Intel PXA255 manual UDC Device Requests, Interrupt Transaction Type, Interrupt Transaction Formats

USB Device Controller

To assemble control transfers, the host sends a control transaction to tell the UDC what type of control transfer is taking place (control read or control write), followed by one or more data transactions. The setup is the first stage of the control transfer. The device must respond with an ACK or no handshake (if the data is corrupted). The control transaction, by default, uses a DATA0 transfer and each subsequent data transaction toggles between DATA1 and DATA0 transfers. A control write to an endpoint uses OUT transactions. Control reads use IN transactions. The transfer direction is the opposite of the last data transaction. The transfer direction is used to report status and functions as a handshake. For a control write, the last transaction is an IN from the UDC to the host. For a control read, the last transaction is an OUT from the host to the UDC. The last data transaction always uses a DATA1 transfer, even if the previous transaction used DATA1.

12.3.5.4Interrupt Transaction Type

The host uses interrupt transactions to query the status of the device. Like bulk transactions, interrupt transactions begin with a setup packet, followed by an optional data packet, then a handshake packet. Interrupt transactions, by default, use DATA0 type transfers. Figure 12-10shows the four types of interrupt transactions.

Table 12-10. Interrupt Transaction Formats

Action	Token Packet	Data Packet	Handshake Packet
Host successfully received data from UDC	IN	DATA0	ACK
UDC temporarily unable to transmit data	IN	None	NAK
UDC endpoint needs host intervention	IN	None	STALL
Host detected PID, CRC, or bit stuff error	IN	DATA0	None
	Packets from UDC to host are boldface

12.3.6UDC Device Requests

The UDC uses its control, status, and data registers to control and monitor the transmit and receive FIFOs for endpoints 1 - 15. The host controls all other UDC configuration and status reporting using device requests that are sent as control transactions to endpoint 0 via the USB. Each setup packet to Endpoint 0 is 8 bytes long and specifies:

•Data transfer direction: host to device, device to host

•Data transfer type: standard, class, vendor

•Data recipient: device, interface, endpoint, other

•Number of bytes to transfer

•Index or offset

•Value: used to pass a variable-sized data parameter

•Device request

Table 12-11shows a summary of all device requests. Refer to the Universal Serial Bus Specification Revision 1.1 for a full description of host device requests.

12-8 Intel® PXA255 Processor Developer’s Manual