USB System Architecture

Hubs

A major function associated with a hub is the distribution and control of cable power. Hubs may derive all power from the upstream cable or may include their own power supply for powering downstream ports. Hubs must also provide voltage regulation and ensure current is limited to downstream ports for safety considerations.

Hubs, like all USB devices, must include descriptors to specify their capabilities. A major portion of a hub’s descriptor definition relates to power-related issues.

Current Budget

A fully rated port must be able to provide five units of current (500ma) to the attached device. Self-powered hubs (including the root hub) having their own local power supply can provide the maximum rated power to each port. However, bus-powered hubs have only the bus power that they receive from the upstream cable to distribute to all of their USB ports. This can severely limit the amount of current that is available for USB devices that attach to bus-powered hub ports. The minimum current available at a port is 100ma.

Hubs specify whether they are self-powered as part of their configuration descriptor as shown in Table 4-1. The shaded area shows a bit-mapped “Attribute” field that defines the hub’s power implementation. Note that bit 7 is now reserved and must be set to 1. This setting in the 1.x environment indicated whether the device was bus powered.

If a device loses its external power source, it must not consume additional power from the bus to make up the deficit such that the bus power consumed exceeds the amount reported in the “MaxPower” field of the configuration descriptor. If the device cannot continue to operate with local power removed, it will no longer respond to accesses and software will be notified of the failure. USB system software may be able to detect that local power has been removed by checking device status.

76

Chapter 4: USB Cable Power Distribution

Table 4-1: Source of Hub Power Defined in Configuration Descriptor

77

USB System Architecture

Over-Current Protection

USB ports must be current limited due to safety regulations. No more than five amps of current can be supplied to a single port due to personal safety concerns. Current protection can be ganged to multiple ports or done on a per port basis as long as the current protection satisfies the 5a limit. Note also that a bus-pow- ered hub has only the power it receives from the cable to distribute to USB ports. In this instance, no current limiting is needed.

Voltage Drop Budget

Power may be supplied to USB peripheral devices via the cable. Voltage at a powered hub port can be no less than 4.75Vdc, while voltage at a bus-powered hub may be no lower than 4.40Vdc as illustrated in Figure 4-1. Consequently, USB devices must operate properly with as little as 4.40 volts at the upstream end of their cable.

Figure 4-1: Minimum Cable Voltage and Voltage Drop Budget

+RVW RU

3RZHUHG +XE

9 PLQ

%XV 3RZHUHG +XE

9 PLQ

%XV 3RZHUHG )XQFWLRQ

78

Chapter 4: USB Cable Power Distribution

Power Switching

Hubs apply power to ports in one of the following ways:

•Direct power — not switched

•Ganged switching to all ports in common

•Individual port power switching

The power switching supported by a given hub is specified within the hub’s endpoint zero descriptor. Table 4-2 on page 79 shows a portion of the hub’s endpoint descriptor. Note that data bits D0 and D1 at offset 3 define the power switching mode supported by the hub.

Table 4-2: Power Switching Mode Supported Is Defined by the Hub Class Descriptor

79