AND-OR-invert


AND-OR-invert logic and AOI gates are two-level compound logic functions constructed from the combination of one or more AND gates followed by a NOR gate. Construction of AOI cells is particularly efficient using CMOS technology, where the total number of transistor gates can be compared to the same construction using NAND logic or NOR logic. The complement of AOI logic is OR-AND-invert logic, where the OR gates precede a NAND gate.

Overview

Most logic optimization result in a sum-of-products or product-of-sums logic expression.
AOI is used for sum-of-products, the variables are ANDed to form minterms which are ORed together then inverted:
  • is known as a AOI 2-1 gate.
  • is known as a AOI 2-2 gate.
  • is known as a AOI 3-3 gate.
  • is known as a AOI 4-4 gate.
  • is known as a AOI 4-3-2 gate.
  • and other variations.

    Examples

AOI gates perform one or more AND operations followed by an OR operation then an inversion.

2-1 AOI gate

The 2-1 AOI gate can be represented by the following boolean equation and truth table:

2-2 AOI gate

Real world examples of an 2-2 AOI gate are found in the CD4085B, SN74LS51, SN5450 logic ICs.
The 2-2 AOI gate can be represented by the following boolean equation and truth table:

3-3 AOI gate

Real world examples of an 3-3 AOI gate is found in the SN74LS51 logic IC.
The 3-3 AOI gate can be represented by the following boolean equation and truth table:
Its logic table would have 64 entries, but is not shown.

4-4 AOI gate

Real world examples of an 4-4 AOI gate is found in the CD4048B logic IC.
The 4-4 AOI gate can be represented by the following boolean equation and truth table:
Its logic table would have 256 entries, but is not shown.

Extensions to multiple levels

It is possible to create multi-level compound gates, which combine the logic of AND-OR-Invert gates with OR-AND-invert gates. An example is shown below. The parts implementing the same logic have been put in boxes with the same color.

Electronic implementation

AND-OR-invert and OAI gates can be readily implemented in CMOS circuitry. AOI gates are particularly advantaged in that the total number of transistors is less than if the AND, NOT, and OR functions were implemented separately. This results in increased speed, reduced power, smaller area, and potentially lower fabrication cost. For example, a 2-1 AOI gate can be constructed with 6 transistors in CMOS, compared to 10 transistors using a 2-input NAND gate, an inverter, and a 2-input NOR gate.
In NMOS logic, the lower half of the CMOS circuit is used in combination with a load device or pull-up transistor.
AOI gates are similarly efficient in transistor–transistor logic.
;Examples
CMOS 4000-series logic family:
  • CD4085B = dual 2-2 AOI gate
  • CD4086B = single expandable 2-2-2-2 AOI gate"
  • CD4048B = single expandable 8-input 8-function with three-state output, 8 choices for gate type: 8 NOR / 8 OR / 8 NAND / 8 AND / 4-4 AND-OR-Invert / 4-4 AND-OR / 4-4 OR-AND-Invert / 4-4 OR-AND
TTL 7400-series logic family:
  • SN5450 = dual 2-2 AOI gate, one is expandable
  • SN74LS51 = 2-2 AOI gate and 3-3 AOI gate
  • SN54LS54 = single 2-3-3-2 AOI gate