0. Introduction
This resource will cover what logic gates are, some examples of logic gates and what logic gates can be used for, the resource is targeted mainly at people who know the basics of electronics, but have little or no previous experience with logic gates or any form of digital electronics. Across this resource you will see lots of 1's and 0's, these are used to represent the binary states of high and low voltage.
1 and 0 are not always represented by high and low respectively. high and low refer to the voltage level; true, false, 0, 1, on and off are abstract concepts that we can apply how we wish.
For example, consider inverted logic, or signals that are asserted when pulled low. In both cases you need to carefully consider what voltage level means "true" or "on".
1. Basic information
Logic gates come in the form of integrated circuits, these integrated circuits contain several transistors in different configurations to create logic gates. There are several types of logic gate, each using 1, 2 or sometimes even more inputs to produce an output based on this input.
As I said before, not all logic comes in the form of integrated circuits. Often we use logic functions without them.
1.1 Construction and types
There are several ways logic gates are constructed, the 2 most popular are called TTL (transistor transistor logic) this uses bipolar transistors to create the logic gate and the other is called CMOS (complementary metal oxide semiconductor) this uses MOSFET (metal oxide semiconductor field effect transistor) transistors, CMOS logic gates are often preferable to TTL's as CMOS generally require less supply voltage to operate. MOSFETS (the type of transistor used in CMOS logic gates) are extremely sensitive to ESD (electrostatic discharge), ESD's can be several thousand volts, far more than enough to fry the thin wafer of insulation in the MOSFET. Ensure you do not build up any static and ground yourself (preferably with a anti-static wrist strap).
Apart from the very long ans rambling sentence, you have an inaccuacy in that CMOS migh be preferred due to low power consumption (but that may not be true at high speeds) and CMOS may operate over a wider supply range, e.g. 3 to 15V as compared to 4.5 to 5.5V
All IC's (integrated circuit) have a little mark called a clocking notch, this is used to find where pins of certain numbers are, pin numbers count counter-clockwise. below is a picture of an IC, showing which pins belong to which numbers.
As I said earlier, I've never heard it called a clocking notch. Can you provide a reference to that?
2. Analogue and digital electronics
Logic gates work in a form of electronic signal known as digital, the voltage can only represent discrete amounts, high or low. Modern computers use digital signals. Analogue signals can vary, they are non-discrete, some of the first computers used analogue signals.
This needs to be earlier as you've already talked about high and low.
Truth tables are used to show the logic of a logic gate, what inputs will produce what output. Truth tables are very simple to read
OK up to here...
, they consist of 2 or 3 columns (some contain more) with a series of 1's and 0's. These ones and zeros represent high and low voltages, there are only two values in truth tables as they work in a binary, digital signal. The binary part means it uses base 2, we use base 10, the values can only be a 1 or a 0. The columns on the left (there are usually 2) are the inputs, truth tables will show all the combinations of inputs, for a 2 input logic gate this is:
That is rambling and not very informative.
0(low voltage/off),0(low voltage/off)
0(low voltage/off),1(high voltage/on)
1(high voltage/on),0(low voltage/off)
1(high voltage/on),1(high voltage/on)
As I mentioned earlier, you need to avoid conflating voltage levels with meanings. Logic gates are defined for positive logic, and you'll find that datasheets will explicitly state that high = 1 and low = 0 for the definition of their function. When you define a truth table, you can use 0 and 1 or true and false. When you talk about a physical gate you need to talk about high and low. It is the way that we assign high and low to 0 and 1 that define a gate as being nand or nor (for example)
Then the column on the right will show the corresponding output next to the series of inputs, the out put can only be a 1 or a 0. You can see some truth tables in the next section
But you need to remember that there can be more than one output.
6.Schmitt gates
You may see when searching for logic gates some of them are labelled Schmitt ..... gate/trigger. The property that Schmittt gates/ triggers possess is extremely important when feeding an analogue signal into a logic gate, the simple explanation is that turning on (going from 0 to 1) and turning off (going from 1 to 0) will happen at slightly different levels of voltages, this can be used to prevent rapid switching, where a circuit turns on and off extremely fast. E.g. a dark activated light, when it goes dark the light is set to come on, however when the light does come on it raises the light level slightly, with a standard logic gates this would cause the input and output to rapidly fluctuate between a 1 and a 0. With a schmitt logic gate/ trigger, under normal circumstances, this would not happen. For a more in depth explanation see:
here
There is no such thing as a schmitt gate. An input to a gate may have a schmitt trigger. And in some logic functions only select inputs will have them.
To talk about schmitt trigger inputs you have to mention hysteresis and you also need to have mentioned the valid input ranges for gates.
