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Public:Schematic Conventions
See ((Layout Standards)) for related information about developing your layout properly once the schematic is done.
See ((KiCad)) for information on setting up the program before you get to work on these things.
Our team is in the process of creating and implementing standards for all of our electrical design and CAD. This is being done in an attempt to reduce errors in manufactured boards, make it easier for other members to edit boards that were already created, and make the finished CAD look presentable.
{REMARKSBOX(type="warning" title="Under Construction")}
All content below this point is under construction. Follow all conventions given here and be aware they may not cover all situations.
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{REMARKSBOX(type="note" title="Note for KiCad 5")}The latest versions of KiCad include a few differences in settings from the older versions. Since we are all using the latest version, please be aware there may be settings you can't find and update these instructions as you find any changes in the software.{REMARKSBOX}
{REMARKSBOX(type="note" title="Units")}Millimeters are fantastic and we encourage using metric dimensions whenever possible, however BAC's standards are written in mils (1/1000 inch) or inches so we present all minimum and recommended dimensions in both mm and mil.{REMARKSBOX}
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General Advice for Clean Schematics
Be consistent with whatever conventions you choose, if we do not require a specific standard!
If you think something is hard to read or confusing, and you are following whatever conventions listed here, please post your dilemma in the #elec-cad channel on ((Slack)).
Net Naming
All nets with more than two pins should be named with a descriptive name.
Net names should only contain capital letters, numbers, and certain symbols including underscores and plus or minus. Net names should never contain spaces.
More guidance on net naming will be provided at a later time
Power Rails
No VCC/VDD. Net names should indicate the polarity and voltage of the rail. If the zero voltage reference of the rail is not the net named "GND", the net name of the power rail must reflect this.
- Voltage Rail should be near the top of the schematic
- *Voltage symbol usually points upwards
- *In the case of a voltage output, e.g. from a DC/DC converter, voltage symbol may point sideways
- GND Rail should be near/at bottom
- *GND symbol always points downwards
- If there is a decimal point in the voltage, use the convention of replacing the decimal with V for voltage, e.g. +3V3 describes +3.3V
I2C/SPI
Will expand on this later
UART
It is very easy to get mixed up with UART nets, be thorough and double check your work!
UARTx_Source_to_Destination
Examples
Put some examples here
Graphical Considerations
Schematics are the most critical piece of documentation for a PCB. They are read by both ECAD software and fellow engineers. The schematic should clearly illustrate its functionality, be easy to read, and contain enough information so that it is easy to order parts and assemble the finished PCB.
More information will be provided at a later time
Sheets
Paper size
ANSI A (8.5" x 11")
We need to be able to print out all of our current schematics at any time as a backup.
Diptrace archives may not use any standard paper size, as we did not enforce the standard on our boards when we used it.
How to set the correct size:
- Click the "Edit Page settings" icon:
{IFRAME(width="250" height="200" frameborder="0")}https://uofi.app.box.com/embed/s/6n13iwu67xk1hi4ylx9eq5j6pxgk9pnu?sortColumn=date&view=list{IFRAME} * Make sure "A 8.5x11in" and "Landscape" are selected
- All of the other options should be changed per page, more on those in Title Block
How to export the pages to PDF for printing:
- Click the Plot Schematic icon:
{IFRAME(width="275" height="200" frameborder="0")}https://uofi.app.box.com/embed/s/psrlxqdceb52ze23c7iyluk1gonu8wgc?sortColumn=date&view=list{IFRAME} * Make sure the output setting is PDF
- Click "Plot All Pages"
Title Block
Kicad Default, Diptrace --TBD-- now defunct, do not use
Include information in these fields:
Coming soon...
Wire Crossings
Long answer short, dots on all 3-way intersections, no dots on 4-way intersections. A dot means that the wires connect, no dot means that there's no connection. Keep reading to find out why those statements should not contradict each other in your design. If you encounter a 4-way connection, in which both wires crossing at the perpendicular angle are connected to each other, separate it into two 3-way connections instead.
Take a look at these pictures for some examples:
{IFRAME(width="275" height="300" frameborder="0")}https://uofi.app.box.com/embed/s/rm2rqogc5yeyjyercjagkw1a9zluu5gl?sortColumn=date&view=list{IFRAME} * Left vertical wire connects to 3 but not 1.
- Right vertical wire connects to 1 only
- 2 connected to GND and not 3
{IFRAME(width="275" height="300" frameborder="0")}
https://uofi.app.box.com/embed/s/ewxez1by9b3hjv7q8d8qjxzd8i5b0z75?sortColumn=date&view=list{IFRAME} * Left vertical wire connects to 3 but not 1
- Right vertical wire connects to 1 only
- 2 needed to connect to both 3 and GND, so that's separated into 2 different connections: 3 to GND and 2 to 3
- do not simply put a dot on the 4-way intersection to show the connection between 3, GND, and 2
Direct Wire Connections
Wires should be as straight as possible to make it easier for the eye to follow.
Routing from Components
Route lines should come straight out of components, as opposed to immediately making an angle. This makes it easier to move components later on.
Prevent Overlapping
Components, reference designators, and part values must never overlap anything. The only thing that wires can cross is other wires. Text comments should not overlap anything.
Net Coloring
While net coloring can be useful to easily identity and follow nets around a schematic, the schematic should be completely intelligible without net coloring. This is because the schematic will most likely be printed in black and white if it is printed.
Part Information
Using just the schematic, anyone should be able to determine which parts should be ordered to assemble the board. To achieve this, the following guidelines should be followed to ensure that all necessary information is included in the schematic.
Below is a table listing seven Fields which you should see on each part. The fields must be present when viewing the properties for each part, but not necessarily filled in. You must add the fields that are not default in KiCad so they show up on each part's properties ( -+MPN+-, -+DNP+-, -+Note+-).
Windows: Do this by opening Eeschema and going to -+Preferences -> General Options+- and the -+Field Name Templates+- tab.
Mac: Do this by opening Eeschema and going to -+KiCad -> Preferences -> General Options+- and the -+Field Name Templates+- tab.
You can also choose whether or not it is visible on the schematic by default, and you can change any field's visibility as you edit each part.
Pro tip! Go to Tools --> Edit Symbol Fields... to see all the fields for all the parts and edit them in what's basically a spreadsheet but slightly more annoying. (It's still easier than trying to remember which parts you edited individually.)
||Name|Default?|Required on every part?|Included in Field|Not Allowed in Field|Additional Notes
Reference|Y|Y|Auto generated by KiCad| |You can also change the numbers yourself, if you wish. KiCad itself will yell at you if it doesn't think your edit is acceptable.
Value|Y|N|Farads (cap)%%%Ohms (resistor)%%%Amps (fuse)%%%Volts (zener diode)%%%MPN (only for parts which do not also have one of the above values, e.g. ICs)|MPN (for parts which also have one of the above values, e.g. ICs)%%%Descriptions or labels|Basically, a value that makes it easier to parse what the function is should be here, and then the MPN field should have the MPN. Many ICs don't have one defining value like that, so they will not have this field filled.
Footprint|Y|Y|Assigned by KiCad when you associate a footprint| |Even if it's not populated, still associate it with a footprint. You can mess with that property in another field.
Datasheet|Y|N|Link to datasheet| |If you have a MPN, you must link the datasheet.
MPN|N|N|Manufacturer's Part Number| |
DNP|N|N|DNP|Must leave blank if you want it populated.|Really you can put anything you want in here, it doesn't have to be exactly "DNP", and our BOM generator/viewer can handle it however you want, but please be reasonable.
Note|N|N|For parts with a base value, you may add other characteristics in the following order, with spaces in between:%%%Voltage%%%Power%%%Temperature (e.g. X7R or 85C)%%%Tolerance%%%Other%%%For other parts, you may put label or descriptions about function (e.g. CAN LED, RESET, 12V Input)|Size or Dimensions (if you know that, assign a footprint)| ||
We chose the fields to add the least amount of work possible to making schematics, while still including all the necessary information for sourcing and placing components and working with BOM mangement software. Please give feedback on anything you want clarified or changed, on the #elec-cad channel on ((Slack)).
Symbols
Using Them
Capacitors
Below is a diagram of the correct and incorrect capacitor symbols. Never use the small capacitor symbols. You do not need to. Only use the polarized symbol if you have a capacitor which is polarized, such as an electrolytic cap. Ceramic capacitors are the ones used most often on our PCBs, and they are not polarized.
{IFRAME(width="275" height="200" frameborder="0")}
https://uofi.app.box.com/embed/s/f7i1td9fs7bcc8x2s4su12z368nm25pl?sortColumn=date&view=list{IFRAME}
Resistors
Like the picture of the capacitors above, never use the small resistors symbols. This is to keep the resistor size the same across all our boards. Also, make sure to keep the style of your resistors the same throughout your schematic whether that is the US or the European style.
Connectors
Please do not use these connector symbols:
{IFRAME(width="275" height="200" frameborder="0")}
https://uofi.app.box.com/embed/s/cajq5bm57f43d4buu6vyvtkhq5nyg4ec?sortColumn=date&view=list{IFRAME} Instead, use these connector symbols:
{IFRAME(width="275" height="200" frameborder="0")}
https://uofi.app.box.com/embed/s/2z55eo0eii0cfvb7hb123m7ytx3bkc4f?sortColumn=date&view=list{IFRAME}
Making Your Own
Look, please make symbols that aren't shit or look like shit. If you have questions, please refer to the Kicad Library Conventions for now.