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http://www.geocities.com/capecanaveral/lab.../TraceWidth.htmSilicon Labs AN203 C8051FXXX PRINTED CIRCUIT BOARD DESIGN NOTES
4.1. Unused Pins
Many embedded system designs do not use all available
pins on a mixed-signal MCU. The following is a list
of typical unused pin types and what action to take during
PCB design:
Digital general-purpose I/O port pins (GPIO):
Connect directly to digital ground and configured as
open-drain with internal weak pullups disabled to
save power, or they can be left floating and driven to
logic “0” by software.
Analog signals: Connect directly to analog ground,
which reduces susceptibility to radiated noise.
Op-amps: Connect their non-inverting (+) input to
ground and the inverting input (-) to the op-amp
output.