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Programmable Logic Array

Implementation of BCD to Excess-3 using PLA


Programmable logic devices (PLDs) are used for designing logic circuits. PLDs
can be configured by the user to perform specific functions. The different types of PLDs
available are:
• ROM and EPROM.
• Programmable Logic Arrays (PLA)
• Programmable Array Logic (PAL)
• Field Programmable Gate Arrays (FPGA)


ROM consists of an array of semiconductor devices interconnected to store an array of
memory data. Data can be only read, it cannot be changed under normal operating
• Mask programmable ROM
• Erasable programmable ROM (EPROM)
• Electrically erasable PROM (EEPROM)
• Flash


A block diagram of the ROM is shown in the figure 1. There are n inputs and m
outputs. The inputs provide the address for the memory, and the outputs give the data bits
of the stored word that is selected from the address. The number of words in the ROM
device is determined from the fact that n address input lines can specify words. For
example a 4 x 8 ROM has 2 (ie., n=2) Input Address lines. & Can store 4 words each of 8
(ie., m=8) bits.
Internally a ROM consists of a decoder and a memory array. When a pattern of n 0s
and 1s is applied to the decoder inputs, exactly one of the -decoder outputs is ‘1’. This
decoder output line selects one of the words in the memory array, and the bit pattern
stored in this word is transferred to the memory output lines. x m ROM can realize m
functions of n variables , since it can store a truth table with rows and m columns.
Note: ROM doesn’t have data inputs, because it doesn’t have write operation.
Four technologies are used for ROM programming.

Mask programmable ROM

If mask programming is used, then the data array is permanently stored at the time
of manufacture. Preparation of the mask is expensive, so mask programmable ROMs are
economically feasible if large quantity are required within the same data array.

Erasable programmable ROM (EPROM)

If a small quantity of ROMs are required within a given data array, then
EPROMs may be used. EPROMs allow the modification of the data stored as they use a
special charge storage mechanism to enable or disable the switching elements in the
memory array. The data stored in the EPROM is generally permanent until erased using
ultraviolet light.


The electrically erasable PROM (EEPROM) is similar to EPROM except that the
erasure of data is accomplished using electrical pulses instead of ultraviolet light. An
EEPROM can be erased and reprogrammed only a limited number of times.


Flash memories are similar to EEPROMs except that they use a different charge
storage mechanism. They also have built in programming and erase capability so that the
data can be written to the flash memory while it is in place in a circuit without the need
for a separate programmer.

Realization of a Sequential Network with ROM

A sequential network can easily be designed using a ROM and flip-flops. The
combinational part of the sequential network can be realized using a ROM. The ROM
can be used to realize the output functions and the next state functions. The state of the
network can then be stored in a register of D flip-flops and fed back to the input of the
Use of D flip flops is preferable to J-K flip flops, since use of 2 input flip flops
would require increasing the number of outputs. The fact that the D flip flop input
equations would generally require more gates than the J-K equations is of no
consequence, since the size of the ROM depends only on the number of inputs and
outputs and not on the complexity of the equations being realized. For this reason, the
state assignment used is also of little importance, and generally a state assignment in
straight binary order is as good as any.