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PIC USB Interface
The USB spec defines several transfer types. Bulk, isochronous, interrupt, and control.
The PIC USB interfaces uses interrupt transfers, which provide low bandwidth, low latency, and high reliability.
“Interrupt” transfer is a misnomer, the USB host is the only device on the USB which may initiate a transfer. The device driver instructs the host to poll the PIC periodically to see if new data is available. If data is available the host receives it and then issues a real interrupt to the PC.