Schematic capture
the stage in the PCB design process where the circuit or "schematic" is
captured and entered into a computer.
With computer based PCB design and many more computer
simulations being used in the design of electronics circuits, the schematic
capture or schematic entry stage of an electronics circuit into a computer is
now an integral part of the design process. Typically complete software suites
are now used, and the schematic capture stage only needs to be undertaken once,
and the information used not only for being able to produce a drawing of the
circuit, but also data entry into the PCB design as well as circuit simulation,
production of the Bill of Materials (BOM) and many other applications.
Schematic capture in the design process
The schematic capture part of the design process is today undertaken
interactively. Prior to the schematic capture of the design, the initial high
level design must be undertaken. Then in years gone by, breadboards of the
circuit would be made up and made to work before committing to the schematic
stage. Now with highly sophisticated circuit simulation software, the circuit is
designed interactively during the schematic capture stage and the circuit
simulated using software rather than building a hardware version of the circuit.
By using a computer based system for schematic capture, it is
possible to enter very complicated circuits into a computer relatively quickly.
It is also possible to undertake the design of the board and perform circuit
simulations while the basic design is underway. In addition to this, many
circuit capture systems provide a means by which the circuit revisions can be
managed and configuration controlled properly. Where a circuit is being
repeatedly updated, and there may be the possibility of several people working
on different areas, this is of great importance.
Elements entered into a schematic have a shape associated
with them for the schematic. In this way a shape designed for a particular part
will be pre-drawn and appear on the circuit every time that particular type of
part appears on the circuit. When using an end-to-end design suite, the full
shape may also include the PCB outline, pads and the like. In this way the part
number for that part defines all the elements of the part for the design.
Practical aspects of schematic capture
One of the big problems with computer based schematic capture systems now is
that the circuits are often very large and they can become unreadable and
difficult to follow. There is a trend to print the circuits out on A4 or letter
sized paper, and each sheet may only have a few components.
Schematic error checking
It is very important to ensure that any schematic that has been captured is
fully checked. While the simulation and other applications now available as part
of an end-to-end design suite will trap and highlight many errors, some can
still get through. Errors that creep through can be quite subtle. One that has
been seen is where a particular node may be given slightly different names on
different sheets. As the names are different they will not be connected by the
computer. For example a node may be labelled "0v" on one sheet, but could appear
as "gnd" on another. Accordingly it is very important to ensure that errors such
as these do not creep through. Discipline in naming is essential.
Summary
Computer based schematic capture has greatly simplified the process of drawing
circuit diagrams. Circuit schematics can be drawn as the circuit is designed,
and managed in such a way that there is little room for error. While errors can
occur, the level of errors has fallen dramatically with the introduction of
circuit schematic capture software.
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