[Spec] Language feature: Hierarchical bulk connection override for MOS devices

[coderabbitai]

Description

In the Cascode language, MOS device bulk connections need better handling to support both common cases and advanced use cases.

Default Behavior

In most normal circumstances, bulk connections should be implied:

• PMOS devices should get VDD by default
• NMOS devices should get GND/VSS by default

Need for Override

In certain cases, designers need the ability to override the default bulk connections. More importantly, this override capability should be available at higher levels in the hierarchy.

Required Language Feature

We need a language feature that allows users to take a motif or module that uses MOS devices internally, and override the bulk connections from a higher level in the design hierarchy, without having to modify the lower-level module definition.

Context

This issue was raised during review of the CurrentMirror motif in the Cascode Standard Library.

• PR: Language spec revisions and Cascode Standard Library #18
• Comment: Language spec revisions and Cascode Standard Library #18 (comment)
• Requested by: @daniellovell