A USB 2.0 hub communicates with a USB 2.0 host at high speed unless a USB 1.x hub is between the host and hub. When a low- or full-speed device is attached to a USB 2.0 hub, the hub converts between speeds as needed. But speed conversion isn’t all a hub does to manage multiple speeds. High speed is 40× faster than full speed and 320× faster than low speed. It doesn’t make sense for the entire bus to wait while a hub exchanges low- or full-speed data with a device.
The solution is split transactions. A USB 2.0 host uses split transactions when
communicating with a low- or full-speed device on a high-speed bus. What would be a single transaction at low or full speed usually requires two types of split transactions: one or more start-split transactions to send information to the device and one or more complete-split transactions to receive information from the device. The exception is isochronous OUT transactions, which don’t use complete-split transactions because the device has nothing to send.
Split transactions require more transactions to complete a transfer but make better use of bus time because they minimize the time spent waiting for a lowor full-speed device to transfer data. The components responsible for performing split transactions are the USB 2.0 host controller and a USB 2.0 hub that has an upstream connection to a high-speed bus segment and a downstream connection to a low/full-speed bus segment. The transactions at the device are identical whether the host is using split transactions or not. At the host, device drivers and application software don’t have to know or care whether the host is using split transactions because the protocol is handled at a lower level. Chapter 15 has more about how the host and hubs manage split transactions.
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Tags: Is Split, The Solution, Transactions
