UNIT-5
MEMEORY, PROGRAMMABLE LOGIC DEVICES AND LOGIC FAMILIES
Memory is used for storing binary words.
Reading is the process by which the data can be retrieved from memory:
Writing is the process by which the data can be stored in the memory.
Semiconductor memories make use of bipolar and MOS.
Types of memories -
(a) RAM
(b) ROM
RAM is the random access memory or read/write memory. One can read and write in RAM.
ROM is the read only memory. One can only read from ROM nothing can be written in it.
RAM is of two types:
(a) SRAM
(b) DRAM
SRAM is also known as static RAM. In SRAM data will remain stored permanently as long
as power is supplied, they need not required rewriting periodically the data. The basic cell is
SRAM is a flip-flop.
DRAM is also known as dynamic RAM. In DRAM rewriting periodically the data into
memory is required. The basic cell in DRAM is a capacitor.
ROM is of four types:
(a) Masked ROM
(b) PROM
(c) EPROM V
(d) EEPROM V
In masked ROM the data is stored permanently through photo masking during fabrication i.e.
programming is done through masking and metalization process.
PROM is the programmable read only memory. it is programmed using PROM-programmer
and once the data is programmed (i.e. written) cannot be rewrite or changed again.
EPROM is the erasable programmable read only memory. The user can erase and programme
this memory again and again. The EPROM is erased by exposing the chip from inside via
window at the top to ultra violet light.
EEPROM is the electrically erasable. programmable read only memory. its function is similar
to EPROM but instead UV light the data can be erased using electrical signals.
PLDs (Programmable Logic Devices):
There are of three types
(a) PAL
(b) PLA
(c) PGA or FPGA.
PAL are the programmable arary logic in which AND gate arrays are programmable and OR
gate arrays are fixed.
PLAs are programmable logic arrays in which AND-OR gate arrays are used and
programmed for specific logic functions.
FPGA is the field programmable gate array. It consists of logic blocks i.e. an array of circuit
elements.
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