 1996-2000;
G. Borriello, C. Ebeling, S. Burns, 1995,
University of Washington
 [©Prentice Hall] © Prentice Hall 1995, © UCB 1996
Slides for [Rab96]
http://bwrc.eecs.berkeley.edu/Classes/IcBook/instructors.html
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Registers
• Used for storing data
• Structure




N-bit wide
Parallel/serial read/write
Clocked
Static/dynamic implementation
 Multiple read/write ports
possible
 Example: 32-bit wide by 16-bit
deep, dual-port parallel read,
single port parallel write
register file
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16 words
• Register files
32 bits
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...
32
[©Hauck]
202
Implementing Registers Using Logic Gates
• Flip-flops
 Simple SR latch:
S
Q
Q
R
S
Q
R
Q
S
1
1
0
0
R
1
0
1
0
Q
Q
0
1
x
Q’
Q’
1
0
x
 Flip-flops
o JK, D, T
o Clocked
o Master-slave (edge-triggered)
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Implementing Registers in CMOS
• Direct gate implementation too costly
 A master-slave JK flip-flop uses 38 CMOS transistors
• Directly implement in transistors
 Example: clocked SR FF
Q
Q
f
Note: carefully size the
S
S, R and f transistors
so that we can write
Q
f
R
[Rab96] p.342
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Implementing Registers in CMOS (cont.)
• Another example: D latch (register)
 Uses transmission gate
 When “WR” asserted, “write” operation will take place
 Stack D latch structures to get n-bit register
WR
D
Q
Q
WR
WR
WR
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Shift Registers: Idea
• Shift registers are used for iteratively shifting data
 Used in pipelining, bit-by-bit processing, etc.
f
D
D1
D1
f
• Problem?
f
D2
D2
f
D1
f
D3
D3
f
D2
D3
When clock goes high, the data will traverse all
the shift registers chain in one clock cycle!
Solution: use non overlapping clocks f1 and f2.
f1 used by odd gates, f2 by even gates (use
xmission gates after D1’, D2’, D3’).
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Memory (Array) Design
• Array of bits
• Area very important
 Memory takes considerable area in processor chips
 Compaction results in fewer memory chip modules,
more on-chip cache
• Timing and power consumption of memory blocks
have significant impact on the system
• Different types
 RAM (SRAM, DRAM, CAM)
 ROM (PROM, EEPROM, FLASH)
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Memory Design (cont.)
• Static vs. dynamic RAM
 Dynamic needs refreshing
o Refreshing: read, then write back to restore charge
o Either periodically or after each read
• Static (SRAM)
 Data stored as long as supply voltage is applied
 Large (6 transistors/cell)
 Fast
• Dynamic (DRAM)




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Periodic refresh required
Small (1-3 transistors/cell)
Slower
Special fabrication process
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[©Prentice Hall]
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Memory Architecture: the Big Picture
• Address: which one of the M words to access
• Data: the N bits of the word are read/written
S0
S1
S2
SM-2
SM-1
word
select
lines
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Storage
cells
...
Address
decoder
A0
A1
Word M-2
Word M-1
...
Ak-1
N bits
Decoder
...
Word 0
Word 1
Word 0
Word 1
...
...
SM-2
SM-1 Word M-2
Word M-1
k = log2 (M)
EE 5324 - VLSI Design II - © Kia Bazargan
S0
S1
S2
N bits
210
Memory Access Timing: the Big Picture
• Timing:
 Send address on the address lines,
wait for the word line to become stable
 Read/write data on the data lines
Read Cycle
READ
Read Access
Write Cycle
Read Access
WRITE
Write Access
Data Valid
Data Written
DATA
[©Prentice Hall]
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Memory Cell: Static RAM (8 transistors)
• 8-transistor cell
 Bit_i is the data bus
 Sj is the word line
biti
Sj
biti
– Bit’ used to reduce delay
• Bus drivers
Sj
– Sense Amplifier (inverter
with high gain) used for
fast switching
– Make sure inverters in
cell are weaker than the
combination of “write
buffer” and pass
transistor
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Rd/WR
Very big
driver
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[©Hauck]
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Memory Cell: Static RAM (6 transistors)
• 6-transistor cell
 Must adjust inverters for
input coming through
n-type pass gate
biti (BL)
Sj (WL)
biti
• Bus drivers
 Must adjust senseAmp for
input coming through
n-type pass gate
 Harder to drive 1 than 0
through write buffer (high
resistance via n-transistor)
 One side is sending 0
anyway (bit or bit’) 
written correctly
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Rd/WR
[©Hauck]
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6-Transistor SRAM Cell: Layout
 WL is word line (select line Sj)
 BL is bit line (biti)
VDD
M2
M2
M5
Q
M4
Q
Q
Q
M1
M6
M3
GND
M1
BL
WL
Vdd
M4
M3
M5
BL
BL
M6
WL
BL
[Rab96] p.578
[©Prentice Hall]
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6-Transistor Memory Array
bit0
• 8 words deep RAM,
2 bits wide words
S0
• To write to word j:
S1
 Set Sj=1, all other S
lines to 0
 Send data on the
global bit0, bit0’, bit1,
bit1’
bit0
bit1
bit1
S7
• To read word k:
 Set Sk=1, all other S
lines to 0
 Sense data on bit0
and bit1.
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Rd/WR
bit0
Rd/WR
bit0
EE 5324 - VLSI Design II - © Kia Bazargan
bit1
bit1
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Dynamic RAM 4-Transistor Cell
• 4-transistor cell
• Dynamic charge
storage must be
refreshed
• Dedicated busses for
reading and writing
data in
data out
WR
Rd
Keeps its
value for
about 1ms
[©Hauck]
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Dynamic RAM 3-Transistor Cell
• 3-transistor cell
 No p-type transistors
yield a very compact
layout for cell
 No Vdd connection
 Sense Amplifier must
be able to quickly
detect dropping voltage
precharge
data in
data out
WR
Rd
– Precharge data_out’
to generate ‘1’
outputs
[©Hauck]
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Dynamic RAM 3-Transistor Cell: Timing
precharge
WR
data in
Rd
WR
X
Vdd-VT
data in
data out
data out
X
Vdd
Vdd
DV
Rd
Value stored at node X when writing a “1”=VWR-VTn
[Rab96] p.586
[©Prentice Hall]
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Dynamic RAM 3-Transistor Cell: Layout
Din
Dout Rd
Din
WR
M1
Rd
Dout GND
M3
M3
M2
M2
WR
M1
[Rab96] p.586
[©Prentice Hall]
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Dynamic RAM 1-Transistor Cell
• 1-transistor cell
 Storage capacitor is source
of cell transistor
 Special processing steps to
make the storage
capacitor large
 Charge sharing with bus
capacitance
Bi
Precharge to
middle voltage
level
Si (WL)
Storage
capacitor
(Ccell << Cbus)
 Extra demand on sense
amplifier to detect small
changes
 Destructive read (must
write immediately)
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[©Hauck]
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Dynamic RAM 1-Transistor Cell: Timing
Write "1"
WL
X
GND
BL
Vdd/2
Read "1"
BL
Vdd-VT
WL
X
Cs
Vdd
Vdd/2
sensing
CBL
• Write: Cs is charged/discharged
• Read
 Voltage swing is small (~250 mV)

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DV = VBL - VPRE = (VX - VPRE) . Cs / (Cs+CBL)
EE 5324 - VLSI Design II - © Kia Bazargan
[Rab96] p.587
[©Prentice Hall]
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Dynamic RAM 1-Transistor Cell: Observations
• DRAM memory cell is single-ended
• Read operation is destructive
• Unlike 3T cell, 1T cell requires presence of an
extra capacitance that must be explicitly included
in the design
 Polysilicon-diffusion plate capacitor
 Trench or stacked capacitor
• When writing a “1” into a DRAM cell, a threshold
voltage is lost
 Set WL to a higher value than Vdd
[©Prentice Hall]
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Dynamic RAM 1-Transistor Cell: Layout
Capacitor
Metal word line
poly
poly
n+
n+
M1 word
line
SiO2
Field Oxide
Inversion layer
induced by
plate bias
Diffused
bit line
Polysilicon
gate
Polysilicon
plate
(a) Cross-section
(b) Layout
Used Polysilicon-Diffusion Capacitance
Expensive in Area
[©Prentice Hall]
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Dynamic RAM 1-Transistor Cell: Layout
[©Prentice Hall]
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Dynamic RAM 1-Transistor Cell: Layout
Capacitor
Word line
Dielectric
Insulating Layer Cell plate layer
Cell Plate Si
Capacitor Insulator
Refilling Poly
Transfer gate
Isolation
Storage electrode
Storage Node Poly
Si Substrate
2nd Field Oxide
Trench Cell
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Stacked-capacitor Cell
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[©Prentice Hall]
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RAM Cells: Summary
• Static
 Fastest (no refresh)
 Simple design
 Right solution for small memory arrays such as register
files
• Dynamic
 Densest: 1T is best and is the way to go for large
memory arrays
 Built-in circuitry to step through cells and refresh
(can do more than one word at a time)
 Sense amplifier needed for fast read operation
[©Hauck]
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Multi-Port RAM Cells
bus_B
bus_A
row-bus_A
row-bus_B
bus_B
bus_A
• Idea: add more input and output transistors
• Can be applied to all variants
 Usually not done for 1T cells
[©Hauck]
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Multi-Port RAM Cells Array
• 7 words deep,
2 wide words,
dual port mem
• To read from
word j and write
“d1d0” to word k
simultaneously:
SA0
SB0
SA1
...
bus_B1
bus_A1
bus_A1
bus_B1
EE 5324 - VLSI Design II - © Kia Bazargan
bus_B0
bus_A0
bus_A0
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...
bus_B0
SB1
 Set SAj=1, and all
other SA’s=0
SA7
 Set SBk=1, and all
other SB’s=0
 Sense the values on
bus_A0 and bus_A1 SB7
 Write d1d0 to
bus_B0 and bus_B1
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Read Only Memory (ROM) Cells: MOS NOR
• To store constants data or invariant code
• Popular for control implementation
 Store program or state machine
• Programmable logic array structure
• Can be precharged or pseudo-nMos
MOS
NOR
ROM
read1
bit1
bit2
bit3
010
read2
001
[©Hauck]
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ROM Cell: MOS NOR Layout
Metal1 on top of diffusion
WL0
Basic cell
10 l x 7 l
WL1
GND (diffusion)
Polysilicon
Metal1
WL2
WL3
 Only 1 layer (metal-to-diffusion contact mask) is used
to program memory array
 Programming of the memory can be delayed to one of
last process steps
[©Prentice Hall]
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ROM Cell: MOS NOR Alternative Layout
BL0 BL1 BL2 BL3
Basic Cell
8.5l x 7 l
WL0
WL1
WL2
Threshold raising
implant
GND (diffusion)
Metal1 over diffusion
Polysilicon
WL3
 Threshold raising implants disable transistors
[©Prentice Hall]
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ROM Cell: MOS NAND
Pullup devices
BL0
BL1
BL2
BL3
WL0
WL1
WL2
WL3
All word lines high by default with exception of selected row
[©Prentice Hall]
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ROM Cell: MOS NAND: Layout
Diffusion
Polysilicon
Basic cell
5l x 6l
Threshold
lowering
implant
• No contact to Vdd or GND necessary
drastically reduced cell size
• Loss in performance compared to NOR ROM
 Why?
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[©Prentice Hall]
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ROM Cells: Summary
• Mask programmability
• Precharged vs. pseudo-nMos
• NAND cell, NOR cell
 Area
 Speed
• Other types: EEPROM, etc.
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Memory Cell Array Interface: Example
• Memory parameters:
 16-bit wide
 1024-word deep
 Address = 00000010012
A0
A1
A2
A3
...
A9
Decoder
• Accessing word 9
0
1
0
1
0
…
0
0 S0
0 S1
0 S2
0
1 S9
0
...
0
Word 0
Word 1
Word 2
...
Word 9
.
.
.
S1022
S1023 Word 1022
0
Word 1023
16 bits
SenseAmp /
Drivers
16 bits
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Memory Cell Array Layout
S0
S1
S2
• Memory performance (speed)
• Memory area
 Cell array layout
A0
A1
...
Ak-1
• How to layout the cells array?
 Linear is bad:
o Long data busses  large capacity
o A lot of cells connected to data bus
o Decoder will have a lot of logic
levels
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EE 5324 - VLSI Design II - © Kia Bazargan
Decoder
 Storage cell speed (read, write)
 Data bus capacitance
 Periphery: address decoders,
sense amplifiers, buffers
...
Word 0
Word 1
Word 2
...
SM-2
SM-1 Word M-2
Word M-1
N bits
SenseAmp /
Drivers
N bits
240
Memory Cell Array Layout (cont.)
• Group the M words into M/L rows, each containing L words
• Benefits?
...
Ak-1
Row Decoder
Alog L
Alog L+1
S0..L-1
Word 0
Word 1
SL..2L-1
Word L Word L+1
S2L..3L-1 Word 2L Word 2L+1
...
SM-L..M-1 Word M-L
...
...
N bits
N bits
SAmp/Drv SAmp/Drv
A0
N bits
...
Alog L-1
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N bits
...
...
...
Word L-1
Word 2L-1
Word 3L-1
...
...
...
Word M-1
...
...
N bits
SAmp/Drv
...
N bits
address:
L bits
k-L bits
Column Decoder + MUX
N bits
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Memory Cell Array Access Example
• word=16-bit wide(N), row=8 words(L), address=10 bits (k)
• Accessing word 9= 00000010012
Row Decoder
1 A3
0 A4
0
… ...
0 A9
S0..7
S8..15
S16..23
S1016-1023
L=8 words
Word 0
Word 8
Word 16
Word 1
Word 9
Word 17
...
...
...
...
...
...
...
Word 1016
...
...
Word 1023
...
...
16 bits
SAmp/Drv
...
16 bits
16 bits
16 bits
SAmp/Drv SAmp/Drv
1 A0
0 A1
0 A2
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Word 7
Word 15
Word 23
16 bits
16 bits
M/L =
1024/8=
128 rows
Column Decoder + MUX
16 bits
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Hierarchical Memory Structure
• Taking the idea one step further
 Shorter wires within each block
 Enable only one block addr decoder power savings
Row
Address
Column
Address
Block
Address
Blk EN
Blk EN
Blk EN
Blk EN
Global Bus
SAmp/ Global drivers/
Drv sense amplifiers
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[Rab96] p. 558
243
Decreasing Word Line Delay
• Word line delay comes into play!
 We used to have long busses, made 2D array 
shorter busses
 But, longer word lines!
• How to decrease the delay on the word lines?
 Break the word line by inserting buffers
 Place the decoder in the middle
Polysilicon word line
Metal word line
(a) Drive the word line
from both sides
Spring 2006
Polysilicon word line
Metal bypass
(b) Use metal bypass
EE 5324 - VLSI Design II - © Kia Bazargan
[©Prentice Hall]
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Decreasing Word Line Delay (cont.)
• Place the decoder in the middle
• Add buffers to outputs of decoder
memory
cell array
d
e
c
o
d
e
r
memory
cell array
k
Address lines
[©Hauck]
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Row Decoder Implementation
• Collection of 2k high fan-in (k inputs) logic gates
• Regular and dense structure
• N(AND) decoder
WL0 = A0.A1.A2.A3.A4.A5.A6.A7.A8.A9
WL511 = A0.A1.A2.A3.A4.A5.A6.A7.A8.A9
• NOR decoder
WL0 = A0+A1+A2+A3+A4+A5+A6+A7+A8+A9
WL511 = A0+A1+A2+A3+A4+A5+A6+A7+A8+A9
[©Prentice Hall]
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Row Decoder Implementation (cont.)
Precharge
devices
GND
GND
WL3
WL3
WL2
WL2
Vdd f
A0
A0
A1
WL1
WL1
WL0
WL0
A1
Dynamic 2-to-4 NOR
Decoder
A0
A0
A1
A1
f
2-to-4 MOS Dynamic
NAND Decoder
[©Prentice Hall]
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Row Decoder Implementation (cont.)
WL1
WL0
A0A1 A0A1 A0A1 A0A1
A0
A1
A1
A0
A2A3 A2A3 A2A3 A2A3
A2
A3
A3
A2
Splitting decoder into two or more logic layers
produces a faster and cheaper implementation
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[©Prentice Hall]
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Column Multiplexers: Tree-Based Decoder
• Route many inputs to
a single output
 Inputs come from
different words, same
bit position
• Series transistors are
slow
 On the critical path too
• Area?
w0 w1 w2 w3 w4w5 w6 w7
A0'
A0
A1'
A1
A2'
 One-bit very small, but
have to repeat the
“decoding” for all bit
positions.
Spring 2006
Bit position i of
L word columns
A2
EE 5324 - VLSI Design II - © Kia Bazargan
Bit position i
[©Hauck]
249
Column Multiplexers: Faster Implementation
• Decode address into one-hot signals
• Each bit passes through single n-device or pass
gate
• Column decoding done in parallel w/ row decoding
Alog L1
Decoder
A0
A1
...
bit 0
bit 1
[©Hauck]
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Content Addressable Memory (CAM)
• Instead of address, provide data  find a match
 Applications: cache, physical particle collider
• Needs “Encoder”:
 Inverse function of decoder
 Take a one-hot collection of signals and encode them
m bits
2n
rows
content
addressable
memory
cell array
m
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e
n
c
o
d
e
r
n
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[©Hauck]
252
Content Addressable Memory Cell
• Read and write like normal 6T memory cell
• Match signal is precharged to 1,
pulled to 0 if no match
 Send data on bit’ and data’ on bit for matching
 Match remains 1 iff all bits in word match
row
select
match
bit
bit'
[©Hauck]
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Encoders
content
addressable
memory
cell array
e
n
c
o
d
e
r
row
select
match
bit
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bit'
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Content Addressable Memory (CAM)
• Writing is done as normal SRAM
 Address decoder needed
 Drive row select
• ½ n log n transistors on the address lines (in
encoder)
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Outline
• Registers (flip-flops), shift registers
• Memory interface
• Memory cells
 Static memory cell
 Dynamic memory cell
 ROM cells
• Address decoders
• Content addressable memory (CAM)
• Non volatile memory cells
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Non-Volatile Memory Cells
• Programmable after fabrication
• Keep their configuration even after the supply
voltage is disconnected
• Basic idea:
 Use a floating strip of polysilicon between the
substrate and the gate
 Put charges on the floating gate
 Increase threshold voltage  disable the device
• Different types based on the erasure method
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Floating-Gate Transistor (FAMOS)
Floating gate
Gate
D
Drain
Source
G
t ox
t ox
n+
p
Substrate
S
n+
(a) Device cross-section
(b) Schematic symbol
[©Prentice Hall]
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Floating-Gate Transistor: Programming
20 V
10V 5V
0V
20 V
-5 V
- -
S
- -
5V
0V
-2.5 V
- - - -
D
Avalanche injection.
S
5V
- - - -
D
Removing
programming voltage
leaves charges
trapped
S
D
Programming
results in
higher VT
[©Prentice Hall]
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FLOTOX EEPROM
Floating gate
Source
Gate
Drain
I
-10 V
20-30 nm
n+
p
Substrate
10 V
n+
VGD
10 nm
(b) Fowler-Nordheim
I-V characteristics
(a) Flotox transistor
BL
WL
V DD
(c) EEPROM cell during a read operation
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[©Prentice Hall]
260
FLASH EEPROM
Control gate
Floating gate
erasure
n+ source
Thin tunneling oxide
programming
n+ drain
p-substrate
[©Prentice Hall]
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Cross-Section of NVM Cells
Courtesy Intel
[©Prentice Hall]
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Characteristics of Some NVM Cells
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