Intel IXP400 Transaction Formats, Data Packet Format, Handshake Packet Format, Bits 023 Bytes

Intel® IXP400 Software

Access-Layer Components: USB Access (ixUSB) API

Data packets follow token packets, and are used to transmit data between the host and UDC. There are two types of data packets as specified by the PID: DATA0 and DATA1. These two types are used to provide a mechanism to guarantee data sequence synchronization between the transmitter and receiver across multiple transactions.

During the handshake phase, both communicate and agree which data token type to transmit first. For each subsequent packet transmitted, the data packet type is toggled (DATA0, DATA1, DATA0, and so on). A data packet consists of a sync, a PID, from 0 to 1,023 bytes of data, and a CRC16 field, as shown in Table 52. Note that the UDC supports a maximum of 8 bytes of data for an Interrupt IN data payload, a maximum of 64 bytes of data for a Bulk data payload, and a maximum of 256 bytes of data for an Isochronous data payload.

Table 52. Data Packet Format

8 Bits	8 Bits	0–1,023 Bytes	16 Bits
Sync	PID	Data	CRC16

Handshake packets consist of only a sync and a PID. Handshake packets do not contain a CRC because the PID contains its own check field. They are used to report data transaction status, including whether data was successfully received, flow control, and stall conditions. Only transactions that support flow control can return handshakes.

The three types of handshake packets are: ACK, NAK, and STALL.

•ACK — Indicates that a data packet was received without bit stuffing, CRC, or PID check errors.

•NAK — Indicates that the UDC was unable to accept data from the host, or it has no data to transmit.

•STALL — Indicates that the UDC is unable to transmit or receive data, and requires host intervention to clear the stall condition.

Bit stuffing, CRC, and PID errors are signaled by the receiving unit by omitting a handshake packet. Table 53 shows the format of a handshake packet.

Table 53. Handshake Packet Format

8 Bits	8 Bits
Sync	PID

22.3.2Transaction Formats

Packets are assembled into groups to form transactions. Four different transaction formats are used in the USB protocol. Each is specific to a particular endpoint type: bulk, control, interrupt, and isochronous. Endpoint 0, by default, is a control endpoint and receives only control transactions.

The host controller initiates all USB transactions, and transmission takes place between the host and UDC one direction at a time (half-duplex).

Bulk transactions guarantee error-free transmission of data between the host and UDC by using packet-error detection and retry. The host schedules bulk packets when there is available time on the bus. The three packet types used to construct bulk transactions are: token, data, and handshake.

Programmer’s Guide	IXP400 Software Version 2.0	April 2005
	Document Number: 252539, Revision: 007	299