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IXP400 Software
Access-Layer Components: USB Access (ixUSB) API
April 2005 IXP400 Software Version 2.0 Programmer’s Guide
298 Document Number: 252539, Revision: 007
Endpoint 0, by default, is used only to communicate control transactions to configure the UDC
after it is reset or hooked up (physically connected to an active USB host or hub). Endpoint 0’s
responsibilities include:
The USB protocol uses differential signaling between the two pins for half-duplex data
transmission. A 1.5-KΩ pull-up resistor is required to be connected to the USB cable’s D+ signal to
pull the UDC+ pin high when polarity for data transmission is needed.
Using differential signaling allows multiple states to be transmitted on the serial bus. These states
are combined to transmit data, as well as various bus conditions, including: idle, resume, start of
packet, end of packet, disconnect, connect, and reset.
USB transmissions are scheduled in 1-ms frames. A frame starts with a SOF (Start-Of-Frame)
packet and contains USB packets. All USB transmissions are regarded from the host’s point of
view: IN means towards the host and OUT means towards the device.
22.3.1 Packet Formats
USB supports four packet types:
A token packet is placed at the beginning of a frame, and is used to identify OUT, IN, SOF, and
SETUP transactions. OUT and IN frames are used to transfer data., SOF packets are used to time
isochronous transactions, and SETUP packets are used for control transfers to configure endpoints.
An IN, OUT and SETUP token packet consists of a sync, a PID, an address, an endpoint, and a
CRC5 field.
For OUT and SETUP transactions, the address and endpoint fields are used to select which UDC
endpoint is to receive the data, and for an IN transaction, which endpoint must transmit data. A
PRE (Preamble) PID precedes a low-speed (1.5 Mbps) USB transmission. The UDC supports full-
speed (12 Mbps) USB transfers only. PRE packets signifying low-speed devices are ignored as
well as the low-speed data transfer that follows.
A Start Of Frame (SOF) is a special type of token packet that is issued by the host at a nominal
interval of once every 1 ms +/- 0.0005 ms. SOF packets consist of a sync, a PID, a frame number
(which is incremented after each frame is transmitted), and a CRC5 field, as shown in Table 51.
The presence of SOF packets every 1ms prevents the UDC from going into suspend mode.
• Connection • Endpoint configuration • Disconnect
• Address assignment • Bus enumeration
• Token • Handshake
• Data • Special
Table 50. IN, OUT, and SETUP Token Packet Format
8 Bits 8 Bits 7 Bits 4 Bits 5 Bits
Sync PID Address Endpoint CRC5
Table 51. SOF Token Packet Format
8 Bits 8 Bits 11 Bits 5 Bits
Sync PID Frame Number CRC5