Intel PXA26X 12.3.4 Packet Formats

Intel® PXA26x Processor Family Developer’s Manual 12-5

Universal Serial Bus Device Controller

The PID is 1 byte wide and always follows the sync field. The first four bits contain an encoded

value that represents packet type (token, data, handshake, and special), packet format, and type of

error detection. The last four bits contain a check field that ensures the PID is transmitted without

errors. The check field is generated by performing a ones complem ent of the P ID. The UDC XO Rs

the PID and CRC fields and takes the action prescribed in the USB standard if the result does not

contain all ones, which indicates an error has occurred in transmission.

Use the address and endpoint fields to access the UDC’s 16 endpoin ts. The address field contains

seven bits and permits 127 unique devices to be placed on the USB. After the USB host signals a

reset, the UDC and all other devices are assigned the default address, zero. The host is th en

responsible for assigning a unique address to each device on the bus. Addresses are assigne d in the

enumeration process, one device at a time. After the host assigns the an address to the UDC, the

UDC only responds to transactions directed to that address. The addres s field follows the PID in

every packet transmitted.

When the UDC detects a packet that is addressed to it, it uses th e end point field t o det ermin e which

of the UDC’s endpoints is being addressed. The endpoint field contains four bits. Encodings for

endpoints 0 (0000b) through 15 (11 11b) are allowed. The endpoint field follows the address field.

The frame number is an 11-bit field incremented by the host each time a frame is transmitted.

When it reaches its maximum value, 2047 (0x7FF), its value rolls over. Frame number is

transmitted in the SOF packet, which the host outputs in 1ms intervals. Device controllers use the

frame number field to control isochronous transfers. Data fields transmit the packet data between

the host and the UDC. A data field consists of 0 to 1023 bytes. Each byte is transmitted least

significant bit first. The UDC generates an interrupt to indicate th at a start-of-frame event has

occurred.

CRC fields detect errors introduced during token and data packet transmi ssion, and are applied to

all the fields in the packet except the PID field. The PID contains its own 4-bit ones complement

check field for error detection. Token packets use a 5-bi t CRC (x5+x2+1) called CRC5 and data

packets use a 16-bit CRC (x16+x15+x2+1) called CRC16. For both CRCs, the checker resets to all

ones at the start of each packet.

12.3.4 Packet Formats

USB supports four packet types: token, data, handshake, and special. A PRE (preamble) PID

precedes a low-speed (1.5Mbps) USB transmission. The UDC supports high-speed (12Mbps)

USB transfers only. PRE packets that signify low-speed devices and the low-speed data transfer

that follows such PRE packets are ignored.

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 packets transfer data, SOF packets time isochronous

transactions, and SETUP packets are used for control transfers to configure endpo ints. A token

packet consists of a sync, a PID, an address, an endpoint, and a CRC5 field (see Table12-3). For

OUT and SETUP transactions, use the address and endpoint fields to select the UDC endpoint that

receives the data. For an IN transaction, use the address and endpoint fields to select the UDC

endpoint that transmits da t a .