For the design of any electronic system, it is essential to understand the operation of the integrated circuits (ICs) involved; in this case, we will analyze digital devices called logic gates.
We will review the basic concepts and operation using integrated circuits of the 74LS series of logic gates: AND, OR, NOT, NAND, NOR, and XOR.
Logic gates are commercially available in the form of integrated circuits. The best-known technologies on the market include:
TTL: Transistor-Transistor-Logic
CMOS: Metal-Oxide-Semiconductor Complementary Logic
ECL: Emitter-Coupled Logic
CMOS technology is based on MOS transistors, while TTL and ECL technologies are based on bipolar transistors.
What is a logic gate?
A logic gate is a digital device that performs a binary operation with one or two logic states (0,1) at the input and obtains a Boolean result (0,1) at the output of the system. They can be combined with each other to obtain new functions.
The basic gates are AND, OR and NOT, and their complementary gates are NAND, NOR. There is also an XOR auxiliary gate and its XNOR auxiliary gate.
The most commonly used TTL series for these gates is the 74LSXX series (LS: Low Power Schottky), some of its features include:
Power supply: 5V ± 0.25, it is convenient to use a regulated power supply, and connect a 0.1μF ceramic capacitor through the power supply near the IC to eliminate the spikes generated by the switching.
Input: It is recommended to connect any unused input to +Vcc to eliminate noise, as these ICs have floating inputs.
Output: They can provide 16mA, but can also provide more than 2mA.
Gate propagation time: It takes about 10ns for the signal to pass through the gate.
Frequency: 35MHz under ideal conditions.
Power consumption: a few milliwatts.
Each gate is associated with a symbol, a truth table, and a Boolean operation; it expresses the output state for each possible input combination.
Depending on the result given by the operation, the results can be divided into:
Product of sums (maxterms): Boolean expression that brings together input variables that have logical zero at output.
Sum of products (minterms): Boolean expression that brings together input variables that have logical values at output.
How are they used?
The IC of the above logic gate has the following internal configuration, which we provide for the connections later:
Connection diagram
To make the following connections, we need:
330 ohm and 1K ohm resistors
LED
Wire
8-bit dip switch
74LS02 NOR
74LS86 XOR
74LS00 NAND
74LS08 AND
74LS32 OR
74LS04 NOT
The connections for AND, NAND, XOR, OR gates are the same, just change the IC appropriately, as shown below:
For the 74LS02-NOR gate, the connections are as follows:
Finally, the 74LS04 NOT integrated circuit:
