THE LOGICAL FAMILIES
In digital electronics, the logic family is a circuit-level embodiment of a digital logic component. In integrated circuits it represents a set of logic gates made by means of a particular design, characterized by the same logic levels and supply voltage. In the past, logic families were implemented as individual components, each of which containing one or more elementary logic functions, which formed the basis for the implementation of more complex logic blocks. Each logic family is characterized by a main logic function, which corresponds to the basic logic gate of the family, which is usually the NOT function (logic inversion).
COMPATIBILITY BETWEEN LOGICAL FAMILIES
The typical feature of a logic family is that components or cells belonging to the same logic family are compatible with each other, i.e. they can be connected directly. This does not generally apply to components or cells belonging to different logic families: The term “logic family” is also used to refer to a set of techniques used to implement logic within Very large scale integration circuits, such as CPUs, memories and other complex logic functions. The widely used logic families are CMOS and TTL . TTL will be covered in this series of posts on digital electronics.
TABLE OF LOGIC FAMILIES FOR INTEGRATED CIRCUITS
TTL FAMILY
Transistor-transistor logic ( TTL ) was the first integrated circuit (IC) technology to be deployed globally in a variety of applications, such as computers, industrial controls, laboratory instrumentation, consumer electronics, music equipment, etc.
With TTL technology it is possible to realize many functions, such as (the list is not exhaustive):
- logic gates such as AND, OR, NAND, NOR, XOR, NOT (inverters)
- flip-flop
- latch
- counters
- adders, multipliers and ALUs
- shift register
- timer
- ROM memories
THE LOGICAL GATE
A logic gate can be represented as a container having one or more inputs and a single output. The logic state of the output depends at a certain instant on the logic states assumed by the inputs, ie the gate output is controlled by the input states. Digital logic is controlled through the use of multiple transistors which are critical both in building logic gates and modern equipment such as a multi-core microprocessor.
TABLE OF THE TRUTH OF A LOGICAL GATE
With two inputs we have four possible combinations, 2n = 4, by grouping these combinations in a table we obtain what is called the logic gate truth table.
|
A |
B |
Y |
|
0 |
0 |
0 |
|
0 |
1 |
0 |
|
1 |
0 |
0 |
|
1 |
1 |
1 |
We have obtained the truth table of an AND logic gate.
TTL AND CMOS
We have said that there are two fundamental logic families, the TTL and the CMOS . In practice they differ according to their manufacturing technology, the name emphasizes the type of transistor with which they are made. In these posts we will consider the TTL (Transistor Transistor Logic) family which uses a BJT bipolar transistor analyzed in a previous post . Even the supply voltages are different in the two families, in practice the family TTL is powered at 5V While the CMOS at a variable voltage between 3 and 18V. This makes us understand the difference between the two logic families in terms of recognition of the logic voltage levels.
TTL LOGIC LEVELS
INPUT AND OUTPUT CHARACTERISTICS
OUTPUT LOAD OF A GATE (FAN-OUT)
In order for the logical acknowledgment intervals to be respected, a gate can drive only a limited number of devices on its output. Otherwise, if this is not respected, a reduction in the VOH voltage is produced. One standard TTL port can drive up to 10 standard TTL ports at most without affecting the VOH voltage. The logic levels assumed by the output (Low and High) produce currents with two travel directions. When VOH has a High level, current flows out one port and into the others. This is called the source current. When the output has a logic level Low (VOL) it is the output itself that absorbs current from the load, this is called sink or absorption current.
WHERE:
- IOH Output current High
- IIH Input current High
WHERE:
- IIL Input current Low
- IOL Output current Low
This 4 currents, like the voltages, are supplied by the manufacturer to determine the FAN-OUT, i.e. the number of ports that can be connected at the output.
POSITIVE AND NEGATIVE LOGIC
In Boolean algebra we speak of positive and negative logic to indicate two different conventions that associate the Boolean values zero and one to the voltages of an electrical or combinatorial circuit.
- Positive logic . This convention maps the lowest voltage level to logic zero and the highest voltage level to logic one. It is the classical logic mainly followed.
- Negative logic . This convention associates the lowest voltage level with logic one and the highest voltage level with logic zero. It is a logic opposite to positive logic.
Positive logic has the advantage of matching the answers of the logic gates (OR, AND, NOT) with the respective logic functions of disjunction, conjunction and negation.
Positive logic is the one predominantly adopted in the design of combinatorial circuits.
Note . In some exceptional cases it may still be more efficient to follow the negative logic.
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