Mask ROM (Mask ROM)
When a memory IC is manufactured in the factory, programs are
written to it. A mask is a plate of glass imprinted with patterns
of wirings and transistors used in the IC manufacturing process.
Users have a semiconductor manufacture custom-build a mask to manufacture
a memory IC.
Advantages
- Suitable for volume production
- Low cost
Disadvantages
- Long lead-time from ordering to completion
- Not reprogrammable once built
EPROM (Erasable & Programmable ROM)
Stored data can be erased by ultraviolet irradiation. To this
end, a special package with a glass window is used. An EPROM writer
is used to write to EPROM. EPROM is erasable and programmable about
100 times.
Advantage
- Erasable and programmable and thus convenient for testing and debugging
Disadvantage
- Expensive because of the use of a special package
OTPROM (One Time Programmable ROM)
An EPROM chip housed in an inexpensive plastic package. Stored
data cannot be erased by ultraviolet irradiation because no glass window
can be attached to the plastic package. OPTROM can be written only
once, but it comes by far cheaper than EPROM. Programs are debugged
in EPPROM and, when finalized, moved to OTPROM for volume production.
Advantage
- Cheaper than EPROM and suitable for small-batch production
Disadvantage
- Not erasable and programmable
EEPROM (Electrically Erasable & Programmable
ROM)
EEPROM can be electrically erased and programmable, and can be reprogrammed
when mounted on a board as a finished product. EEPROM is reprogrammable
about tens of thousand times.
Advantages
- Onboard reprogrammable
- Ready for infinite times of reprogramming
Disadvantage
- Expensive
Flash memory (Flash Memory)
A variation of flash memory, which is cheaper and larger-sized.
Advantage
- Cheaper and larger-sized than EEPROM
Disadvantages
- Unable to write address by address, unlike EEPROM
- Memory IC divided into blocks for erasure and reprogramming block
by block
Static RAM (Static RAM)
RAM with its storage circuit built of flip-flops. Given a supply
voltage, static RAM preserves stored data intact. It dissipates least
power when out of use. Because six transistors are used to build
its flip-flops, static RAM offers less storage capacity than does ROM.
Advantages
- Fast
- Low power consumption and suitable for battery backup
Disadvantages
- Expensive
- Small storage capacity
Dynamic RAM (Dynamic RAM)
Simplified storage circuitry with only one capacitor and one transistor
to provide each bit of memory, and hence larger-sized than static RAM.
Charges on the capacitors, however, drain with time, resulting ultimately
in loss of stored data. Before such loss, all
stored data must be read out and refreshed. Power alone does not
allow dynamic RAM to retain stored data but requires refreshing for that
purpose.
Advantage
- Cheap and large-sized
Disadvantage
- Refreshing required
Table 1.1 summarizes features of key ROM and RAM devices.
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