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Slave Clock Control Unit
Copyright © 12/13/06 Marvell
CONFIDENTIAL
Doc. No. MV-TBD-00 Rev. A
December 13, 2006
Document Classification: Proprietary Information
Page 161
Not approved by Document Control. For review only.
7.2.7
Performing Peripheral Frequency Changes
Distinct from the core frequency changes discussed in
, the clock-control unit allows the
frequencies of select peripheral clocks on the chip to be changed on an individual basis. These frequencies are
changed by simply writing to the appropriate fields in the ACCR register. For most of the units, the procedure is
as simple as writing the fields and then checking the corresponding field in the ACSR status register to determine
if the operation has completed. The DDR memory controller, which is one of the units whose frequencies can be
changed in this fashion, requires more careful handling via software, both before and after the frequency change.
For details, consult the Dynamic Memory Controller chapter in the Vol. II: PXA300 Processor Developers
Manual: Memory Configuration.
In all cases, software must wait for the appropriate status field in ACSR to be updated before proceeding further;
otherwise, results are unpredictable. However, it is entirely legitimate for software to change more than one
frequency at a time, in which case all fields must be updated before proceeding further.
Warning:
When user changes frequency for two or more modules at same time, all frequencies have to
change in the same direction or stay the same
7.2.8
Changing PLL State
The ACCR register contains controls for enabling and disabling the core and system PLLs. Specifically, these are
ACCR[XPDIS] and ACCR[SPDIS]. They are meant for saving power when the clocks on-chip are being sourced
from the ring oscillator.As with the frequency change described in
, software must wait for the
corresponding status bits in the ACSR register to be updated before proceeding further; otherwise, results are
unpredictable. Refer to
for more information on entering D0CS mode and to
for
more info on exiting D0CS mode.
Note:
Software must not exit D0CS mode until the PLLs are locked and ready for use or the system
hangs. Refer to the ACSR[XPLCK] and ACSR[SPLCK] bits in
Subsystem Clock Status Register (ACSR)”
7.2.9
Core Idle Mode
Idle mode lets users stop the core clocks during periods of processor inactivity while continuing to monitor
interrupt service requests. The generation of all other clocks remains unchanged, so that when an interrupt to the
core occurs, the core is re-activated quickly at the point where it entered idle mode. Idle mode does not stop the
bus clocks and does not alter any of the peripheral clocks. When the core is in idle mode, all other on-chip
resources can be active. Before the core enters idle mode, enable any interrupts that can be used as idle-mode
wake-up events.
No power domain state changes are associated with C1 entry. All power supplies are unaffected by this mode.
Entry into the core idle mode is performed by setting the mode bits in the PWRMODE register to idle mode. The
following occurs when these bits are written:
•
All core activity is stopped, and all core interrupt requests to the core are held.
•
All core loads are completed, and core stores are sent to the core system bus.
•
The core clock is stopped. The core PLL remains enabled.