Schematic Symbols - Page 1
Forum section: electronics / projects

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: October 23, 2014, 07:22:32 pm
content: I need to create a few schematic symbols and board footprints for some 100 and 144 pin microcontrollers in KiCad. These parts are TQFP packages with pins on all four sides.For the schematic symbols, is it generally preferable to group the signals in functional groups (such as PortA, PortB, VCC pins, Vss pins), or should I follow the layout of the physical chip?

author: 8086
author_rank: Super Contributor
timestamp: October 23, 2014, 07:32:50 pm
content: Entirely your choice. There are reasons to go either way.Personally, I usually group by function.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 23, 2014, 07:36:08 pm
content: Entirely up to you. I'd group power and ground (for the logic), oscillator, configuration, junk like that into one symbol, then each port by itself. Usually you end up with quite long pin names, or you just put the basic name used in your app, ie PA0 becomes AIN0 if it's analog in.If you have extra power pins dedicated to something, put those on the function symbol, not the main power one.Of course you need to make sure that all the various symbols across different pages package into one footprint.

author: graynomad
author_rank: Regular Contributor
timestamp: October 24, 2014, 12:01:37 am
content: I always do both, a physical replica for starters because it's easier to check against the data sheet, then a copy with pins split into function groups, usually as a multi-part component. I sometimes use the first one just to get a component on the board while playing in the very early stages of a design, but it's the second one I use when things start getting real.I know a lot of people just use a physical model and let the CAD sort it out using the net names, and while that can be useful if you want to know what function is on pin 56 for example you do have the data sheet for that and a grouped version is much clearer. My personal thinking is that just dropping a square box with 144 pins onto a schematic is the lazy way out, the purpose of a schematic is to make the design clear to a human and grouping the pins helps in that regard.

author: TerraHertz
author_rank: Super Contributor
timestamp: October 25, 2014, 04:39:26 am
content: I'll vote for functional grouping. It makes the schematic so much cleaner.Also there's no reason why all the pins have to be on one visual entity. I'd quite often break major functional pin groups of one part up into different elements. So I could have all the bus I/O and control lines on one, general purpose I/O on another, clock gen and stuff like that on another, etc. Then you can group things in optimal places in the schematic, without lots of long lines, or bus ports.If you think about how the schematic is going to be arranged from the start, you can partition the large pinout components to suit.

author: zapta
author_rank: Super Contributor
timestamp: October 25, 2014, 03:59:01 pm
content: Quote from: TerraHertz on October 25, 2014, 04:39:26 am+1freeelectron mentioned once an interesting idea of being able to move the pins around while in the schematic editor. Don't know if it is available in any EDA package. In Eagle I often go back and force between the schematic and library editors to optimize the pin arrangement for the schematic.

author: graynomad
author_rank: Regular Contributor
timestamp: October 25, 2014, 10:33:22 pm
content: Moving pins on the schematic is possible with Altium and IIRC was possible right back in the dim dark past with DOS-based Protel. IMO it's a very important feature that should be available in all packages.