
Public Version
PRCM Functional Description
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3.5.4.6
USBHOST/USBTLL Save-and-Restore Management
Both USBHOST and USBTLL support a save-and-restore mechanism. When the device enters into off
mode (that is, all power domains, except the WKUP power domain, are off), the user may want to save
the USBHOST context and restore it when exiting off mode. The save-and-restore mechanism for the HS
USB Host is enabled by setting the PRCM.
[4] SAVEANDRESTORE bit; for
the USBTLL, it is configured by the PRCM.
[4] SAVEANDRESTORE bit. The save
mechanism is initiated as the power domain transitions from active to off state (or to OSWR state for the
USBTLL), whereas the restore mechanism is initiated as the power domain transitions from off to active
power state.
shows the generic save-and-restore sequence applicable to both the USBHOST and the
USBTLL. The sequence follows:
1. When the PRCM module intends to initiate a sleep transition over the power domain (the USBHOST
power domain for the HS USB Host subsystem and the CORE power domain for the USBTLL module),
it sends a sleep request to the power domain. When the domain modules acknowledge the sleep
request, the functional and interface clocks to the modules are gated.
2. The PRCM module enables the SAR_FCLK functional clock and initiates the save sequence for the
module. SAR_FCLK is gated when the save sequence completes.
3. The PRCM module switches the power domain state to off power state. When a wake-up event occurs,
the PRCM module switches on the power domain.
4. The power domain local reset is asserted and then released by the reset manager.
5. The PRCM module enables the SAR_FCLK functional clock, and then initiates the restore sequence
for the module. SAR_FCLK is gated when the restore sequence completes.
6. The PRCM module enables the functional and interface clocks to the modules and releases the sleep
request. The modules acknowledge, and the wake-up sequence completes.
NOTE:
The PRCM sleep request and the module acknowledge signals are part of a hardware
communication interface to ensure the correct sleep and wake-up sequence.
shows the save-and-restore sequence. It does not provide the exact timing delays between
the switching of the signals and serves only to highlight the sequence of events.
366
Power, Reset, and Clock Management
SWPU177N – December 2009 – Revised November 2010
Copyright © 2009–2010, Texas Instruments Incorporated