20 FX3 Programmers Manual, Doc. # 001-64707 Rev. *C
Introduction to USB
Figure 2-8. ACK Transaction Packet
Data packets actually carry data. These are made up of two parts: a data header and the actual data.
The structure of a data packet is shown on the right.
Isochronous Time Stamp packets contain timestamps and are broadcast by the host to all active
devices. Devices use timestamps to synchronize with the host. These do not have any routing
information. The structure of an ITP is shown in Figure 2-10.
Figure 2-9. Example Data Packet
Figure 2-10. ITP Structure
OUT transfers are initiated by the host by sending a data packet on the downstream bus. The data
packet contains the device routing address and the endpoint number. If the transaction is not an
isochronous transaction, then, on receiving the data packet, the device launches an
acknowledgement packet, which also contains the next packet number in the sequence. This
process continues until all the packets are transmitted unless an endpoint responds with an error
during the transaction. In transfers are initiated by the host by sending an acknowledge packet to the
device containing the device, endpoint address and the number of packets that the host expects.
The device then starts sending the data packets to the host. The response from the host
acknowledges the previous transfer while initiating the next transfer from the device.
One important modification in the USB 3.0 specification is uni-casting in place of broadcasting.
Packets in USB 2.0 were broadcast to all devices. This necessitated every connected device to
decode the packet address to check if the packet was targeted at it. Devices had to wake up to any
USB activity regardless of its necessity in the transfer. This resulted in higher idle power. USB 3.0
packets (except ITP) are uni-casted to the target. Necessary routing information for hubs is built into
the packet.
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