Open Source PIC.org

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USB

USB Device

Open Source USB Device Software Stacks
Microchip This is a solid stack for Device. They also have PC drivers. Compatible with PIC18, PIC24 and PIC32

USB Host

Open Source USB Host Software Stacks
Microchip This is a good stack for the Host but its a little big. Right now there are no other stacks out there. Good overall Stack. Compatible with the PIC24 and PIC32

USB On-The-Go

Open Source USB OTG Software Stacks
Microchip Microchip announced that this is coming soon. Will be compatible with the PIC24 and PIC32.