4.1 Symbol Creation
This section has been written by Ales Hvezda for the gEDA
project and later adapted for Lepton.
4.1.1 Overview
This document describes the creation of component symbols, including
style conventions, and hints/tips and things to look out for when
drawing symbols for the gEDA/gaf system.
4.1.2 Component Symbol Creation
Component symbols (from here on known as symbols) are drawn
using lepton-schematic
. You can start with a blank page or a known-to-be
good symbol that is similar to the one you want to create.
4.1.2.1 Preparing
Before you open lepton-schematic
to create your symbol, there are a few
questions you should ask yourself, that will effect how you create
your symbol:
- Is this symbol purely for aesthetic purposes (a graphic)? If so,
several steps are simplified.
- Do I want to simulate the component in question, with e.g. SPICE?
- Do I want to use this symbol in a schematic that includes power
distribution? This will become important when deciding what pins to
draw.
- Does this symbol represent a component that comes several in a
package? This is called slotting in gEDA’s language, and has
special rules.
Once you’ve answered those questions, the following guide will be more
useful to you.
To start building your symbol:
- Open
lepton-schematic
.
- The default empty page has boxes on it. Use the selection tool (the
one that looks like a black mouse cursor) to select everything on the
page. Hit the Delete key to delete it.
- Zoom in at least one step (View → Zoom, or hit
the Z key).
- Make sure the snap-to-grid functionality is ON (Options
→ Toggle Snap On/Off, or O S).
- Make sure grid snap size is set to 100 (Options
→ Snap Grid Spacing..., or O Shift-S).
4.1.2.2 Drawing your Symbol
Use the ‘line’, ‘box’, ‘circle’, and ‘arc’ tools
(in the ‘Add’ menu) to create a simplified visual representation
of your device. Make sure that the only color you use is
graphic
(green). Do not place any pins on the device until the
next step.
To check what color you are using to draw, go to Edit
→ Color... (or hit E O).
4.1.2.3 Placing Pins on your Symbol
If you have decided your symbol is graphical in nature (e.g., does not
connect to anything, is just artwork), you can skip this step.
Add your first pin.
- Select Add → Pin, or hit A P. Once that
is done, click from the end of your pin, to one of the edges of your
symbol. A ’0’ and the word ’unknown’ should pop up near your new pin.
Please note that the direction you draw your pins is important. If
you draw them starting from the edge of your symbol to outside of your
symbol, you will end up with text outside of the symbol!
- Click on the selection tool, and use it to select the pin you just
drew. The pin and text with it should change colors, and boxes should
be drawn around the ends of the pin.
- Select Edit → Edit..., or hit E E (or
right click, and hit ‘Edit...’). This will bring up the edit
dialog for this pin.
In the Edit dialog for a pin, you should see four attributes
(name=value pairs) already associated with your new pin. These
attributes can be edited by clicking on the attribute in the list at
the top of the edit window, then clicking on the field you want to
edit. Note that when selected, the ‘Add Attribute’ section of
this dialog does not reflect what has been selected, as it is only
meant for addition, not for editing.
Let’s look at each attribute already associated with your pin, and
edit some of the values. I recommend expanding the size of the edit
window until all attributes are visible.
- All pins should have a pinseq attribute. This number dictates
the order pins are looked at on this symbol in SPICE,
lepton-netlist
, and
other tools. For our first pin, start this sequence number at
1
, incrementing it on each new pin, as we add them.
- All pins should have a pinnumber attribute. This is the number
of the pin, on the package of the item you are drawing a symbol for.
pinnumber values can be alphanumeric (i.e. like
E
or
C
).
- All pins should have a pinlabel attribute. This attribute is
the name or label of the pin (not the pin number).
- All pins should have a pintype attribute. This attribute
describes the kind of a pin.
- pinseq and pinnumber attributes of several pins can be
automatically numbered by Attributes →
Autonumber Text....
Possible pintype values are: in, out, io,
oc, oe, pas, tp, tri, clk,
pwr. See pintype attribute for more info.
4.1.2.4 Adding attributes to your symbol
Every symbol needs some attributes (name=value pairs) in order
to be useful in lepton-schematic
, and in other tools.
First we’re going to add all of the attributes we want, then make the
ones that need to be invisible invisible.
- A device=DEVICENAME attribute should be placed somewhere in the
symbol. The devicename should consist of nothing but capital
letters, dashes, numbers, and underscores. If the object is a graphic
then device= should be set to none (device=none).
- A graphical=1 attribute should exist somewhere in a symbol
which is purely graphical (such as a title block or decon
symbol). Symbols which have this attribute have no electrical or
circuit significance. Don’t forget to set device=none per the
previous step.
- A description=TEXT attribute should exist somewhere in the
symbol. The TEXT value should provide a one line description
of the symbol.
- A footprint=PACKAGENAME attribute should exist somewhere in the
symbol which might be used with a pcb netlister. PACKAGENAME
is a pcb footprint or package type like
DIP14
or DIP40
.
Please see the footprint naming conventions page for further detail. If you are
using gEDA / PCB, also see the
PCB documentation and
gnetlist/docs/README.pcb for more info on this attribute and how it is
used.
- A refdes= attribute should be placed inside all non-graphical
symbols. This attribute should default to U? for integrated
circuits, and T? for transistors. Make only the value visible
and it will be promoted (attached to the outside of the symbol (so it
can be edited)) when the symbol is placed in a schematic.
- A numslots= attribute should be placed inside of all symbols,
to prevent a warning when you will check your symbol by gsymcheck.
numslots=1 works fine.
Now that those have been added, make the numslots,
footprint, description, graphical and
device attributes invisible. Leave only the value of the
refdes attribute visible, and place it somewhere at the top of
your symbol.
4.1.2.5 Finishing
- Add a label to your symbol by selecting Add →
Text... or pressing A T. Type out your label, and move it to
the top of your symbol.
- Translate the symbol to the origin using Edit →
Symbol Translate... or press E T.
- Enter
0
into the entry field and press ‘OK’.
- Save the symbol using File → Save or
File → Save As...
- Place the symbol in one of the directories specified as a symbol
library in a suite configuration file (one of the gafrc files).
For an explanation of how to set up symbol libraries, see the
Legacy configuration. Once this is done, the symbol should be
visible immediately and can be selected and placed with the
Add → Select Component... menu item. On
Debian, placing files in /usr/share/gEDA/sym/local/ works, but
requires permissions setup.
Note: Translating the symbol to the origin is a required step. To translate
a symbol elsewhere, enter a offset (in mils) which is a even multiple
of 100. Make sure all pins are snapped to a 100 mil grid point.
4.1.3 Common Symbol Naming Conventions
- Symbols are named: symbolname-#.sym
- Symbols end with a .sym extension.
- Symbols have a -# where
#
is a number. Typically
#
is 1
but if there are multiple symbols for a device
then this number simply increments.
- Symbol names are typically lowercase but letters which are part of a
part number are uppercase.
- The above case rule can be broken if the filename looks incorrect or
wrong.
4.1.4 Symbol Requirements
This section describes the various requirements which must be met in
order to create a valid symbol which will display and netlist in the
gEDA/gaf system.
4.1.4.1 Pins
- All pins should have a pinseq attribute. This number dictates
the order pins are looked at on this symbol in spice,
lepton-netlist
, and
other tools. This is just a sequence number, and should increment
sequentially starting at 1.
- All pins should have a pinnumber attribute attached to them.
This is the number of the pin, on the package of the item you are
drawing a symbol for. The pin number can be alphanumeric (i.e. like
E
or C
).
- All pins should have a pinlabel attribute attached to them.
This attribute is the name or label of the pin (not the pin number).
This attribute is also used when a symbol is used in a hierarchical
schematic.
- All pins should have pintype attribute attached to them. This
attribute describes the kind of a pin. Possible values are:
in, out, io, oc, oe, pas,
tp, tri, clk, pwr. See pintype attribute for more info.
4.1.4.2 Add attributes
For more information on the attributes presented here, please see the
Master attribute list.
- A device=DEVICENAME attribute should be placed somewhere in the
symbol and made invisible. Typically the devicename is in all
caps (capital letters). This attribute should not be used as a label.
Use a separate text object for the label. If the object is a graphic
then device= should be set to none (device=none).
- A graphical=1 attribute should exist somewhere in a symbol
which is purely graphical (such as a title block or decon
symbol). Symbols which have this attribute have no electrical or
circuit significance. Don’t forget to set device=none per the
previous requirement.
- A description=TEXT attribute should exist somewhere in the
symbol. The TEXT value should provide a one line description
of the symbol.
- A footprint=PACKAGENAME attribute should exist somewhere in the
symbol which might be used with a pcb netlister. PACKAGENAME
is a pcb footprint or package type like DIP14 or DIP40. Please see
the footprint naming
conventions page for further detail. If you are using gEDA / PCB,
also see the PCB
documentation and gnetlist/docs/README.pcb for more info on this
attribute and how it is used.
- A refdes=U? attribute should be placed inside all non-graphical
symbols. Make only the value visible and it will be promoted
(attached to the outside of the symbol, so it can be edited) when the
symbol is placed in a schematic.
- The label= and name= attributes should not exist
anywhere in the symbol. They are obsolete.
- The netname= attribute should not be attached anywhere in the
symbol. It is only used in schematics.
4.1.4.3 Slotting
- If a component has multiple slots in a package (such as a 7400 (NAND)
which has 4 NANDs per package) then you need a numslots=#
attribute. The # is the number of slots the device has.
numslots= should be exist somewhere in the symbol and made
invisible. Additional slot related required attributes are described
below.
- If a component has multiple slots in a physical package then you also
need to include a slotdef=#:#,#,#... for every slot. The first
# corresponds to the slot number. If a device has 4 slots then
there should be slotdef=1:..., slotdef=2:...,
slotdef=3:..., and slotdef=4:..., attributes existing
somewhere in the symbol and made invisible. The subsequent #
have a one-to-one correspondence to pinseq=# attributes and
specify what corresponding pinnumber=# should be when that slot
is set. See the attached 7400-1.sym as an
example of how this should all work.
- It is recommended that all symbols which have slots have a
slot=1 attribute inside the symbol.
4.1.5 Checking a Symbol
Running gsymcheck will check that all of these requirements are met.
gsymcheck will output fatal errors which are quite serious and must be
corrected. gsymcheck will also output warnings on things which should
be fixed but are not fatal.
4.1.6 Style
This section describes the style which is used in the standard
gEDA/gaf symbol library.
4.1.6.1 Text
- All text labels should all be 10 pt in size.
- Text (labels not attributes) should be color number 9 (text | green).
- Use the special overbar encoding (e.g. pinlabel=\_Reset\_) to
express inverse logic levels.
4.1.6.2 Attributes
- pinnumber attributes should be displayed as pin numbers for
pins (not pinseq) and should be 8 pt in size.
- Attached attributes should be color number 5 (attributes | yellow).
The color is set automatically to yellow if the text item is attached.
- The only exception to this is pinlabel= attributes, those
should be color number 9 (text | green). If every text item within a
symbol is yellow, the symbol looks too yellow.
- Attributes can be attached to some part of the symbol. Toplevel
attributes (like the device= or net= attributes) used to
be required to be attached to something to be attributes, but now they
just have to exist in the symbol file as name=value.
- Expanding a bit on the last sentence, as long as the text item has
the format name=value, it is considered an attribute.
Attributes inside a symbol do not have to be attached to anything.
In order to see hidden attributes in
lepton-schematic
select
Edit → Show/Hide Inv Text.
- There is a symbol content versioning system in libgeda which is based
on the symversion= attribute. Please see the symversion attribute for more information on using this versioning scheme.
4.1.6.3 Graphics
- Lines, boxes, arcs, and any other graphics should be color number 3
(graphic | green).
- Polarity bubbles should be color number 6 (logic bubble | cyan).
- If you are unsure on how to make a new symbol look or how big to make
a new symbol, look at the existing symbols to get a feel for the
appropriate appearance and size.
4.1.6.4 Pins
- Pins should all be 300 mils (3 grid spaces) long.
- For pins which are next to a logic bubble, make the pins 200 mils (2
grid spaces) long and then make the logic bubble 100 mils in diameter.
In order to draw a 100 mil diameter circle, you will need to change
the snap spacing to 50.
- A pin has two ends: one end has a red endpoint and one end that does
not. The red endpoint is where nets can be connected. You can either
rotate the pin so that this active end is in the right place or
manually edit the symbol file changing the “whichend” parameter on
the pin object. See pin for more info.
- Be that all endpoints of pins which are meant to be connected to are
on the 100 mil grid. The endpoint which is not active can be off the
grid if necessary.
- Pins should be color number 1 (pins | white).
- Leave 400 mils (4 grid spaces) between (vertically) pins, unless you
are drawing a special symbol, then just try to make it look good.
- Pin number attributes should be 50 mils above (or below; which ever
makes the most sense) the pin which they are attached to.
- Input pins belong on the left and output pins belong on the right of
the symbol.
- Please do not mix inputs and outputs on the same side of the symbol,
unless absolutely necessary.
- You can have pins on the top or bottom of a symbol.
- The order for rows of pins (buses) should be LSB (least significant
bit) to MSB (most significant bit). When drawing pins which are part
of a bus, make sure the LSB of the bus is at the top (or for pins on
top/bottom of a symbol, left of the rest of the other pins). Look at
74/74181-1.sym for a correct example of this order (
A0
on top through A3
and B0
on top through B3
).
Violating this rule will make connecting buses much more difficult.
- When placing pins on logic gates, be sure to place the smallest pin
numbers toward the top (or left) and then increment going down (or
across).
4.1.6.5 Electrical
- Do not draw power and ground pins. That information will be conveyed
using attributes (see the net= attribute mini-HOWTO).
- The above rule can be broken if necessary, but keep in mind most of
the standard library does not have power pins showing.
- Keep in mind, symbols are supposed to be symbolic, they do not
represent the physical package that the device comes in.
- There is some disagreement on above, so this is okay too: arrange the
pins on a symbol logically so that they promote an uncluttered
schematic. Note that this is frequently not the same pin arrangement
as the physical device.
4.1.7 Hints and Tips
This section describes some hints and tips which will make your symbol
creation experience easier.
- Pins MUST be snapped on the 100-spaced grid.
- Avoid drawing things off of the grid. If you do, you cannot move the
object(s) using the move command (if the grid is on) since the object
will be snapped to the grid. [This was an old bug, which I think has
been fixed, but avoid doing this anyway]. Use the symbol translate
command instead (or move the object with grid snap off).
- It is okay to set the grid finer than 100 (say 10 or 25) when creating
symbols, just remember to set this back to 100 once you are ready to
translate the symbol to the origin.
- If you want to translate a symbol from the origin to elsewhere, then
use the Edit → Symbol Translate... command and
enter a non zero number. Make sure this number is a multiple of 100
(i.e. 1000, or 1100).
- Remember that pins are special objects; if you want to add a pin, make
sure it is a pin and not a line or net. Use the Add
→ Pin command to place a pin.
- Don’t include nets or buses inside symbols. That is not supported and
doesn’t make much sense anyway.
- pinnumber should be the visible attribute for pins, along with
pinlabel if any.