Texas Instruments OMAP5912 Reference Manual page 1402

USB Device Controller
3.3 Non-Isochronous, Non-Setup OUT (USB HOST -> MPU) Transactions
110 Universal Serial Bus (USB)
Non-isochronous, non-setup OUT transactions refer to USB transactions
where data is moved from the USB host to the MPU, the USB handshaking
protocols are in effect, and data transmission is ensured. These types of
transactions apply to all OUT transactions on bulk and interrupt endpoint
types, and to non-setup transactions on control endpoints.
Figure 4 shows the various USB protocol conditions that can occur during
non-isochronous, non-setup OUT transactions. The diagram shows the three
phases that can occur in an OUT transaction, the direction of information flow
for each phase, when endpoint interrupts are generated, and the resulting
STAT_FLG bits for the endpoint. The top three cases show the normal USB
handshaking modes: acknowledge (good data received), NAK (device not
ready to receive data), and STALL (device in a condition where the endpoint
cannot handle OUT transactions). The last case is an abnormal instance
where the token packet or the data packet was received with errors. The RX
FIFO only contains valid receive data under the first (acknowledge) case.
SPRU761A