Page 113 - Digital Electronics by harish

DDR SDRAM power

DDR SDRAM provides an improvement in speed, but, the power dissipation is high.
The power required by a DDR SDRAM is related to the number of rows that are open at any
one time. Thus to gain the fastest operation, it is necessary to open a number of rows
together, but this consumes more power.

4.7 Read only memory

ROM (Read Only Memory) is a form of semiconductor memory that retainsits
contents even when the power supply is switched off. So, ROM is called Non-volatile
memory. In ROM, we can read the data any number of times but data can be written to once
during the manufacturing process only. ROM is used to store the “boot” or start-up program
(so called firmware) that acomputer executes when powered on. ROM is also used in systems
with fixed functionalities, e.g.controllers in cars, household appliances etc.

There are some special varieties of ROM such as PROM, EPROM, EEPROM and
Flash memory.

PROM (Programmable Read Only Memory) : PROM is like ROM but allows end-users to
write their own programs anddata. It requires special PROM writing equipment. The users
can only write-once to PROM.

EPROM (Erasable Programmable Read Only Memory) : With EPROM we can erase
(using strong ultra-violet light) the contentsof the chip and rewrite it with new contents,
typically several thousand times. It is commonly used tostore the “boot” program of a
computer, known as the firmware.

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EEPROM (Electrically Erasable Programmable Read Only Memory)or E PROM: As
the name implies the contents of EEPROMs are erasedelectrically. EEPROMs are also
limited to the number of erase-writes that can be performed (e.g,1,00,000) but support
updates (erase-writes) to individual bytes whereas EPROM updates the wholememory and
only supports around 10,000 erase-write cycles.

FLASH memory: It is a cheaper form of EEPROM where updates (erase-writes) can only be
performedon blocks of memory, not on individual bytes. Flash memories are found in USB
Pen drive, memory cardsand typically range in size from 1GB to 32GB. The number of
erase/write cycles to a block istypically several hundred thousand times.

Types of ROM

 ROM
o Written during manufacture
o Very expensive for small volumes
 Programmable ROM (PROM)
o Read-only
o Write-once
o Needs special equipment to program
o Convenience
 Erasable Programmable (EPROM)
o R/W
o Have to erase before write
o Erased by UV

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