Embedded Systems: Memory

Embedded Systems: Memory

Unit Structure

Objectives

1. Introduction

2. Types of Memory

3. Types of RAM

SRAM

DRAM

4. Types of ROM

MASKED

PROM

EPROM

5. Types of Hybrid Memory

NVRAM

FLASH

EEPROM

6. DIRECT MEMORY ACCESS (DMA)

OBJECTIVES

After reading this chapter you will understand:

Different types of memory available

Types of RAM

Types of ROM

Types of Hybrid Memory

1. INTRODUCTION

There are different types of memories available to be used in computers as well as embedded system.

This chapter guides the reader through the different types of memories that are available and can be used and tries to explain their differences in simple words.

2 TYPES OF MEMORY

There are three main types of memories, they are

RAM (Random Access Memory)

It is read write memory.

Data at any memory location can be read or written.

It is volatile memory, i.e. retains the contents as long as electricity is supplied.

Data access to RAM is very fast

ROM (Read Only Memory)

It is read only memory.

Data at any memory location can be only read.

It is non-volatile memory, i.e. the contents are retained even after electricity is switched off and available after it is switched on.

Data access to ROM is slow compared to RAM

HYBRID

It is combination of RAM as well as ROM

It has certain features of RAM and some of ROM

Like RAM the contents to hybrid memory can be read and written

Like ROM the contents of hybrid memory are non volatile

The following figure gives a classification of different types of memory

3 TYPES OF RAM

  There are 2 important memory device in the RAM   family.

SRAM (Static RAM)

DRAM (Dynamic RAM)

SRAM (Static RAM)

It retains the content as long as the power is applied to the chip.

If the power is turned off then its contents will be lost forever.

DRAM (Dynamic RAM)

DRAM has extremely short Data lifetime(usually less than a quarter of second). This is true even when power is applied constantly.

A DRAM controller is used to make DRAM behave more like SRAM.

The DRAM controller periodically refreshes the data stored in the DRAM. By refreshing the data several times a second, the DRAM controller keeps the contents of memory alive for a long time.

4 TYPES OF ROM

There are three types of ROM described as follows:

Masked ROM

These are hardwired memory devices found on system.

It contains pre-programmed set of instruction and data and it cannot be modified or appended in any way. (it is just like an Audio CD that contains songs pre-written on

it and does not allow to write any other data)

The main advantage of masked ROM is low cost of production.

PROM (PROGRAMMABLE ROM )

This memory device comes in an un-programmed state i.e. at the time of purchased it is in an un-programmed state and it allows the user to write his/her own program or code into this ROM.

In the un-programmed state the data is entirely made up of 1’s.

PROMs are also known as one-time-programmable (OTP) device because any data can be written on it only once. If the data on the chip has some error and needs to be modified this memory chip has to be discarded and the modified data has to be written to another new PROM.

EPROM (ERASABLE-AND-PROGRAMABLE ROM)

It is same as PROM and is programmed in same manner as a PROM.

It can be erased and reprogrammed repeatedly as the name suggests.

The erase operation in case of an EPROM is performed by exposing the chip to a source of ultraviolet light.

The reprogramming ability makes EPROM as essential part of software development and testing process.

5 TYPES OF HYBRID MEMORY

There are three types of Hybrid memory devices:

EEPROMs

EEPROMs stand for Electrically Erasable and Programmable ROM.

It is same as EPROM, but the erase operation is performed electrically.

Any byte in EEPROM can be erased and rewritten as desired

Flash

Flash memory is the most recent advancement in memory technology.

Flash memory devices are high density, low cost, nonvolatile, fast (to read, but not to write), and electrically reprogrammable.

Flash is much more popular than EEPROM and is rapidly displacing many of the ROM devices.

Flash devices can be erased only one sector at a time, not byte by byte.

NVRAM

NVRAM is usually just a SRAM with battery backup.

When power is turned on, the NVRAM operates just like any other SRAM but when power is off, the NVRAM draws enough electrical power from the battery to retain its content.

NVRAM is fairly common in embedded systems.

It is more expensive than SRAM.

6. DIRECT MEMORY ACCESS (DMA)

DMA is a technique for transferring blocks of data directly between two hardware devices.

In the absence of DMA the processor must read the data from one device and write it to the other one byte or word at a time.

DMA Absence Disadvantage: If the amount of data to be transferred is large or frequency of transfer is high the rest of the software might never get a chance to run.

DMA Presence Advantage: The DMA Controller performs entire transfer with little help from the Processor.

Working of DMA

The Processor provides the DMA Controller with source and destination address & total number of bytes of the block of data which needs transfer.

After copying each byte each address is incremented & remaining bytes are reduced by one.

When number of bytes reaches zeros the block transfer ends & DMA Controller sends an Interrupt to Processor.