Memory

Section 7.2

Types of Memories

• Definitions–Write: store new information into

memory– Read: transfer stored information out of

memory

• Random-Access Memory (RAM)– Can read and write

• Read-Only-Memory (ROM)– Read only

Interesting Facts

• Interesting fact of RAM: the time it takes to transfer information to or from any desired random location is always the same

• A word =a group of bits– A group of 8 bits is called a byte.–

Two Types of RAM

• Static RAM (SRAM): Stored information remains valid as long as power is applied to the unit.

• Dynamic RAM (DRAM): Stores binary information in the form of electric charges on capacitor provided inside the chip by MOS transistors.

74LS189 RAM

64 bits=16 4-bits words.

Block Diagram of a Memory Unit

74LS189 RAM

[A3,A2,A1,A0]=address inputs[D3,D2,D1,D0]=data inputs[S3,S2,S1,S0]=outputsME,WE control the direction of transferVCC=powerGND=ground

Logic Diagram memory cell

Each word is enabled by the 4-input AND

Write →Read

Logic Diagram memory cell

Each word is enabled by the 4-input AND

Switch Characteristics

Switching Time Waveforms

17 nS 23 nS

-7 nS for address-14 nS for data

A negative hold time means that the address/data can change before the rising edge of WE because the thereis internal delay through the chip.

Write (ME=0, WE=0)

0

0

10

11

11

D1

D2D3D4 [hi Z?]

READ (ME=0, WE=1)

1

0

01

Complement of data stored

HOLD (ME=1, WE=X)

X

1

0Hi-Z output

Memory Description in Verilog

memory depth: 64 words

word length4-bits

Need 6 bits address for 26=64 words.

Enable ReadWrite

1 1 Read

1 0 Write

0 X Hi-Z