FX3 Programmers Manual, Doc. # 001-64707 Rev. *C 15
Introduction to USB
Isochronous data is time-critical and used to stream data similar to audio and video. An isochronous
packet may contain up to 1023 bytes at full-speed, or up to 1024 bytes at high-speed. Time of
delivery is the most important requirement for isochronous data. In every USB frame, a certain
amount of USB bandwidth is allocated to isochronous transfers. To lighten the overhead,
isochronous transfers have no handshake and no retries; error detection is limited to a 16-bit CRC.
Control transfers configure and send commands to a device. Because they are so important, they
employ the most extensive USB error checking. The host reserves a portion of each USB frame for
control transfers.
2.1.3.4 Protocol Layer
The function of the protocol layer is to understand the type of transfer, create the necessary packet
IDs and headers, packet long data and generate CRCs, and pass them on to the link layer. Protocol
level decisions similar to packet retry are also handled in this layer.
All communication over USB happen in the form of packets. Every USB packet, consist of a Packet
ID (PID). These PIDs may fall into one of the four different categories and are listed here.
The PIDs shown in bold are additions that happened in the USB 2.0 specification.
Figure 2-1. USB Packets
A regular pay load data transfer requires at least three packets: Token, Data, and Ack. Figure 2-1
illustrates a USB OUT transfer. Host traffic is shown in solid shading, while device traffic is shown
cross-hatched. Packet 1 is an OUT token, indicated by the OUT PID. The OUT token signifies that
data from the host is about to be transmitted over the bus. Packet 2 contains data, as indicated by
the DATA1 PID. Packet 3 is a hand-shake packet, sent by the device using the ACK (acknowledge)
PID to signify to the host that the device received the data error-free. Continuing with Figure 2-1, a
second transaction begins with another OUT token 4, followed by more data 5, this time using the
DATA0 PID. Finally, the device again indicates success by transmitting the ACK PID in a handshake
packet 6.
SETUP tokens are unique to CONTROL transfers. They preface eight bytes of data from which the
peripheral decodes host device requests. At full-speed, start of frame (SOF) tokens occur once per
millisecond. At high speed, each frame contains eight SOF tokens, each denoting a 125-µs
microframe.
Four handshake PIDs indicate the status of a USB transfer: ACK (Acknowledge) means 'success';
the data is received error-free. NAK (Negative Acknowledge) means 'busy, try again.' It is tempting to
assume that NAK means 'error,' but it does not; a USB device indicates an error by not responding.
PID Type PID Name
Tok en IN , O UT, SOF, SE TU P
Data DATA0, DATA1, DATA2, MDATA
Handshake ACK, NAK, STALL, NYET
Special PRE, ERR, SPLIT, PING
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