Ch. 7

Memory and Programmable Logic

Memory and Programmable Logic

Random-Access MemoryMemory DecodingError Detection and CorrectionRead-Only MemoryProgrammable Logic ArrayProgrammable Array LogicSequential Programmable Devices

Memory

Memory – A device to which binary information is transferred

for storage.Type of memory

– random access memory , RAM– read-only memory, ROM

Write operation– Storing new information into memory

Read operation– Transferring the stored information out of memory

RAM

RAM– The time it takes to transfer information to or from

any desired random location is always the same

Storage unit–byte– byte ： 8 bits

– Length of a word ： multiple of 8 bits

– word ： represent a number, an instruction, alphanumeric character

Capacity of memory–total number of bytes

Block diagram of memory unit

k address lines ：select one particular word

read, write ： specify the direction of transfer

n data input line ：provide the information to be stored in memoryn data output line ：supplying the information coming out of memory

Capacity of memory

Range of in memory size– 210~232 words

bytes– K=210 、 M=220 、 G=230 。– 64K=216 、 2M=221 、 4G=232 。

Memory 1K x 16– 10 bits address ， 16 bits in each word

Determine the no. of bits for address

mk 2 k: no. of address bitsm: total number of words

Control inputs to memory chip

memory enable read/write memory

operation 0 x None 1 0 Write to select

word 1 1 read from

selectd word

Memory cycle timing waveforms

access time– the time required to select a word and read it

cycle time– the time required to complete a write cycle

access time 、 cycle time– equal to a fixed number of CPU clock

See Fig. 7-4

Types of memory

The mode of access of a memory– RAM-volatile

• Static RAM(SRAM)– internal latch – easier to used and shorter read and write time

• Dynamic RAM(DRAM) – electric charges on capacitor– less power consumption– larger storage capacity

– ROM-nonvolatile • Read/write time depend on the distance between

the magnetic reader/writer and the data

Memory Decoding

Decoder – select the memory word specified by the input

address

2-dimensional coincident decoding is a more efficient decoding scheme for large memories

Memory cell

One bit memory cell

4X4 RAM

Coincident Decoding－ two-dimensional selection scheme

Decoder with k input and 2k output requires 2k AND gates with k input

k input decoder can be implemented by two k/2 input decoders with one for column and another for row

e.g., 10×1024 decoder can be implemented by two 5×32 decoders

Example for two-dimensional decoder

Address multiplexing64K-word memory

Read-Only Memory

ROM ： permanent binary information is stored

k input, n output ROM

ROM

No data inputIntegrated circuit ROM have one or more

enable inputSometimes come with three-state outputs

to facilitate the construction of large arrays of ROM

Internal logic of 32X8 ROM

ROM truth table

Table 7-3 32×8 ROM truth table 輸入 輸出

I4 I3 I2 I1 I0 A7 A6 A5 A4 A3 A2 A1 A0

0 0 0 0 0 1 0 1 1 0 1 1 0

0 0 0 0 1 0 0 0 1 1 1 0 1

0 0 0 1 0 1 1 0 0 0 1 0 1

0 0 0 1 1 1 0 1 1 0 0 1 0 . . .

.

.

.

1 1 1 0 0 0 0 0 0 1 0 0 1

1 1 1 0 1 1 1 1 0 0 0 1 0

1 1 1 1 0 0 1 0 0 1 0 1 0

1 1 1 1 1

0 0 1 1 0 0 1 1

Programmomg the ROM according to Taable 7-3

× denote a connection in place of a dot used for permanent connection

Example 7-1

輸入 輸出

A2 A1 A0 B5 B4 B3 B2 B1 B0

十進位

0 0 0 0 0 0 0 0 0 0

0 0 1 0 0 0 0 0 1 1

0 1 0 0 0 0 1 0 0 4

0 1 1 0 0 1 0 0 1 9

1 0 0 0 1 0 0 0 0 16

1 0 1 0 1 1 0 0 1 25

1 1 0 1 0 0 1 0 0 36

1 1 1

1 1 0 0 0 1

49

Design a combinational circuit with 3-input using a ROM.Output = square(input)

ROM implementation of Example 7-1

Types of ROMs

The required path in a ROM may be programmed in four different ways. – mask programming (mask ROM)

• Mask is done by Fab. company during the last fabrication

• Customer must fill out the truth table

• High cost

– programmable read-only memory(PROM)• allows users to program in Lab.

• the program is irreversible

Types of ROMs

– Erasable PROM(EPROM)• by ultraviolet light

– electrically-erasable PROM(EEPROM or E²PROM) ，

• by electrical signal • can be erased without removing it from tis socket

Types of PLD (Programmable Logic Device)

Programmable Logic Array (PLA)

similar to PROM does not provide full decoding and does

not generate all the mintermsdecoder is replaced by an array of AND

gate

PLA with 3 inputs, 4 product terms, and two outputs

PLA Programming Table

PLA Programming Table consists of three sections

– 1st, list the product terms numerically

– 2nd, specify the required path between inputs and AND gates

– 3rd, specifies the paths between the AND and OR gates

outputs

inputs (T) (C) Product

term A B C F1 F2

BA 1 1 0 - 1 -

AC 2 1 - 1 1 1

BC 3 - 1 1 - 1

CBA 4 0 1 0

1 -

Example 7-2

Implement the following two Boolean functions with a PLA:

Simplified by K-map ：

)7,6,5,0(),,(

)4,2,1,0(),,(

2

1

CBAF

CBAF

CBAACABF

BCACABF

2

1 )(

Solution of Example 7-2