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Spinpoint M8U-Internal Product Manual REV 3.4
45
INSTALLATION
6.2.3 Power Distribution
This section describes the USB power distribution. Our Storage Device is Bus-powered hubs.
6.2.3.1 Overview
The power source and sink requirements of different device classes can be simplified with the introduction of
the concept of a unit load. A unit load is defined to be 100 mA. The number of unit loads a device can draw is
an absolute maximum, not an average over time. A device may be either low-power at one unit load or high
power, consuming up to five unit loads. All devices default to low-power. The transition to high-power is
under software control. It is the responsibility of software to ensure adequate power is available before
allowing devices to consume high-power.
Classes of Devices
The USB supports a range of power sourcing and power consuming agents; these include the following:
Root port hubs: Are directly attached to the USB Host Controller. Hub power is derived from the same
source as the Host Controller. Systems that obtain operating power externally, either AC or DC, must
supply at least five unit loads to each port. Such ports are called high-power ports. Battery-powered
systems may supply either one or five unit loads. Ports that can supply only one unit load are termed
lowpower ports.
Bus-powered hubs: Draw all of their power for any internal functions and downstream facing ports from
VBUS on the hub’s upstream facing port. Bus-powered hubs may only draw up to one unit load upon
power-up and five unit loads after configuration. The configuration power is split between allocations to the
hub, any non-removable functions and the external ports. External ports in a bus-powered hub can supply
only one unit load per port regardless of the current draw on the other ports of that hub. The hub must be
able to supply this port current when the hub is in the Active or Suspend state.
Self-powered hubs: Power for the internal functions and downstream facing ports does not come from
V
BUS. However, the USB interface of the hub may draw up to one unit load from VBUS on its upstream
facing port to allow the interface to function when the remainder of the hub is powered down. Hubs that
obtain operating power externally (from the USB) must supply five unit loads to each port. Battery powered
hubs may supply either one or five unit loads per port.
Low-power bus-powered functions: All power to these devices comes from VBUS. They may draw no
more than one unit load at any time.
High-power bus-powered functions: All power to these devices comes from V
BUS. They must draw no
more than one unit load upon power-up and may draw up to five unit loads after being configured.
Self-powered functions: May draw up to one unit load from V
BUS to allow the USB interface to function
when the remainder of the function is powered down. All other power comes from an external (to the USB)
source.
No device shall supply (source) current on V
BUS at its upstream facing port at any time. From VBUS on its
upstream facing port, a device may only draw (sink) current. They may not provide power to the pull-up
resistor on D+/D- unless V
BUS is present. When VBUS is removed, the device must remove power from the
D+/D- pull-up resistor within 10 seconds. On power-up, a device needs to ensure that its upstream facing port
is not driving the bus, so that the device is able to receive the reset signaling. Devices must also ensure that
the maximum operating current drawn by a device is one unit load, until configured. Any device that draws
power from the bus must be able to detect lack of activity on the bus, enter the Suspend state, and reduce its
current consumption from V
BUS.
6.2.3.2 Bus-powered Hubs
Bus-powered hub power requirements can be met with a power control circuit such as the one shown in
Figure 6-12. Bus-powered hubs often contain at least one non-removable function. Power is always available
to the hub’s controller, which permits host access to power management and other configuration registers
during the enumeration process. A non-removable function(s) may require that its power be switched, so that
upon power-up, the entire device (hub and non-removable functions) draws no more than one unit load.