22 FX3 Programmers Manual, Doc. # 001-64707 Rev. *C
Introduction to USB
2.2.5 Power Management
USB 3.0 provides enhanced power management capabilities to address the needs of
battery-powered portable applications. Two "Idle" modes (denoted as U1 and U2) are defined in
addition to the "Suspend" mode (denoted as U3) of the USB 2.0 standard.
The U2 state provides higher power savings than U1 by allowing more analog circuitry (such as
clock generation circuits) to be quiesced. This results in a longer transition time from U2 to active
state. The Suspend state (U3) consumes the least power and again requires a longer time to wake
up the system.
The Idle modes may be entered due to inactivity on a downstream port for a programmable period of
time or may be initiated by the device, based on scheduling information received from the host. Such
information is indicated by the host to the device using the flags "Packet pending," "End of burst,"
and "Last packet." Based on these flags, the device may decide to enter an Idle mode without having
to wait for inactivity on the bus. When a link is in one of these Idle states, communication may take
place via low-frequency period signaling (LFPS), which consumes significantly lower power than
SuperSpeed signaling. In fact, the Idle mode can be exited with an LFPS transmission from either
the host or device.
The USB 3.0 standard also introduces the "Function Suspend" feature, which enables the power
management of the individual functions of a composite device. This provides the flexibility of
suspending certain functions of a composite device, while other functions remain active.
Additional power saving is achieved via a latency tolerance messaging (LTM) mechanism imple-
mented by USB 3.0. A device may inform the host of the maximum delay it can tolerate from the time
it reports an ERDY status to the time it receives a response. The host may factor in this latency toler-
ance to manage system power.
Thus, power efficiency is embedded into all levels of a USB 3.0 system, including the link layer, pro-
tocol layer, and PHY. A USB 3.0 system requires more power while active. But due to its higher data
rate and various power-efficiency features, it remains active for shorter periods. A SuperSpeed data
transfer could cost up to 50 percent less power than a hi-speed transfer. This is crucial to the battery
life of mobile handset devices such as cellular phones.
2.3 Reference Documents
Some of this chapter’s contents have been sourced from the following documents:
■ Universal Serial Bus 3.0 Specification, Revision 1.0
■ Universal Serial Bus Specification, Revision 2.0
■ On-The-Go Supplement to the USB 2.0 Specification, Revision 1.3
To Next Page
Loading...