2020-08-08 15:47:34 +02:00
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<title>CREATING A CIRCUIT SCHEMATIC</title>
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<!-- slide title -->
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<h1>CREATING SYMBOLS</h1><br>
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2023-02-03 11:58:00 +01:00
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<h3> Creating a subcircuit symbol</h3>
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2020-08-08 15:47:34 +02:00
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<p>
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2023-02-03 11:58:00 +01:00
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Suppose you have just finished creating a circuit and you now want to create a
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symbol for it so you can use this circuit as a sub block in other schematics:
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</p>
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<img src="creating_symbols01.png">
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<p>
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Above schematic contains <kbd>VPP</kbd>, <kbd>PLUS</kbd>, <kbd>MINUS</kbd>,
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<kbd>VSS</kbd>, <kbd>VNN</kbd> input pins and <kbd>OUT</kbd> output pin.<br>
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If you press the <kbd>a</kbd> key xschem will generate a symbol automatically.
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</p>
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<img src="creating_symbols02.png">
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<p>
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If your schematic is called <kbd>mos_power_amplifier.sch</kbd> the symbol will be saved
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in the same place as the schematic and named <kbd>mos_power_amplifier.sym</kbd>.<br>
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If you open a new empty schematic and use the <kbd>Insert</kbd> or <kbd>Shift-I</kbd> key
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to insert a symbol and select the <kbd>mos_power_amplifier.sym</kbd> you get this:
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</p>
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<img src="creating_symbols03.png">
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<p>
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If you select the symbol instance and press <kbd>q</kbd> you see the instance <kbd>name</kbd>
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attribute; the name attribute specifies an unique name in current schematic. There can not be two
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<kbd>x2</kbd> instances in a schematic. If you copy the placed instance to get two of them the new
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one will be automatically renamed (to <kbd>x3</kbd> or <kbd>x1</kbd>, or any available unique name).
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</p>
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<img src="creating_symbols06.png">
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<p>
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If you descend into the symbol and press <kbd>q</kbd> you see the following attributes:
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</p>
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<img src="creating_symbols04.png">
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<pre class="code"style="width:500px;margin-left:40px;">
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type=subcircuit
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format="@name @pinlist @symname"
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template="name=x1"
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2025-07-21 10:41:45 +02:00
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</pre><br>
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<p class="important">
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It is now possible to use @model instead of @symname to use a custom specified subcircuit name instead
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of the symbol filename, see the following example for an opamp subcircuit with a gain parameter:
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</p><br>
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<pre class="code"style="width:500px;margin-left:40px;">
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type=subcircuit
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format="@name @pinlist @model gain=@gain"
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template="name=x1 gain=100 model=opamp"
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</pre><br>
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<p> the above extension is allowed only for spice and spectre netlists.</p>
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2023-02-03 11:58:00 +01:00
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<p>
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These attributes are using by xschem to generate the subcircuit netlist line. the <kbd>format</kbd>
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attribute tells xschem that a line containing the instance name (<kbd>@name</kbd>, replaced by
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<kbd>x2</kbd>), the list of attached nets
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(<kbd>@pinlist</kbd>, replaced by the nets attached to the symbol i/o ports in the
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order they are declared in the subcircuit)
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and the symbol name (<kbd>@symname</kbd>, replaced by <kbd>mos_power_amplifier</kbd>).<br>
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The <kbd>type</kbd> attribute tells xschem that the symbol is a subcircuit (not a terminal symbol)
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and netlister should further descend into the corresponding schematic to complete the netlist.<br>
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The <kbd>template</kbd> attribute defines default values for attributes when the symbol is
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placed in a schematic. For example if you place an instance of this symbol in an empty schematic
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the instance <kbd>name</kbd> attribute will be set to <kbd>x1</kbd>. If there is already an
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<kbd>x1</kbd> instance xschem will automatically rename the instance to a unique name.
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<br><br>
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You can manually edit the symbol to change its shape or change the pin ordering.<br>
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If you change the pin positions always move the pin (the red square) and the label together.
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</p>
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<img src="creating_symbols05.png">
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<p>
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If you select one pin (the small red square box) and press 'q' you see the pin attributes:<br>
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<kbd>name</kbd> specifies the pin name.<br>
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<kbd>dir</kbd> specifies the pin direction (<kbd>in, out, inout</kbd>).<br>
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It is good practice to verify that the pin <kbd>name</kbd> attribute matches the name of the text label next to it.
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</p>
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<p class="important">
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If you edit the text label next to a pin the pin name attribute will be changed automatically.
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</p>
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<img src="creating_symbols07.png">
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<br>
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<h3> Creating a new symbol and schematic by cloning</h3>
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<p>
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Another useful approach to create a new component (both symbol and schematic view) is to 'clone' it
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2020-08-08 15:47:34 +02:00
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from a similar existing component: after copying a component to a different place in the schematic,
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press the edit property bindkey (<kbd>q</kbd> key) and set a new name for the symbol, set also the
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<kbd>copy cell</kbd> checkbox:
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</p>
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2021-10-13 17:33:10 +02:00
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<img src="symbol_generation5.png">
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2020-08-08 15:47:34 +02:00
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<p>
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After pressing <kbd>OK</kbd> a copy (both schematic and symbol views) of the previously selected
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component will be created. After this clone operation modifications can be made on the newly
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created schematic and symbol views without affecting the original component.
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</p>
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2021-10-13 17:33:10 +02:00
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<img src="symbol_generation6.png">
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2020-08-08 15:47:34 +02:00
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<p> for more info on symbols see the <a href="tutorial_create_symbol.html">Tutorial</a> </p><br>
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<!-- end of slide -->
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