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I'm about to start using a chip that has one positive and two ground connections - I've only used chips with one positive and one ground before.
How should I attach the decoupling capacitor(s?) to this?
Should I just have one capacitor across pin 20 and 19?
If you are using something inferior to a PCB like a breadboard then the decoupler(s) should go across pins 19 and 20 with pin 8 connected to pin 19 using a short wire.
If using a single sided PCB then ditto what I said about the breadboard.
Anything with two layers or more use a ground plane and have the cap(s) close to pin 20 with its earth-end via'd thru to the ground plane.
If your layout is done correctly, you have a Vss plane under the PIC, so a decoupling cap across the Vdd and Vss pins (19 & 20) is what you need. Depending on the value of the decoupling cap you might want some additional capacitance there too, like a 0.1uF and a .01uF. Of course read the datasheet to see if they have any specific decoupling recommendations and if so follow those. As Andy said if you have a single sided or no PCB you need a low inductance connection (fat trace or big wire) between the two Vss pins.
