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Attributes System

LibrePCB uses a sophisticated attributes system which makes it possible to display dynamically determined information in schematics and boards, or to provide additional information for the BOM.

Example: Symbol

Let's start with a short example. A symbol as defined in the library could look like this:

library_symbol.png
Library Symbol

It contains various texts on different layers, and each of them will be replaced by a dynamically determined text while the symbol is part of a schematic:

Text Graphics Layer In schematics substituted by following value
{{NAME}} sym_names Designator of the symbol's component in a schematic (e.g. "R5")
{{VALUE}} sym_values Most important attribute of the symbol's component (e.g. "100nF" for a capacitor, or "LM358N" for an OpAmp)
Note: With which attribute's value {{VALUE}} is substituted is controlled by the corresponding component, not by the symbol itself.
{{PARTNUMBER}} sym_attributes Part number of the linked device (e.g. "LM358N")
Note: Such arbitrary attributes should normally not be added to symbols! Instead, the {{VALUE}} attribute should be used in most cases.

The {{NAME}} and {{VALUE}} are the most important attributes of a symbol, nearly every symbol will have text items to display them in schematics. They also have their own graphics layer, so it's possible to show or hide the names and values of all symbols in a schematic by changing the visibility of these two layers.

For text items which display other attributes (e.g. {{PARTNUMBER}}, {{RESISTANCE}} or {{FOOBAR}}) the graphics layer sym_attributes should be used. There are no more layers to further distinguish between different attributes.

Exactly the same system also applies to footprints, with the only exception that different graphics layers are used (top_names, bot_names, top_values, ...).

Built-In and User-Defined Attributes

There are two different kinds of attributes:

Built-In Attributes: These attributes are automatically defined by the application and cannot be removed. Some of these attributes values can still be modified by the user, but some are fully defined by the application and are not editable.

User-Defined Attributes: On some scopes (see next chapter), the user can also add arbitrary attributes in addition to the built-in ones. They always have higher priority than the built-in ones, so the user is (theoretically) able to override built-in attributes if necessary.

Scopes

Attributes can be defined at various objects, with different scopes:

Object of class Scope
librepcb::project::Project Whole project (global)
librepcb::project::ComponentInstance One component, its device and symbols
librepcb::project::Schematic One schematic page and its symbols
librepcb::project::SI_Symbol One schematic symbol
librepcb::project::Board One board, its devices and components
librepcb::project::BI_Device One device and its package

Lookup Order

If the value of an attribute needs to be determined for one specific purpose (for example to substitute a text in a symbol), the lookup is done in a specific order. The red arrows of the following diagram show how this order is defined. It also lists all built-in attributes and on which scopes it is possible to add user-defined attributes.

attributes_system.png
Attributes System

Multiple Key Substitution

Sometimes the {{VALUE}} text in a symbol or footprint should be substituted by the most exact attribute of a component or device. For example a microcontroller symbol in the schematic should show the exact part number of the corresponding device (Prio. 1). But if no part number is available, the device name should be shown instead (Prio. 2). And if the component does not even have a device assigned, the component name should be shown (Prio. 3). This table shows these three cases for the STM32F103C8T7TR:

Prio. Component Name Device Name Device Part Number Desired Substitution of {{VALUE}}
1 "STM32F103C" "STM32F103CxT" "STM32F103C8T7TR" "STM32F103C8T7TR"
2 "STM32F103C" "STM32F103CxT" N/A "STM32F103CxT"
3 "STM32F103C" N/A N/A "STM32F103C"

To get this "best-match-attribute" behavior, the {{VALUE}} attribute of a component can be set to the value "{{PARTNUMBER or DEVICE or COMPONENT}}". So the 'or' keyword after a key means that a fallback key is following if the first key is not set to a non-empty value. As soon as the first value of the specified keys is not empty, all following keys are ignored.

Of course this system applies to arbitrary attributes, not only to {{VALUE}}.

Allowed Characters

Attribute keys must only consist of the characters 0-9, a-z, A-Z and '_'. In addition, key names must not start with a digit.

Visibility in Schematics and Boards

Generally the user wants to select which attributes of a component should be visible in the schematic and in the board. Sometimes schematics and boards should even show different attributes of the same component, for example a capacitor should show the value "100nF/50V" in the schematic, but only "100nF" in the board because of the limited space.

By default, symbols and footprints show all the attributes which are contained in a text item in their library elements (like {{NAME}}, {{VALUE}} and {{PARTNUMBER}} in the symbol above). But in schematics and boards, the user can remove every single of these text items to hide specific attributes. Or to show more attributes, new text items (with arbitrary values) can be added to symbols and footprints. New text items are then added with some default properties (layer, text size, position, ...) which the user needs to adjust afterwards.

Anyway, adding attributes other than {{NAME}} and {{VALUE}} to library symbols and footprints should be used very rarely to keep them as clean and simple as possible. For the most important attributes, the {{VALUE}} text item should be used.

Example Use-Cases

Here are some examples how the attribute system works for standard use-cases:

Use-Cases Symbol /
Footprint
Component Attributes /
Device Attributes
Component Defaults /
Device Defaults
Component Values /
Device Values
Schematic /
Board
Resistor
Capacitor
Inductor
resistor_symbol.png
{{VALUE}}"{{RESISTANCE}} {{TOLERANCE}} {{POWER}}"
resistor_schematic.png
{{RESISTANCE}}"10Ω"
{{TOLERANCE}}"1%"
{POWER}}"5W"
resistor_footprint.png
{{VALUE}}"{{RESISTANCE}}"
resistor_board.png
Diode
Transistor
OpAmp
diode_symbol.png
{{VALUE}}"{{ PARTNUMBER or DEVICE }}"
diode_schematic.png
diode_footprint.png
N/A
diode_board.png
LED
led_symbol.png
{{VALUE}} "{{ PARTNUMBER or DEVICE }}
{{COLOR}}"
led_schematic.png
{{COLOR}}
led_footprint.png
{{VALUE}}"{{COLOR}}"
led_board.png
{{COLOR}}"RED"
Connector
connector_symbol.png
{{VALUE}}"{{ PARTNUMBER or DEVICE }}"
connector_schematic.png
connector_footprint.png
N/A
connector_board.png
Microcontroller
microcontroller_symbol.png
{{VALUE}}"{{ PARTNUMBER or DEVICE or COMPONENT }}"
microcontroller_schematic.png
microcontroller_footprint.png
N/A
microcontroller_board.png
Sheet Frame
sheetframe_symbol.png
N/A
sheetframe_schematic.png

Implementation

Every class which provides some attributes need to inherit from librepcb::AttributeProvider. Please read the documentation of librepcb::AttributeProvider to see how this class works in detail.

The substitution of attributes in strings is done by the class librepcb::AttributeSubstitutor.