CREATING A CIRCUIT SCHEMATIC

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# CREATING SYMBOLS

### Creating a subcircuit symbol

Suppose you have just finished creating a circuit and you now want to create a symbol for it so you can use this circuit as a sub block in other schematics:

Above schematic contains `VPP`, `PLUS`, `MINUS`, `VSS`, `VNN` input pins and `OUT` output pin.
If you press the `a` key xschem will generate a symbol automatically.

If your schematic is called `mos_power_amplifier.sch` the symbol will be saved in the same place as the schematic and named `mos_power_amplifier.sym`.
If you open a new empty schematic and use the `Insert` or `Shift-I` key to insert a symbol and select the `mos_power_amplifier.sym` you get this:

If you select the symbol instance and press `q` you see the instance `name` attribute; the name attribute specifies an unique name in current schematic. There can not be two `x2` instances in a schematic. If you copy the placed instance to get two of them the new one will be automatically renamed (to `x3` or `x1`, or any available unique name).

If you descend into the symbol and press `q` you see the following attributes:

    type=subcircuit
    format="@name @pinlist @symname"
    template="name=x1"

These attributes are using by xschem to generate the subcircuit netlist line. the `format` attribute tells xschem that a line containing the instance name (`@name`, replaced by `x2`), the list of attached nets (`@pinlist`, replaced by the nets attached to the symbol i/o ports in the order they are declared in the subcircuit) and the symbol name (`@symname`, replaced by `mos_power_amplifier`).
The `type` attribute tells xschem that the symbol is a subcircuit (not a terminal symbol) and netlister should further descend into the corresponding schematic to complete the netlist.
The `template` attribute defines default values for attributes when the symbol is placed in a schematic. For example if you place an instance of this symbol in an empty schematic the instance `name` attribute will be set to `x1`. If there is already an `x1` instance xschem will automatically rename the instance to a unique name.
You can manually edit the symbol to change its shape or change the pin ordering.
If you change the pin positions always move the pin (the red square) and the label together.

If you select one pin (the small red square box) and press 'q' you see the pin attributes:
`name` specifies the pin name.
`dir` specifies the pin direction (`in, out, inout`).
It is good practice to verify that the pin `name` attribute matches the name of the text label next to it.

If you edit the text label next to a pin the pin name attribute will be changed automatically.

### Creating a new symbol and schematic by cloning

Another useful approach to create a new component (both symbol and schematic view) is to 'clone' it from a similar existing component: after copying a component to a different place in the schematic, press the edit property bindkey (`q` key) and set a new name for the symbol, set also the `copy cell` checkbox:

After pressing `OK` a copy (both schematic and symbol views) of the previously selected component will be created. After this clone operation modifications can be made on the newly created schematic and symbol views without affecting the original component.

for more info on symbols see the Tutorial