I'm building a Pandora device based on the Pi which uses the P1 header with Adafruit's Pi T-Cobbler breakout.
The 3.3V pin (P1-01) powers 6 momentary buttons each with the following circuit:
The 5V pin (P1-02) powers an LCD through the following circuit:
So the question is: is there any reason that I should use different ground pins for the circuits on the two different power rails? For example, should I use ground pin P1-06 for the buttons and ground pin P1-09 for the LCD circuit?
It is generally considered good engineering practice to use separate grounds for differing classes of connections. This minimises crosstalk and excess voltage drop.
In practice, with the Pi, I doubt the current draw would be high enough to cause problems, or the speed high enough to cause crosstalk. If you were using very long leads to connect to the devices this is more important.
I would use separate connections for the power supply regulator and signal grounds. Good design costs nothing, and can avoid potential problems.
No, the pins are all connected to ground, even on the early Rev 1 boards.
You shouldn't have currents large enough to have a significant potential difference across the various pins.
All ground pins are supposed to be connected, so there's no difference which pin (or combination of pins) you use for your schematics.
If you want to be extra careful, you may use multimeter to make sure it reads exactly 0V between the ground pins you're about to connect to different parts of your design.
