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High-Speed USB Host Subsystem
22.2.4.2.4 VBUS Management and Emulations
In transceiver configurations, an actual USB cable is present, including an actual 5 V VBUS supply line.
On the other hand, in TLL configurations, the physical USB lines are emulated and have no physical
existence. This is especially true for the VBUS line, which distributes the 5-V power provided by the
default host (or A-device) to the entire bus. VBUS is also used for signaling purposes, and those features
must be emulated:
•
A peripheral detects the presence of a host by detecting the presence of VBUS.
•
USB OTG defines an elaborate voltage-sensing scheme to dynamically switch on and off VBUS (start
and stop sessions). In the context of TLL, this bring no power saving compared to simple suspend.
•
In particular, USB OTG uses VBUS as a wake-up source (VBUS-pulsing SRP) for the default
peripheral (or B-device).
For more information on how sideband controls are integrated, see
and
and the
related explanations.
22.2.4.2.4.1 VBUS Control and Status for Transceiver (Non-TLL) Configurations
In non-TLL modes, VBUS exists, and the problem is to propagate control and status to/from the actual
VBUS manager IC (typically the transceiver itself).
Only serial transceiver configurations are concerned in the case of the high-speed USB host subsystem in
the device.
22.2.4.2.4.1.1 VBUS Management in Serial Transceiver Configurations
VBUS management is not standardized in transceiver configurations. The chosen implementation is
described below. See also
•
VBUS control required for host and OTG operation (VBUS drive, VBUS pullup "charge", VBUS
pulldown "discharge") is assumed to be taken care of separately from the USBTLL module (that is, by
software and straight to the power IC, which can be the transceiver itself, especially in OTG cases).
•
VBUS status must be sampled by the appropriate hardware (again, most of the time the transceiver
itself) and reported by software to the USBTLL module, using the USBHOST.
DRVVBUS and CHRGVBUS bits, as indicated in
lists the values to write to the USBHOST.
register depending on the
VBUS status observed by the transceiver on the actual VBUS line. The same register fields are also used
in TLL configuration, and have been named according to that second configuration. In transceiver
configurations the fields actual signification is:
•
DRVVBUS: Set to 1 to report a VBUS level greater than VBUS valid.
•
CHRGVBUS: Set to 1 to report a VBUS level greater than Session valid
Table 22-47. VBUS Level Software Reporting for Serial Transceiver Configuration
VBUS Status
USBHOST.
[15]
DRVVBUS Bit
CHRGVBUS Bit
VBUS valid
1
1
Session valid (A/B)
0
1
Session not valid
0
0
Session end
0
0
22.2.4.2.4.2 VBUS Emulation for TLL Configurations
The TLL VBUS emulation sums up all actions on the VBUS line, obtains a voltage level, reported in the
VBUS status bits following the protocol. The level depends on the immediate VBUS actions and has no
memory of previous levels, whereas a real VBUS line behaves like an RC circuit and takes time to charge
and discharge. This causes the following differences:
•
The TLL level always jumps abruptly from session valid to session end (and back) with no transient
time in between (where session is neither valid nor ended) as in real life.
3277
SWPU177N – December 2009 – Revised November 2010
High-Speed USB Host Subsystem and High-Speed USB OTG Controller
Copyright © 2009–2010, Texas Instruments Incorporated
