add more documentation on parametric subcircuits
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stefan schippers
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@ -17,7 +17,7 @@ p{padding: 15px 30px 10px;}
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<!-- navigation buttons -->
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<a href="creating_symbols.html" class="prev">PREV</a>
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<a href="xschem_man.html" class="home">UP</a>
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<a href="commands.html" class="next">NEXT</a>
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<a href="subckt_with_parameters.html" class="next">NEXT</a>
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<!-- slide title -->
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<h1>COMPONENT PARAMETERS</h1><br>
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<p>
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@ -0,0 +1,148 @@
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<!DOCTYPE html>
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<html>
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<head>
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<title>CREATING A PARAMETRIC SUBCIRCUIT</title>
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<link rel="stylesheet" type="text/css" href="xschem_man.css" />
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<style type="text/css">
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/* Local styling goes here */
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p{padding: 15px 30px 10px;}
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</style>
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</head>
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<body>
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<!-- start of slide -->
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<div class="content">
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<!-- navigation buttons -->
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<a href="parameters.html" class="prev">PREV</a>
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<a href="xschem_man.html" class="home">UP</a>
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<a href="commands.html" class="next">NEXT</a>
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<!-- slide title -->
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<h1>CREATING A PARAMETRIC SUBCIRCUIT</h1><br>
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<p>
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Let's suppose we want to design an OPAMP macromodel taking the following parameters:
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</p>
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<ul>
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<li>GAIN: The differential maximum small signal gain of the opamp.</li>
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<li>AMPLITUDE: The peak to peak swing ot the opamp output.</li>
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<li>OFFSET: the offset of the output when input differential signal is zero.<br>
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For example giving AMPLITUDE=10 and OFFSET=5 will result in an output swing from 0 to +10V.</li>
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<li>ROUT: the output resistance.</li>
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<li>COUT: the output capacitance. Together with ROUT defines a RC time constant (dominant pole).</li>
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</ul>
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<p>
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The image below shows the circuit. A 'B' voltage-type source with an hyperbolic tangent
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function is used because it has continuous derivative and a realistic shape.
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</p>
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<img src="subckt_with_parameters01.png">
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<p>
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after drawig the schematic a symbol is created. The easiest way is to press the 'a' key in the
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schematic to automatically create the symbol, then descend into the symbol and do some artwork to
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reshape it to represent an opamp.<br>
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After reshaping the symbol edit its global attributes and add handling of subcircuit parameters
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in the <kbd>format</kbd> and <kbd>template</kbd> attributes as shown below:
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</p>
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<img src="subckt_with_parameters02.png">
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<p>
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the symbol has the following global attributes:
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</p>
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<pre class="code">
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type=subcircuit
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format="@name @pinlist @symname OFFSET=@OFFSET AMPLITUDE=@AMPLITUDE GAIN=@GAIN ROUT=@ROUT COUT=@COUT"
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template="name=x1 OFFSET=0 AMPLITUDE=5 GAIN=100 ROUT=1000 COUT=1p"
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</pre>
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<p>
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The <kbd>format</kbd> string defines how the instantiation will look in the spice netlist,
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The following is the resulting spice netlist line and how it is generated from the format string:
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</p>
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<pre class="code">
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x1 REFD DRIVED IN comp_ngspice OFFSET=5 AMPLITUDE=10 GAIN=100 ROUT=1000 COUT=1p
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-- -------------- ------------ -------- ------------ -------- --------- -------
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| | | | | | | |
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| | | | | | | COUT=@COUT
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| | | | | | ROUT=@ROUT
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| | | | | GAIN=@GAIN
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| | | | |
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| | | | AMPLITUDE=@AMPLITUDE
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| | | |
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| | | OFFSET=@OFFSET
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| | @symname
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| |
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| @pinlist
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@name
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</pre>
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<p>
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The <kbd>template</kbd> string defines initial values for these parameters when you first instantiate this
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component:
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</p>
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<pre class="code">
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template="name=x1 OFFSET=0 AMPLITUDE=5 GAIN=100 ROUT=1000 COUT=1p"
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</pre>
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<img src="subckt_with_parameters03.png">
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<p>
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As you can see in above image the placed component has instance parameters set to the same values
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listed in the <kbd>template</kbd> string. You may then change these values according yo your needs.
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The values set in the instance affect that specific component instance behavior. Multiple instances
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can be placed, each with it's own set of parameter values.
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As you can see the @param values in the format string are replaced with the actual value set in
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the instance attributes.
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</p>
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<p>
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When a netlist is generated the following lines are generated regarding this comp_ngspice symbol:
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</p>
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<pre class="code">
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* sch_path: /home/schippes/xschem-repo/trunk/xschem_library/examples/classD_amp.sch
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...
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... other components ....
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...
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x1 REFD DRIVED IN comp_ngspice OFFSET=5 AMPLITUDE=10 GAIN=100 ROUT=1000 COUT=1
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...
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...
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* expanding symbol: comp_ngspice.sym # of pins=3
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** sym_path: /home/schippes/xschem-repo/trunk/xschem_library/ngspice/comp_ngspice.sym
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** sch_path: /home/schippes/xschem-repo/trunk/xschem_library/ngspice/comp_ngspice.sch
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.subckt comp_ngspice PLUS OUT MINUS OFFSET=0 AMPLITUDE=5 GAIN=100 ROUT=1000 COUT=1p
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*.ipin PLUS
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*.ipin MINUS
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*.opin OUT
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B1 IOUT 0 V = {OFFSET + AMPLITUDE/2*(tanh(V(IPLUS,IMINUS)*GAIN*2/AMPLITUDE))}
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R1 OUT IOUT ROUT m=1
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C3 OUT 0 COUT m=1
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V1 IPLUS PLUS 0
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.save i(v1)
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V2 IMINUS MINUS 0
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.save i(v2)
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.ends
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...
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...
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.end
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</pre>
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<p>
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You see the <kbd>.subckt</kbd> line contains the subcircuit parameters and their values:
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The values present in the <kbd>.subckt</kbd> line are overridden by instance attribute values
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if given.
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</p>
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<pre class="code">
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.subckt comp_ngspice PLUS OUT MINUS OFFSET=0 AMPLITUDE=5 GAIN=100 ROUT=1000 COUT=1p
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------------ -------------- -----------------------------------------------
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| | |
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| | |
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| | parameters from template attributes:
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| | |
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| | _______________________________________________
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| | template="name=x1 OFFSET=0 AMPLITUDE=5 GAIN=100 ROUT=1000 COUT=1p"
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| port list
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symbol name
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</pre>
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<br>
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<br>
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<!-- end of slide -->
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<div class="filler"></div>
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</div>
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<!-- frame footer -->
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<iframe seamless src="xschem_footer.html" class="footer_iframe" >
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</body>
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</html>
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@ -43,6 +43,7 @@
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<li> <a href="creating_schematic.html">Creating a circuit schematic</a></li>
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<li> <a href="creating_symbols.html">Creating symbols</a></li>
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<li> <a href="parameters.html">Component parameters</a></li>
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<li> <a href="subckt_with_parameters.html">More info on creating a parametric subcircuit</a></li>
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<li> <a href="commands.html">Editor commands</a></li>
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<li> <a href="netlisting.html">Netlisting</a></li>
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<li> <a href="net_probes.html">Net Probes</a></li>
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