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Read only memory (ROM) 只读存储器
ROM is nonvolatile memory (非易失 性存储器)；that is ,its contents are preserved even if no power is applied.
Memory-not!
(P800)
Address inputs
Static RAM (SRAM)
The memory device is a latch ,the data is stable, and the access time is short, but the circuit is complex and expensive !
The data can only be write in the manufacture process, can not be changed by the users !
Read only memory (ROM)
P-ROM: Programmable ,add a fuse to the diode.
The floating gate can be charged to write “0” and uncharged to write “1” !
Random access memory (RAM)
Use memory device to store data bit, and use word line to control its input/output from bit line .
Data bits are stored in a structured way
Once a word line is selected, all the bits in this line can be in output through the bit lines !
Memory
Example: When a2a1a0= 000, y4y3y2y1= 1000; When a2a1a0= 101, y4y3y2y1= 0110;
y 1 m 7 m 4 m 2 m 1 a 2 a 1 a 0 y 3 m 7 m 5 m 3 m 2 m 1 a 2 a 1 a 0
Read only memory (ROM)
AND-OR structure: Each word line is an output of AND gate; Each bit line is an output of OR gate.
A1
阵 列
. （译码器） A1
A0
.W 3 A0 输出缓冲器
. D3 位
DD 33
EN
. . D2
线
EN
. D 1
D2
D2
D1
D1
EN
. D0
DD00
EN
EN
W0=1,ACTIVE HIGH
W0 A1A0
与门阵列输出表达式
或门阵列输出表达式
W0 A1A0
D0W 0W2
W1 A1A0
D 1W 1W 2W 3
Memory
A memory can be treated as a combinational circuit !
A truth table is stored in the memory array ! Each bit line is a function output !
Memory
Example:
The fuse can be melted by the user to write “0” !
W0
W1 W2 W3
VC C
1
A1
1 ..
1
A0
1
.
或..
门
阵 列
.
（编码器）
..
. . .. . .. .. .. .. W 0 字 W 1 线 W 2 .. ..
.. .. .. . ..
.
.
与
门
▪ Bit line:
▪ The bit line corresponds to one output bit of the ROM.
Read only memory (ROM)
Mask ROM: use a diode or transistor as memory connection.
Active-high decoder output (word line )
W2 A1A0
W3A1A0
D 2W 0W 2W 3 D3W1W3
ROM存储内容的真值表
地址
A1
A0
0
0
0
1
1
0
1
1
存储内容
D3
D2
D1
D0
0
1
0
1
1
0
1
0
0
1
1
1
1
1
1
0
Memory
Construct with a binary decoder and a memory array !
Memory
... ...
Memory
Word line
W0Βιβλιοθήκη A0 A1W1...
decoder
Wi
...
Memory cells
0 cell 1 cell
i cell
...
...
An1
W 2n -1
n
2 -1
cell
...
Bit line D0 D1
Db 1
Data Output
▪ Word line :
▪ each decoder output is called a “word line”because it selects one row or word of the table stored in the ROM.
memory
Memory
▪ we usually reserve the word “memory” to refer to bits that are sorted in a structured way,usually as a two-dimensional array in which one row of bits is accessed at a time.(P799)
9.1.4 Commercial ROM Types
▪ Mask ROM ▪ PROM ▪ EPROM ▪ (OTP)ROM ▪ EEPROM ▪ Flash EPROM
Read only memory (ROM)
EPROM(可擦除可编程只读存储器): Erasable PROM ,use floating-gate technology. Include EEPROM and FLASH .