Product manual

Spinpoint M9TU-USB 3.0 Product Manual REV 1.0
37
USB INTERFACE AND USB COMMANDS
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 V
BUS 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 VBUS 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.
Power switching on any non-removable function may be implemented either by removing its power or by
shutting off the clock. Switching on the non-removable function is not required if the aggregate power drawn by
it and the Hub Controller is less than one unit load. However, as long as the hub port associated with the
function is in the Power-off state, the function must be logically reset and the device must appear to be not
connected. The total current drawn by a bus-powered device is the sum of the current to the Hub Controller, any
non-removable function(s), and the downstream
facing ports.