Download Semiconductor Memory

Semiconductor Memory
ECE 3710 Fall 2006
1
Memory: basic concepts
• Stores large number of bits
…
m x n: m words of n bits each
k = Log2(m) address input signals
or m = 2k words
e.g., 4,096 x 8 memory:
m words
–
–
–
–
m × n memory
…
n bits per word
• 32,768 bits
• 12 address input signals
• 8 input/output data signals
memory external view
r/w
• Memory access
2k × n read and write
memory
enable
– r/w: selects read or write
– enable: read or write only when asserted
– multiport: multiple accesses to different locations
simultaneously
A0
…
Ak-1
…
Qn-1
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
Q0
2
•
Traditional ROM/RAM distinctions
–
ROM
•
–
Advanced ROMs can be written to
•
–
Life of
product
Battery
life (10
years)
e.g., EEPROM
Ideal memory
OTP ROM
EPROM
EEPROM
FLASH
NVRAM
Nonvolatile
In-system
programmable
SRAM/DRAM
Near
zero
Write
ability
e.g., NVRAM
Write ability
–
Mask-programmed ROM
Tens of
years
Advanced RAMs can hold bits without
power
•
•
read and write, lose stored bits without
power
Traditional distinctions blurred
–
•
read only, bits stored without power
RAM
•
•
Storage
permanence
Write ability and storage permanence
Manner and speed a memory can be
written
During
External
External
External
fabrication programmer, programmer, programmer
1,000s
OR in-system,
only
one time only
1,000s
of cycles
of cycles
External
In-system, fast
programmer
writes,
OR in-system,
unlimited
block-oriented
cycles
writes, 1,000s
of cycles
Storage permanence
–
Ability of memory to hold stored bits
after they are written
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
Write ability and storage permanence of memories,
showing relative degrees along each axis (not to scale).
ECE 3710 Fall 2008
3
Writability
•
Ranges of writability
– High end
• processor writes to memory simply and quickly
• e.g., RAM
– Middle range
• processor writes to memory, but slower
• e.g., FLASH, EEPROM
– Lower range
• special equipment, “programmer”, must be used to write to memory
• e.g., EPROM, OTP ROM
– Low end
• bits stored only during fabrication
• e.g., Mask-programmed ROM
•
In-system programmable memory
– Can be written to by a processor in the microcomputer system using the memory
– Memories in high end and middle range of write ability
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
4
Storage permanence
•
Range of storage permanence
– High end
• essentially never loses bits
• e.g., mask-programmed ROM
– Middle range
• holds bits days, months, or years after memory’s power source turned off
• e.g., NVRAM
– Lower range
• holds bits as long as power supplied to memory
• e.g., SRAM
– Low end
• begins to lose bits almost immediately after written – refreshing needed
• e.g., DRAM
•
Nonvolatile memory
– Holds bits after power is no longer supplied
– High end and middle range of storage permanence
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
5
ROM: “Read-Only” Memory
External view
A0
Ak-1
– Store software program for general-purpose
processor
– Store constant data (parameters) needed by system
– Implement combinational circuits (e.g., decoders)
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
2k × n ROM
enable
…
• Nonvolatile
• Can be read from but not written to, by a
processor in an microcomputer system
• Traditionally written to, “programmed”,
before inserting to microcomputer system
• Uses
ECE 3710 Fall 2008
…
Qn-1
Q0
6
Example: 8 x 4 ROM
•
•
•
•
Horizontal lines = words
Vertical lines = data
Lines connected only at circles
Decoder sets word 2’s line to 1 if
address input is 010
• Data lines Q3 and Q1 are set to 1
because there is a “programmed”
connection with word 2’s line
• Word 2 is not connected with data
lines Q2 and Q0
• Output is 1010
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
Internal view
8 × 4 ROM
word 0
enable
3×8
decoder
word 1
word 2
A0
A1
A2
word line
data line
programmable
connection
Q3 Q2 Q1 Q0
ECE 3710 Fall 2008
7
Mask-programmed ROM
• Connections “programmed” at fabrication
– set of masks
• Lowest write ability
– only once
• Highest storage permanence
– bits never change unless damaged
• Typically used for final design of high-volume systems
– spread out NRE (non-recurrent engineering) cost for a low unit cost
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
8
OTP ROM: One-time programmable ROM
• Connections “programmed” after manufacture by user
–
–
–
–
user provides file of desired contents of ROM
file input to machine called ROM programmer
each programmable connection is a fuse
ROM programmer blows fuses where connections should not exist
• Very low write ability
– typically written only once and requires ROM programmer device
• Very high storage permanence
– bits don’t change unless reconnected to programmer and more fuses
blown
• Commonly used in final products
– cheaper, harder to inadvertently modify
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
9
EPROM: Erasable programmable ROM
•
Programmable component is a MOS transistor
–
–
–
–
–
•
Transistor has “floating” gate surrounded by an insulator
(a) Negative charges form a channel between source and drain
storing a logic 1
(b) Large positive voltage at gate causes negative charges to
move out of channel and get trapped in floating gate storing a
logic 0
(c) (Erase) Shining UV rays on surface of floating-gate causes
negative charges to return to channel from floating gate restoring
the logic 1
(d) An EPROM package showing quartz window through which
UV light can pass
0V
floating gate
drain
source
(a)
+15V
(b)
source
drain
Better write ability
5-30 min
– can be erased and reprogrammed thousands of times
•
Reduced storage permanence
– program lasts about 10 years but is susceptible to
radiation and electric noise
•
Typically used during design development
source
(d)
.
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
drain
(c)
ECE 3710 Fall 2008
10
Sample EPROM components
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
11
Sample EPROM programmers
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
12
EEPROM: Electrically erasable
programmable ROM
• Programmed and erased electronically
– typically by using higher than normal voltage
– can program and erase individual words
• Better write ability
– can be in-system programmable with built-in circuit to provide higher
than normal voltage
• built-in memory controller commonly used to hide details from memory user
– writes very slow due to erasing and programming
• “busy” pin indicates to processor EEPROM still writing
– can be erased and programmed tens of thousands of times
• Similar storage permanence to EPROM (about 10 years)
• Far more convenient than EPROMs, but more expensive
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
13
FLASH
• Extension of EEPROM
– Same floating gate principle
– Same write ability and storage permanence
• Fast erase
– Large blocks of memory erased at once, rather than one word at a time
– Blocks typically several thousand bytes large
• Writes to single words may be slower
– Entire block must be read, word updated, then entire block written back
• Used with embedded microcomputer systems storing large data
items in nonvolatile memory
– e.g., digital cameras, MP3, cell phones
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
14
Class Project Program Memory
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
15
Example
Show how to use a 128K x 8 FLASH to implement an 8K x 8 FLASH.
A13 – A16
13
A0 – A12
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
16
RAM: “Random-access” memory
external view
• Typically volatile memory
r/w
– bits are not held without power supply
•
2k × n read and write
memory
enable
Read and written to easily by microprocessor
during execution
• Internal structure more complex than ROM
A0
…
Ak-1
…
Qn-1
– a word consists of several memory cells, each
storing 1 bit
Q0
internal view
I3 I2 I1 I0
– each input and output data line connects to each
cell in its column
4×4 RAM
enable
– rd/wr connected to every cell
– when row is enabled by decoder, each cell has logic
that stores input data bit when rd/wr indicates write
or outputs stored bit when rd/wr indicates read
2×4
decoder
A0
A1
Memory
cell
rd/wr
To every cell
Q3 Q2 Q1 Q0
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
17
Basic types of RAM
• SRAM: Static RAM
memory cell internals
– Memory cell uses flip-flop to store bit
– Requires 6 transistors
– Holds data as long as power supplied
SRAM
Data'
• DRAM: Dynamic RAM
– Memory cell uses MOS transistor and
capacitor to store bit
– More compact than SRAM
– Retains data for only 2 – 4 ms
– “Refresh” required due to capacitor leak
Data
W
DRAM
Data
W
• word’s cells refreshed when read
– Slower to access than SRAM
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
18
RAM variations
• PSRAM: Pseudo-static RAM
– DRAM with built-in memory refresh controller
– Popular low-cost high-density alternative to SRAM
• NVRAM: Nonvolatile RAM
– Holds data after external power removed
– Battery-backed RAM
• SRAM with own permanently connected battery
• writes as fast as reads
• no limit on number of writes unlike nonvolatile ROM-based memory
– SRAM with EEPROM or FLASH
• stores complete RAM contents on EEPROM or FLASH before power turned off
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
19
Class Project data/stack memory
HM6264ALP
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
20
Example:
HM6264 & 27C256 RAM/ROM devices
• Low-cost low-capacity memory
devices
• Commonly used in 8-bit
microcontroller-based
embedded systems
• First two numeric digits indicate
device type
– RAM: 62
– ROM: 27
• Subsequent digits indicate
capacity in kilobits
– 64 Kbits = 8 Kbytes
– 256 Kbits = 32 Kbytes
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
11-13, 15-19
2,23,21,24,
25, 3-10
22
data<7…0>
addr<12...0>
11-13, 15-19
data<7…0>
27,26,2,23,21,
addr<14...0>
24,25, 3-10
22
/OE
27
/WE
20
/CS1
26
CS2 HM6264
20
/OE
/CS
27C256
block diagrams
Device
Access Time (ns)
HM6264
85-100
27C256
90
Standby Pwr. (mW)
.01
.5
Active Pwr. (mW)
15
100
Vcc Voltage (V)
5
5
device characteristics
Read operation
Write operation
data
data
addr
addr
OE
WE
/CS1
/CS1
CS2
CS2
HM6264 timing
diagrams
ECE 3710 Fall 2008
21
DRAM Structure
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
22
The TMS4464 (64Kx4) DRAM
But a 64K memory needs 16 address lines?
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
23
The TMS4464 DRAM Timing
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
24
DRAM Address Multiplexer
74LS157, quad 2-line to 1-line data selector/multiplexer
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
25
The 41256 (256Kx1) DRAM
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
26
Example: Address decoding
• 16-bit address bus (A0 – A15)
• Desired memory space:
– EPROM 16 Kbytes (0000, ..., 3FFF)
– RAM 8 Kbytes (4000, ..., 5FFF)
Address
A15 A14 A13
Enable inputs
(active low)
/CS_EPROM
/CS_RAM
0000 – 1FFF
0
0
0
0
1
2000 – 3FFF
0
0
1
0
1
4000 – 5FFF
0
1
0
1
0
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
27
Implementing combinational function
• Any combinational circuit of n functions of same k variables
can be done with 2k x n ROM
• How could we implement the previous EPROM/RAM decoder
with a tiny (and fast) 16-bit ROM circuit?
– a = A15, b = A14, c = A13, y = /CS_EPROM, z = /CS_RAM
Truth table
Inputs (address)
a
b
c
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
Outputs
y
z
0
1
0
1
1
0
1
1
1
1
1
1
1
1
1
1
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
8×2 ROM
0
0
1
1
1
1
1
1
enable
c
b
a
y
1
1
0
1
1
1
1
1
word 0
word 1
word 7
z
ECE 3710 Fall 2008
28
Composing memory
•
•
•
Memory size needed often differs from size of readily
available memories
When available memory is larger, simply ignore unneeded
high-order address bits and higher data lines
When available memory is smaller, compose several smaller
memories into one larger memory
–
–
–
Connect side-by-side to increase width of words
Connect top to bottom to increase number of words
• added high-order address line selects smaller memory
containing desired word using a decoder
Combine techniques to increase number and width of words
Increase number of words
2m+1 × n ROM
2m × n ROM
A0
Am-1
Am
…
…
1×2
decoder
…
2m × n ROM
enable
…
…
…
Qn-1
2m × 3n ROM
2m × n ROM
enable
Increase width
of words
A0
Am
…
2m × n ROM
…
…
Q3n-1
2m × n ROM
Q2n-1
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
A
Increase number
and width of
words
…
…
Q0
…
enable
Q0
ECE 3710 Fall 2008
outputs
29
Memory hierarchy
• Want inexpensive, fast
memory
• Main memory
– Large, inexpensive, slow
memory stores entire
program and data
• Cache
– Small, expensive, fast
memory stores copy of likely
accessed parts of larger
memory
– Can be multiple levels of
cache
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
Processor
Registers
Cache
Main memory
Hard disk
CD or DVD
ECE 3710 Fall 2008
30
Cache
• Usually designed with SRAM
– faster but more expensive than DRAM
• Usually on same chip as processor
– space limited, so much smaller than off-chip main memory
– faster access (1 cycle vs. several cycles for main memory)
• Cache operation:
– Request for main memory access (read or write)
– First, check cache for copy
• cache hit
– copy is in cache, quick access
• cache miss
– copy not in cache, read address and possibly its neighbors into cache
• Several cache design choices
– cache mapping, replacement policies, and write techniques
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
31
Dual-port RAM (DPRAM)
• Usually a static RAM circuit with two address and
data bus connections
– Shared RAM for two independent users
• Flexible communication link between two processors
– Master/slave
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
32
Serial memory interface
• Typically EEPROM or FLASH memory
–
–
–
–
Compact package
Low cost
Slow access
A convenient program storage
Serial vs. parallel interface
(32 Kbytes)
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
33
Summary
• Two memory categories:
– Write ability
– Storage permanence
• Various RAM and ROM alternatives for different needs
• Typically byte-wide memory components in embedded
microcomputer systems
• Compose smaller memory components into larger
memory systems
• Hierarchical memory even in many embedded systems
Embedded Systems Design: A Unified Hardware/Software Introduction
© 2000 Vahid & Givargis
ECE 3710 Fall 2008
34