Micro Computer Based System Design

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MCBSD

Micro Computer Based

System Design


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Tuesday, January 24,2012 2

Memory Hierarchy

Tape FloppyCD-ROM

CD-RW

Fixed Disk

Main Memory

Cache

Registers Access speed

Capacity

Cost/Bit


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Semiconductor memory

Characteristics:Access times (read, write, erase):

Faster is better (varies from few ns to hundreds of ns)

Volatility:

Ability to retain data after power is removed.

Power consumption:

Less is better ( varies from nw to mw).

Density:

Larger is better ( bit/sq.micron, transistors/bit).Cost:

Cheaper the better.

Access Times


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Memory organisationMemory words

m n memory

nbits per word

m

words

enable

Memory

2kx n

A0

r/w

Q0Qn-1

Ak-1

memory external view


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Memory organisationA memory chip typically has:

m x n: m words/locations ofnbits each.k = Log2(m) address input pins

orm = 2^k words

e.g., 4,096 x 8 memory: 32,768 bits

12 address input pins

8 bidirectional data pins


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Memory organisation (contd)Problems

A given memory chip has 14 address lines

and 4 data pins. Find:

a) organization b) capacity.

A 512K memory chip has 8 pins for data.

Find: a) organization b) no of address pinson the memory chip.


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Semiconductor MemoryMemory types:

Semiconductor memory

ROM RAM

PROM EPROM EEPROM Flash

PROM

Mask

PROM

SRAM DRAM NV-

RAM


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PROM (programmable ROM)PROM is a user-programmable memory

PROM is also referred to as OTP (one-time programmable).

Programming ROM, also called burningROM- a ROM burner orROM programmer.

For every bit of the PROM, there exists a fuse. If the information

burned into PROM is wrong, that PROM must be discarded since

its internal fuses are blown permanently.


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PROM storage cell

1-bit cell


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EPROM

(erasable programmable ROM)In EPROM, one can program the memory chip and erase it

thousands of times.

A widely used EPROM is called UV-EPROM

UV stands for ultra-violet.

UV-EPROM erasure is time consuming, takes 20

minutes.

All UV-EPROM chips have a window that is used to

shine ultraviolet (UV) radiation to erase its contents.

To burn the ROM, ROM burner use 12.5V or higher

Vpp.


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UV-PROM


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Programming & erasing a

EPROM

To program a UV-EPROM chip :

To burn code or data into EPROM, the ROMburner uses 12.5 volts, Vpp.

To erase a EPROM:

Placed in EPROM erasure equipment to expose it

to UV radiation for 15-20 minutes


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Tuesday, January 24,2012 Priya Sawant 13

EEPROM

(electrically programmable PROM)

EEPROMs method of erasure is electrical

and therefore instant.

A single byte can be erased.PROM does not require an external erasure and

programming device.

Circuitry to program EEPROM is incorporatedon the system board.Vpp -12.5V & 5V.


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Flash memory EPROMPopular since early 1990.

The erasure method is electrical, is done in flash.

Programming can be done on the system board.

So commonly called flash memory.

The major difference between EEPROM and flash

memory is

Entire contents are erased in flash memory.

In some flash memories erasure can be done block

by block.


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Flash memory (contd)Can flash memory replace part of the hard disk asa mass storage medium?

Widely used as a way to upgrade PC BIOS ROM.

Access time of flash memory -100 ns; disk access time- tens of milliseconds.

Flash memorys program/erase cycles must becomeinfinite like hard disks & RAM.

Program/erase cycle refers to the number of times that a chipcan be erased and programmed before it becomes unusable.

The program/erase cycle is 100,000 for flash and EEPROM,1000 for UV-EPROM.


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Mask ROMMask ROM : Not user-programmable, contents are

programmed by the IC manufacturer.

UV-EPROM or flash memory are used in the

development phase, mask ROM is used in the finalversion.

If an error in the data/code is found, the entire batch

must be thrown away.

Many 8051 manufacturers support mask version of theROM.

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