USB request timeout

# The Setup Packet

Every USB device must respond to setup packets on the default pipe. The setup packets are used for detection and configuration of the device and carry out common functions such as setting the USB device’s address, requesting a device descriptor or checking the status of a endpoint.

A USB compliant Host expects all requests to be processed within a maximum period of 5 seconds. It also specifies stricter timing for specific requests :

Standard Device requests without a data stage must be completed in 50ms.
Standard Device requests with a data stage must start to return data 500ms after the request.
Each data packet must be sent within 500ms of the successful transmission of the previous packet.
The status stage must complete within 50ms after the transmission of the last data packet.
 

The SetAddress command (which contains a data phase) must process the command and return status within 50ms. The device then has 2ms to change address before the next request is sent.
These timeout periods are quite acceptable for even the slowest of devices, but can be a restriction during debugging. 50mS doesn't provide for many debugging characters to be sent at 9600bps on an asynchronous serial port or for a In Circuit Debugger/Emulator to single step or to break execution to examine the internal Registers. As a result, USB requires some different debugging methods to that of other microcontroller projects.

Casually reading through the XP DDK, one may note the Host Controller Driver now has a USBUSER_OP_SEND_ONE_PACKET command which is commented to read "This API is used to implement the 'single step' USB transaction development tool." While such a tool has not been released yet, we can only hope to see one soon.

Each request starts with a 8 byte long Setup Packet which has the following format,