Read Only Memory

1
Read Only Memory

• Read only memory is used to store data on a more
  permanent basis
• It is not volatile memory
• It is used to store programs, long term data and
  can sometimes be used in the place of
  combinational logic
• Putting data into ROM is generally referred to as
  programming the device

2
ROM Signals

• ROM devices will have at least the following
  signals
• DATA BUS This is the set of lines which carry
  data into and out of memory. The memory width is
  how many bits cane be written or read
  simultaneously
• ADDRESS BUS Set of lines which is used to
  specify location of data to be written to or read
  from
• WR line Tells RAM it is going to be written tow
  or read from
• OE line Enables the data bus as an output for
  when we read the device
• CS line Used to select device
• Programming signals Used to signal the memory
  device that data is to be programmed. Older
  devices used high voltage (21V) signals to
  program the device

3
Mask ROM

• This type of memory device is manufactured with
  data already stored in it
• It is typically used for very high volume
  production when the data has already been
  thoroughly tested
• It is a cheap option as it avoids having to
  program the devices yourself
• But once the device is manufactured the data can
  not be altered

4
PROM

• Programmable read only memory is almost obsolete
  now
• These devices were programmed by blowing tiny
  fuses inside the device
• Therefore these devices could be programmed only
  once

5
EPROM

• Erasable Programmable read only memory is also
  nearly obsolete technology
• These devices could be programmed and erased by
  shinning ultraviolet (UV) light through a quartz
  window onto of the chip
• This type of packaging was expensive, and erasing
  the device required a UV light source
• They needed to be exposed to the UV source for at
  least 15 minutes
• They also have a fairly limited endurance
• This is the number of times a device can be
  erased and then reprogrammed
• EPROM typically has an endurance of several
  hundred cycles

6
OTP

• One time programmable memory is a cheaper option
  to EPROM
• These devices use EPROM but do not have the
  expensive quartz window in the packaging
• These devices can only be programmed once and are
  generally preferred to PROM

7
EEPROM

• Electrically erasable PROM chips can be
  programmed and erased with an electrical signal
  as opposed to UV light
• This makes these devices a lot more accessible
  and useable
• They are cheap and are used in a lot of different
  devices
• Some microcontrollers have onboard EEPROM to
  store their programs on
• Many modern microcontrollers have a block of
  EEPROM which is used to store configuration data
• Typical EEPROM endurance figures are 1 000 000
  cycles
• It is a low density technology and therefore
  limits its use as bulk storage devices

8
Flash

• Flash memory is also electrically erasable and
  non volatile
• It has much higher densities to EEPROM and
  therefore has become the bulk storage medium of
  choice
• It does however have lower endurance than EEPROM
• This is improved by techniques such as wear
  levelling and delayed writing
• It is cheap to produce

Taken from Wikipedia
9
Programmable Logic Technologies

• Programmable logic is used to build complex
  circuits without having to wire up discrete gates
• They consist of chips which contain many logic
  gates
• The way in which the gates are connected is
  determined by the way the device is programmed
• Programmable logic is commonly used as it is more
  compact and faster than a circuit made out of
  discrete gates
• There has been remarkable progress in the
  development of programmable logic devices over
  the last few years
• Because of the large number of changes there are
  very few standards in terms of signals and device
  compatibility

10
PAL and PLA

• Programmable array logic and programmable logic
  arrays were the earliest form of programmable
  logic
• There had a simple grid structure which was
  programmed by blowing fuses
• They could therefore only be programmed once
• The programming was done in PALASM, ABEL or CUPL

11
GAL and PEEL

• Generic array logic is slightly more complex and
  flexible than PAL
• They are reprogrammable devices and typically
  have a Joint Test and Access Group (JTAG)
  interface for programming
• This is an industry standard (IEEE 1149.1) four
  wire interface which has become the major
  programming and debugging interface for
  programmable logic, microcontrollers and larger
  processors
• Programmable electrically erasable logic works
  very similarly to GAL except that it uses CMOS
  rather than bipolar technology

12
CPLD

• Complex programmable logic device are typically
  made up of many small PALs connected together
  with a programmable interconnection system
• They are much more flexible than any of the
  previous families

13
FPGA

• Field programmable gate arrays are at the high
  end of the market
• They consist of lots of programmable logic blocks
  
• Each block is quite complex and typically
  consists of PAL logic as well as configurable
  flip flops
• They can contain several million gates
• They often have peripherals such as
• Phase locked loop clock multipliers
• Memory blocks
• Communication systems
• Some FPGAs are big enough to hold entire
  microcontrollers
• These are known as soft core microcontrollers and
  are becoming more popular