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ECE332, Week 10
October 31, 2007
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Topics
Memory Write Ability and Storage Permanence
Common Memory Types
Composing Memory
DRAM
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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:
32,768 bits
12 address input signals
8 input/output data signals
Memory access
r/w: selects read or write
enable: read or write only when asserted
multiport: multiple accesses to different
locations simultaneously
m n memory
…
…
n bits per word
m words
enable
2k n read and write
memory
A0…
r/w
…
Q0
Qn-1
Ak-1
memory external view
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Write Ability
Ranges of write ability
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 embedded system using the
memory
Memories in high end and middle range of write ability
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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
e.g. DRAM
Nonvolatile memory
Holds bits after power is no longer supplied
High end and middle range of storage permanence
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ROM: Read-Only Memory
Nonvolatile memory
Can be read from but not written to, by
a processor in an embedded system
Traditionally, “programmed” before
inserting to embedded system
Uses
Store software program for general-
purpose processor
program instructions can be one or
more ROM words
Store constant data needed by system
Implement combinational circuit
2k n ROM
…
Q0
Qn-1
A0
…
enable
Ak-1
External view
