
CHAPTER 14 USBU (UNIVERSAL SERIAL BUS UNIT)
User’s Manual U14579EJ2V0UM
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Figure 14-4. Low-Speed Device Cable and Resistor Connections
Full-speed or low-
speed USB
transceiver
Non-twisted unshielded cable
D+
D
R1
R2
R1
Host or hub port
D+
D
Low-speed USB
transceiver
Low-speed function
Maximum cable length is 3 meters
R1 = 15 k
±5%
R2 = 1.5 k
±5%
Data transfers are scheduled and managed by the host. Therefore, transfers are always initiated by the host. All
transfers consist of at most three packets (token, data, and handshake). The token packet sends information such as
the type and direction of processing, the address, and endpoint to the USB device. The USB device decodes the
address field and determines whether or not it is being accessed itself. A data packet transmits data in the data
transfer direction (either from the host to the device or from the device to the host) indicated by the token packet.
Finally, the receiving side returns a handshake packet to the transmitting side to indicate whether the transfer was
successful. The USB has the following four types of data transfers.
Interrupt transfers:
Small-scale data transfers used to convey information from a USB device to client
software. The HCD executes USB data transfers by issuing tokens to devices
periodically, with a period that satisfies device requests.
Isochronous transfers: Periodic data transfers having a fixed data transfer rate.
Control transfers:
Asynchronous data transfers, which are used to convey configuration, command, or
status information between client software and USB devices.
Bulk transfers:
Asynchronous data transfers, which are used to convey large quantities of information
between client software and USB devices.
With OpenHCI, the data transfer types are further divided into the two categories of periodic and asynchronous.
Interrupt and isochronous transfers are classified as periodic transfers, which are executed at a certain fixed period.
Control and bulk transfers are classified as asynchronous transfers, which are not executed periodically.
To implement this kind of operation, the system must be equipped with a device called the USB host controller and
software called the USB host controller driver (USBHCD). OpenHCI are specifications that define the relationship
between the host controller and HCD. The USBU unit is compliant with the Open HCI Specification Release 1.0 and
Open HCI Legacy Support Interface Specification Release Version 1.01.