Microprocessor memory interfacing basics

Memory Basics
• Memory is generally divided into locations that store a fixed amount
of data. (usually a byte)
• For the entire memory to be useable, the processor should be
capable of generating a unique address for each location.

• The total number of unique addresses that can be generated by the
processor is referred to as its address space.
• The size of the address space is limited by the width of the address
bus by:
Where n is the width of the address space.
• What do we really mean when we refer to a memory size of 1GB?

Address Connections
• All memory devices have address inputs that select a memory
location within the memory device.
• They are usually labelled from the least significant address input to ,
the most significant.
• A 1K device has 10 address pins; labelled .
• Memory addresses are usually represented in hexadecimal.
• Example: 400H represents 1K-bytes. If a memory device is decoded to
begin at address 1000H, and it is a 1K device, what is the address of
the last memory location?
• Others you should keep in mind: 1000H : 4K, 10000H : 64K, etc

Practical Application
Processors have a limit on how
much memory they can make
use of based on the address
space.

Memory can be organized in different ways:
8 x 1 byte
or
8 x 8 bit
16 x 4 bit
64 x 1 bit
capacity of all these is 64 bits

Data Connections
• All memory devices have a set of data outputs or input/outputs. The
figure in slide 4 has a set of common I/O pins.
• The data connections are the points at which data is entered for
writing or extracted for reading.
• Data pins on memory devices are almost always labelled Do – D7 for an
8-bit wide memory device. (Often called byte-wide memory)
• It is however possible to have 16-bits, 4-bits, or just 1-bit wide
memory devices.

• Catalogue listings of memory devices often refer to memory locations
times bits per location.
• Example: 1K x 8, 16k x 1, etc.
• Memory devices are also classified according to the total bit capacity.
• Example: 1K x 8 may be listed as 8K, 64K x 4 as 256K, etc
• Variations occur across manufacturers.

TEST YOUR UNDERSTANDING!
• Why is the data bus bi-directional?
• If a 32 bit microprocessor system is designed to access a memory
system of total of 256 K bytes what is the data bus and the address
bus lengths of the system.

Selection Connections
• Each memory device has an input that selects or enables the device.
• (Refer again to image on Slide 4)
• This input is often called a chip select (CS or ) or chip enable (CE or ).
It is sometimes simply referred to as a Select (S or ) input.
• If this input is active the memory device performs a read or write. If it
is inactive, the memory device is disabled.

Control Connections
• All memory devices have some form of control input(s).
• This input determines what kind of action is performed on the device.
• A ROM usually has one control input, the output connection ( or gate
( input. (Why?)
• A RAM device has one or two control inputs.
• If there exists only one control input it is usually labelled .
• If there are two control inputs, they are labelled and

Read and Write Protocols
Timing Diagrams
• Most common method for describing a communication protocol is by
the use of timing diagrams.
• On a timing diagram, time proceeds to the right on x-axis.
• A control signal is shown with a single line and may by low or high at
some intervals.
• A signal may be active low (e.g., G’, , or G_L).
• The term assert is used to indicate that the signal is made active and
deassert means deactivated. Asserting G means set G=0.

• Deassert for an active low signal:
• Data signals on the other hand are usually represented by a single line
when inactive and a ‘double’ line when active.

TEST YOUR UNDERSTANDING…
•Can you explain the difference between these
three terms:
•Setup time, read time, write time?

Assignment.
 Read about different types of memory systems in use in today’s
industry.
 ROM, RAM, Disk, Cache, etc.

Microprocessor memory interfacing basics

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