Laker Custom Layout: MCell,UDD & PCells

Parameterized devices are used in the design of analog and custom digital integrated circuits to take the place of many fixed cells by allowing the substitution of different values for specified dimensional variables (parameters). A single parameterized device for an NMOS transistor, for example, can be substituted for an almost infinite number of such devices by changing individual parameters such as gate length for each instance (placement) of the device in the design.

Analog circuits are designed in schematic form using symbols that correspond to specific devices. Before layout begins, the parameter values for each device are set by the circuit designer based upon simulations run using models supplied by the foundry. When the parameterized devices are placed, the layout generated by the layout system will automatically reflect the designated values without the user ever having to draw a single shape by hand.  Parameterized devices not only save an enormous amount of manual layout time, but also facilitate additional levels of automation that compound their inherent productivity.

Cadence® is generally attributed with developing the first commercially successful parameterized devices, called “PCells” (parameterized cells). Today the term is so common that “parameterized devices” and “PCells” are virtually synonymous. The Laker Custom Layout System offers several different kinds of parameterized devices in addition to PCells, but “PCell” is such a generic term that it is best to describe PCells first in order to create a reference point for describing other parameterized devices.

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