Endpoint Type Selection

The different types of endpoint have different characteristics. Control transfers consist of messages or requests sent when the host dictates. Interrupt, bulk, and isochronous transfers are said to be stream pipes as they keep generating application defined data.

There is usually only one control pipe, for Endpoint 0. As well as carrying the USB set up and configuration standard transfers, you can use this pipe for the class-defined or your own vendor-defined requests. If need be, define any further control pipes.

Interrupt pipes are usually used for user input data, but they can also retrieve relatively infrequent data from the device. Interrupt pipes can be used in conjunction with isochronous pipes to provide feedback so that the data rate can be varied. As mentioned previously, a device can NAK an interrupt pipe request to indicate that no data has changed. Additionally, Chapter 23 shows that HID devices support the concept of an idle rate. This technique reduces the number of interrupt reports that are generated.

Devices that generate or consume data samples regularly will want to use isochronous transfers. Isochronous transfers can vary in length, and may be used in preference to bulk transfers as they reserve USB bus bandwidth. Isochronous transfers are not error checked, but a receiver can determine if a transmission error occurred by checking the frame number. Make sure that the application or device can cope with losing data. Bulk transfers are error checked, but they have the lowest priority on the bus.

Low-speed devices can only have two endpoints after Endpoint 0. Full-speed devices can have a maximum of 16 input and 16 output endpoints. Check that maximum packet size for your pipe type is acceptable.

Do not forget that if a pipe stalls, a control pipe request must be defined to clear the stall condition.

You might want to consider some of the new USB features, discussed earlier. Check that Windows supports the relevant feature.