2020-12-27 22:59:15 +01:00
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<title>XSCHEM TUTORIAL: Backannotation of ngspice simulation data</title>
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<a href="xschem_man.html" class="home">UP</a>
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<!-- slide title -->
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<h1> TUTORIAL: Backannotation of NGSPICE simulation operating<br>point data into an XSCHEM schematic</h1>
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<p>
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2020-12-28 02:52:44 +01:00
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The objective of this tutorial is to show into the schematic the operating point data (voltages currents,
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other electrical parameters) of a SPICE simulation done with the
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<a href="https://sourceforge.net/projects/ngspice/">Ngspice</a> simulator.
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This tutorial is based on the <kbd>cmos_example.sch</kbd> example schematic located in the <kbd>examples/</kbd>
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directory. Start Xschem from a terminal since we need to give some commands in this tutorial.
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2020-12-27 22:59:15 +01:00
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</p>
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2020-12-28 02:52:44 +01:00
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<img style="box-shadow:none;" src="backannotation1.svg">
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2020-12-27 22:59:15 +01:00
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2020-12-28 02:52:44 +01:00
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<h3> CONFIGURATION </H3>
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<p>
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Open your <kbd>xschemrc</kbd> file (usually <kbd>~/.xschem/xschemrc</kbd>), go to the end of the file,
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Ensure the following tcl file is appended to the list of scripts loaded on startup by Xschem:
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<pre class="code">
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#### list of tcl files to preload.
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lappend tcl_files ${XSCHEM_SHAREDIR}/ngspice_backannotate.tcl
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</pre>
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<h3> SETUP </H3>
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<p>
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Select the 'STIMULI' code block (click on it) and edit its attributes (press <kbd>q</kbd> or
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<kbd>Shift-q</kbd>):
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</p>
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<pre class="code">
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.temp 30
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** models are generally not free: you must download
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** SPICE models for active devices and put them into the below
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** referenced file in netlist/simulation directory.
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.include "models_cmos_example.txt"
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.control
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<span style="color: red;"> op </span>
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<span style="color: red;"> save all </span>
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<span style="color: red;"> write cmos_example.raw </span>
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.endc
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</pre>
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<p>
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The important parts are in red in above text. This ensures all variables are saved into the raw file.
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These instructions are for an interactive ngspice run.<br>
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When done open the <kbd>Simulation-> Configure simulators and tools</kbd> dialog box and ensure the
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<kbd>Ngspice</kbd> simulator is selected (not Ngspice batch). Also ensure the spice netlist mode is
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selected (<kbd>Options -> Spice netlist</kbd>).
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</p>
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<img src="backannotation2.png">
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<h3> SIMULATION </H3>
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<p>
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If you now press the <kbd>Netlist</kbd> followed by the <kbd>Simulate</kbd> button simulation should complete
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with no errors.
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</p>
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<img src="backannotation3.png">
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<p>
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You can close the simulator since we need only the <kbd>cmos_example.raw</kbd> file that is now saved
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in the simulation directory (usually <kbd>~/.xschem/simulations/cmos_example.raw</kbd>).<br>
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Now verify that xschem is able to read the raw file: issue this command in the xschem console:<br>
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<kbd>ngspice::annotate</kbd>
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</p>
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<pre class="code">
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xschem [~] ngspice::annotate
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Raw file read ...
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xschem [~]
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</pre>
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<p>
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If there are no errors we are ready and set.
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</p>
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<h3> PUSH ANNOTATION METHOD </H3>
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<p>
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Start placing some probe elements into the schematic.
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The first element is the <kbd>devices/spice_probe.sym</kbd> component. This must be attached to
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some schematic wires to show the voltage value.
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</p>
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<img style="box-shadow:none;" src="backannotation4.svg">
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<p>
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Place some of these elements on various nets, issue the above mentioned <kbd>ngspice::annotate</kbd>
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command and see the voltage values in the schematic.
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</p>
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<img style="box-shadow:none;" src="backannotation5.svg">
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<p>
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Another useful component is the <kbd>devices/ammeter.sym</kbd> one which allow to monitor branch currents.
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Break some wires and insert this component as shown here:
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</p>
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<img style="box-shadow:none;" src="backannotation6.svg">
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<p class="important">
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IMPORTANT: When inserting current probes the circuit topology changes (new nodes are created) so you need to
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re-create the netlist and re-run the simulation
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</p><br>
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<p>
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Doing again the <kbd>ngspice::annotate</kbd> command after simulation will update the ammeters showing
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the branch currents.
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</p>
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<img style="box-shadow:none;" src="backannotation7.svg">
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<p>
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These voltage and current values are inserted in the probe components as <b>attributes</b> and thus can be saved
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to file. Remember that if you change the circuit the values shown in the probe elements are no longer valid,
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you should update the values with a new simulation + annotate operation when done with the changes.<br><br>
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What i have described so far is the simplest annotation procedure based on a <b>push</b> method: a tcl script
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reads the simulation raw file and 'pushes' voltage and current values into the probe components as instance
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attributes. If you do an edit attribute on one of these elements you see the attribute 'pushed' into it by the
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annotate script. The advantage of this method is that values pushed into probes can be saved to file and are
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thus persistent.
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</p>
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<img src="backannotation8.png">
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<h3> PULL ANNOTATION METHOD </H3>
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<p>
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There is another annotation procedure that is based on a <b>pull</b> method: the probe objects have tcl commands
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embedded that fetch simulation data from a table that has been read by the annotate script.
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</p>
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2020-12-27 22:59:15 +01:00
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2020-12-28 02:52:44 +01:00
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<p>
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To ensure all currents are saved modify the <kbd>STIMULI</kbd> attributes as follows:
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</p>
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<pre class="code">
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.temp 30
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** models are generally not free: you must download
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** SPICE models for active devices and put them into the below
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** referenced file in netlist/simulation directory.
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.include "models_cmos_example.txt"
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.option savecurrents
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.save all
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.control
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op
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write cmos_example.raw
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.endc
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</pre>
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<p>
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Remove all previous probe elements and place some <kbd>devices/ngspice_probe.sym</kbd> components
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and some <kbd>devices/ngspice_get_value.sym</kbd> components.
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the ngspice_probe.sym is a simple voltage viewer and must be attached to a net. The ngspice_get_value.sym
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displays a generic variable stored in the raw file. This symbol is usually placed next to the referenced
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component, but does not need to be attached to any specific point or wire.
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Edit its attributes and set its <kbd>node</kbd> attribute to an existing saved variable in the raw file.
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</p>
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<img src="backannotation9.png">
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<p>
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Run again the simulation and the <kbd>ngspice::annotate</kbd> command and values will be updated.
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</p>
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<img src="backannotation10.svg">
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<p>
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You can add additional variables in the raw file , for example modifying the .save instruction:<br>
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<kbd>.save all @m4[gm] @m5[gm] @m1[gm]</kbd>
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</p>
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<img src="backannotation11.png">
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<p>
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Data annotated into the schematic using these components allows more simulation parameters to be viewed into
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the schematic, not being restricted to currents and voltages. Since these components get data using a pull method
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data is not persistent and not saved to file. After reloading the file just do a <kbd>ngspice::annotate</kbd>
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to view data again.
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</p>
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2020-12-27 22:59:15 +01:00
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2020-12-28 02:52:44 +01:00
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<p>
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There is one last probe component, the <kbd>devices/ngspice_get_expr.sym</kbd>. This is the most complex one, and
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thus also the most flexible. It allows to insert a generic tcl expression using spice simulated data to report
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more complex data. In the example below this component is used to display the electrical power of transistor m3,
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calculated as <kbd>V(GN) * Id(m3)</kbd>.
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</p>
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<img src="backannotation12.png">
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<p>
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The example shown uses this component to display a (meaningless, but shows the usage) gm ratio of 2 transistors:
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</p>
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<img src="backannotation13.png">
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<p>
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The syntax is a bit clompex, considering the verbosity of TCL and the strange ngspice naming syntax, however
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once a working one is created changing the expression is easy.
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</p>
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<p class="important">
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To avoid the need of typing commands in the xschem console a launcher component <kbd>devices/launcher.sym</kbd>
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can be placed with the tcl command for doing the annotation. Just do a <kbd>Ctrl-Click</kbd> on it to trigger
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the annotation.
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</p>
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<img src="backannotation14.png">
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<br>
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<br>
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<br>
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<br>
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2020-12-27 22:59:15 +01:00
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