Programmable logic devices

1
Programmable logic devices
2
Outline

• PLAs
• PALs
• ROMs

3
Programmable Logic Device

• A programmable logic device (PLD) is an
  integrated circuit with internal logic gates that
  are connected through electronic fuses.
• Programming the device involves the blowing of
  fuses along the paths that must be disconnected
  so as to obtain a particular configuration.
• The word programming refers to a hardware
  procedure that specifies the internal
  configuration device.
• The gates in a PLD are divided into an AND array
  and OR array that are connected together to
  provide an AND-OR sum of Product implementation.
• The initial state of a PLD has all the fuses
  intact.

4
PLDs

• There are three types of PLDs. They are differ
  in the placement of fuses in the AND-OR array as
  shown below.

5
Types of PLDs
6
PLDs

• The most flexible PLD is the programmable logic
  array (PLA), where both the AND and OR arrays
  can be programmed. The product terms in the AND
  array may be shared by any OR gate to provide the
  required sum of products implementation.

7
Programmable logic arrays (PLAs)

• A block diagram of the PLA shown below consists
  of n inputs, m outputs, k product terms, and m
  sum terms. The product terms constitute a group
  of k AND gates and the sum terms constitute a
  group of m OR gates.

8
PLAs

• A typical PLA has 16 inputs, 48 product terms,
  and 8 outputs. The number of programmed fuses is
  2n ? k k ? m. There two types of PLAs
• mask-programmable PLA -- custom-made PLA
• field-programmable PLA (FPLA) -- programmed by a
  user

9
Internal Construction of a PLA.
10
Example

• F1 AB' AC and F2 AC BC

A B C F1 F2
0 0 0 0 0
0 0 1 0 0
0 1 0 0 0
0 1 1 0 1
1 0 0 1 0
1 0 1 1 1
1 1 0 0 0
1 1 1 1 1
Product Inputs Inputs Inputs Outputs Outputs
term A B C F1 F2
1 1 0 ? 1 ?
2 1 ? 1 1 1
3 ? 1 1 ? 1
11
(No Transcript)
12
Examples

• Implement the following Boolean expressions
• F0 A B'C', F1 B'C' AB, F2 AC' AB,
• And F3 B'C A
• PLA implementation of a 2-bit multiplier.
• Design a function generator of three inputs that
  implements the logic functions AND, OR, NAND,
  NOR, XOR, and XNOR

13
Programmable Arrays Logic (PALs)

• A programmable array logic (PAL) is a
  programmable logic device with a fixed OR array
  and a programmable AND array. The PAL is easier
  to program but not as flexible as the PLA. The
  typical PAL usually limits the number of inputs
  to an OR gate to 2, 4, 8, or 16.
• Examples
• BCD to Gray-code converter
• Two-bit comparator

14
Read-Only Memories (ROMs)

• An read-only memory (ROM) is a device that
  includes both the decoder and the OR gates within
  a single IC package. The connections between the
  outputs of the decoder and the inputs of the OR
  gates can be specified for each particular
  configuration.
• The ROM is used to implement complex
  combinational circuits within one IC package or
  as permanent storage for binary information.

15
Read-Only Memories (ROMs)

• ROMs come with special internal electronic fuses
  that can be programmed for a specific
  configuration. Once the pattern is established,
  it stays within the unit even when power is
  turned off and on again.
• A ROM consists of n input lines and m outputs
  lines.
• Each bit combination of the input variables is
  called an address.
• Each bit combination that comes out of the output
  lines is called a word.
• The number of bits per word is equal to number of
  output lines m.

16
Read-Only Memories (ROMs)

• An address is essentially a binary number that
  denotes one of the minterms of n variables.
• The number of distinct addresses possible with n
  input variables is 2n.
• An output word can be selected by a unique
  address, and since there are 2n distinct
  addresses in a ROM, there are 2n distinct words
  that are said to be stored in the unit.
• A ROM is characterized by the number of words 2n
  and the number of bits per word m.

17
ROMs
18
Example

• A 32?8 ROM consist of 32 words of 8 bits each.
  This means there are eight output lines and that
  there are 32 distinct words stored in the unit.
  The particular word selected that is presently
  available on the output lines is determined from
  the five inputs lines.
• A ROM is sometimes specified by the total number
  of bits it contains, which is 2n ? m. For
  example, a 2048-bit ROM may be organized as 512
  words of 4 bits each. This means that the unit
  has four output lines and 9 input lines to
  specify 29 512 words. The total number of the
  bits stored in the unit is 512?4 2048.

19
(No Transcript)
20
Example

• Design a combinational circuit using a ROM. This
  circuit accepts a 3 bit number and generates an
  output binary number equal to the square of the
  input number.

Inputs Outputs Outputs Outputs Outputs Outputs Outputs
A2 A1 A0 B5 B4 B3 B2 B1 B0 Decimal
0 0 0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0 1 1
0 1 0 0 0 0 1 0 0 4
0 1 1 0 0 1 0 0 1 9
1 0 0 0 1 0 0 0 0 16
1 0 1 0 1 1 0 0 1 25
1 1 0 1 0 0 1 0 0 36
1 1 1 1 1 0 0 0 1 49
21
A2 A1 A0 F3 F2 F1 F0
0 0 0 0 0 0 0
0 0 1 0 0 0 0
0 1 0 0 0 0 1
0 1 1 0 0 1 0
1 0 0 0 1 0 0
1 0 1 0 1 1 0
1 1 0 1 0 0 1
1 1 1 1 1 0 0
22
Example

• Design a one-bit full adder using ROM.

23
Types of ROMs

• Mask programming
• Programmable read-only memory (PROM)
• Erasable PROM (EPROM)

24
Exercises

• p.380 5.1, 5.2, 5.3