Logic gate symbol vs IC symbol

Question

When I'm using a logic gate (let's say AND) in a schematic, which symbol should I use: the AND symbol or the IC symbol (the 4081, for example)? Or in the case of flip-flops, should I use the flip-flop symbol or the IC symbol?

Answer 1

Please use the logic symbol! The IC is not relevant to the schematic, where it's about the circuit's functionality.

Most EDA (Electronic Design Automation) software will let you create a component consisting of a package for the PCB layout, and a logic symbol for the schematic capture. For a quad AND gate you'll have either 4 identical AND gates and a symbol for the power connections, or 3 common AND gates and one with power connections.

When placing AND gates on your schematic you'll start with gate 1 of IC U1. Next gate you place will be gate 2 of U1, and so on. This way you can place each gate anywhere you want, one at the top left and another one of the same IC bottom right if you want. Making physical connections is a problem for later, when you make the PCB layout.

If your schematic symbol would represent the package you would have to draw all connections for your gates to the same symbol, whether they belong together or not. Your schematic will become illegible. For the PCB that's not a problem; it's not meant to read the schematic from it.

Answer 2

Usually you would use the functional symbol, as it makes reading and understanding the schematic at a glance much easier.
If all you see is various boxes, then you have to check the part numbers to find out what they do.
So unless there is a very good reason for doing otherwise, use the logic symbols.

Answer 3

If the system is complicated, such that you are thinking primarily about functionality, use the logic symbol. This is classic abstraction as a tool for managing complexity. When you start using parts of the same IC for completely unrelated purposes or at distant points in a signal chain, it really shines; similarly with putting all of the power connections on their own sub-symbol. But it is not always the right answer.

If the system is simple, such that functionality is not a challenge but your thoughts even from the beginning are about issues such as trying to put it on a compact two- (or even handmade one-) layer board, then using an IC symbol (ideally with embedded schematic symbols for classic gates/flops) may help keep the process in tune with the physical reality. A symbol that reflects the package pinout can be even more convenient (and helps when probing around debugging, too).

Yes, there are tools such as gate-swapping that aid in revising a schematic to improve layout considerations, but where the circuit is simple enough that showing physical detail will not impede logical understanding, it can be a quite useful approach. Doubly so if you are designing things that will be worked on by people who are not experienced engineers accustomed to customary abstractions.

Answer 4

It depends on point of view of what are you going to do with your circuit/schematic. If you are drawing a schematic for better understanding, you must use actual symbols like gates and flip-flops, not integrated ones, as they help in better understanding. But if you are drawing circuit for a purpose like PCB designing or something like that, you must draw that with appropriate ic and components.