This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Adapted from J. M. Rabaey, A. Chandrakasan and B. Nikolic, Digital Integrated Circuits, 2nd ed. Copyright 2003 Prentice Hall/Pearson.
ECE 261 Krish Chakrabarty 2
Semiconductor Memory Classification
Read-Write MemoryNon-VolatileRead-Write
MemoryRead-Only Memory
EPROM
E2PROM
FLASH
RandomAccess
Non-RandomAccess
SRAM
DRAM
Mask-Programmed
Programmable (PROM)
FIFO
Shift Register
CAM
LIFO
2
ECE 261 Krish Chakrabarty 3
Memory Timing: Definitions
Write cycleRead access Read access
Read cycle
Write access
Data written
Data valid
DATA
WRITE
READ
ECE 261 Krish Chakrabarty 4
Memory Architecture: Decoders
Word 0
Word 1
Word 2
Word N2 2
Word N2 1
Storagecell
M bits M bits
N
w o r d s
S0
S1
S2
SN2 2
A 0
A 1
AK2 1
K 5 log2N
SN2 1
Word 0
Word 1
Word 2
Word N2 2
Word N2 1
Storagecell
S0
Input-Output(M bits)
Intuitive architecture for N x M memoryToo many select signals:
N words == N select signalsK = log2N
Decoder reduces the number of select signals
Input-Output(M bits)
D e c o d e r
3
ECE 261 Krish Chakrabarty 5
Row
Dec
oder
Bit line2L 2 K
Word line
AKAK1 1
AL 2 1
A0
M.2K
AK2 1
Sense amplifiers / Drivers
Column decoder
Input-Output(M bits)
Storage cell
Array-Structured Memory ArchitectureProblem: ASPECT RATIO or HEIGHT >> WIDTH
Amplify swing torail-to-rail amplitude
Selects appropriateword
ECE 261 Krish Chakrabarty 6
Hierarchical Memory Architecture
Advantages:Advantages:1. Shorter wires within blocks1. Shorter wires within blocks2. Block address activates only 1 block => power savings2. Block address activates only 1 block => power savings
Globalamplifier/driver
Controlcircuitry
Global data busBlock selector
Block 0
Rowaddress
Columnaddress
Blockaddress
Block i Block P 2 1
I/O
4
ECE 261 Krish Chakrabarty 7
Read-Only Memory CellsWL
BL
WL
BL
1WL
BL
WL
BL
WL
BL
0
VDD
WL
BL
GND
Diode ROM MOS ROM 1 MOS ROM 2
ECE 261 Krish Chakrabarty 8
MOS OR ROM
WL[0]
VDD
BL[0]
WL[1]
WL[2]
WL[3]
Vbias
BL[1]
Pull-down loads
BL[2] BL[3]
VDD
5
ECE 261 Krish Chakrabarty 9
ROM Example• 4-word x 6-bit ROM
– Represented with dot diagram– Dots indicate 1’s in ROM
Word 0: 010101Word 1: 011001Word 2: 100101Word 3: 101010
ROM Array
2:4DEC
A0A1
Y0Y1Y2Y3Y4Y5
weakpseudo-nMOS
pullups
Looks like 6 4-input pseudo-nMOS NORs
ECE 261 Krish Chakrabarty 10
MOS NOR ROM
WL[0]
GND
BL [0]
WL [1]
WL [2]
WL [3]
VDD
BL [1]
Pull-up devices
BL [2] BL [3]
GND
6
ECE 261 Krish Chakrabarty 11
MOS NOR ROM Layout
Programmming using theActive Layer Only
Polysilicon
Metal1
Diffusion
Metal1 on Diffusion
Cell (9.5λ x 7λ)
ECE 261 Krish Chakrabarty 12
MOS NOR ROM Layout
Polysilicon
Metal1
Diffusion
Metal1 on Diffusion
Cell (11λ x 7λ)
Programmming usingthe Contact Layer Only
7
ECE 261 Krish Chakrabarty 13
MOS NAND ROM
All word lines high by default with exception of selected row
WL [0]
WL [1]
WL [2]
WL [3]
VDD
Pull-up devices
BL [3]BL [2]BL [1]BL [0]
ECE 261 Krish Chakrabarty 14
MOS NAND ROM Layout
No contact to VDD or GND necessary;
Loss in performance compared to NOR ROMdrastically reduced cell size
Polysilicon
Diffusion
Metal1 on Diffusion
Cell (8λ x 7λ)Programmming usingthe Metal-1 Layer Only
8
ECE 261 Krish Chakrabarty 15
NAND ROM LayoutCell (5λ x 6λ)
Polysilicon
Threshold-alteringimplant
Metal1 on Diffusion
Programmming usingImplants Only
ECE 261 Krish Chakrabarty 16
Decreasing Word Line Delay
Metal bypass
Polysilicon word lineK cells
Polysilicon word lineWLDriver
(b) Using a metal bypass
(a) Driving the word line from both sides
Metal word line
WL
9
ECE 261 Krish Chakrabarty 17
Precharged MOS NOR ROM
PMOS precharge device can be made as large as necessary,but clock driver becomes harder to design.
WL [0]
GND
BL [0]
WL [1]
WL [2]
WL [3]
VDD
BL [1]
Precharge devices
BL [2] BL [3]
GND
pref
ECE 261 Krish Chakrabarty 18
Read-Write Memories (RAM)STATIC (SRAM)
DYNAMIC (DRAM)
Data stored as long as supply is appliedLarge (6 transistors/cell)FastDifferential