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]=outputs
ME,WE control the direction of transfer
VCC=power
GND=ground
Logic Diagram
Each word is enabled by the 4-input AND
memory cell
Write →Read
Logic Diagram
Each word is enabled by the 4-input AND
memory cell
Switch Characteristics
Switching Time Waveforms
17 nS
23 nS
A negative hold time means that the address/data
can change before the rising edge of WE because the there
is internal delay through the chip.
-7 nS for address
-14 nS for data
Write (ME=0, WE=0)
0
0
1
D1
D2
D3
D4
0
1
1
1
1 [hi Z?]
READ (ME=0, WE=1)
1
0
0
1
Complement
of data stored
HOLD (ME=1, WE=X)
X
1
0
Hi-Z output
Memory Description in Verilog
Need 6 bits address for 26=64 words.
memory depth: 64 words
word length
4-bits
Enable
ReadWrite
1
1
Read
1
0
Write
0
X
Hi-Z