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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 155
Not approved by Document Control. For review only.
Set the T-bit to change the frequency of the core. When this bit is set and ACCR[XN] > 1, the core switches to
the turbo frequency. After the T-bit is set, it clears itself automatically. The processor uses the current
ACCR[XN] value each time the T-bit is set to calculate the turbo frequency (13 MHz * XL * XN). For example:
If XL = 16 for a run-mode frequency of 208 MHz (13 MHz * 16) and XN=2, setting the T-bit results in a
turbo-mode frequency of 416 MHz (13 MHz * 16 * 2). XN must be loaded with 1 and the T-bit set again to exit
the 416 MHz turbo mode frequency and return to 208 MHz (13 MHz * 16 * 1) core frequency. If the T-bit is set
with ACCR[XN] = 1, the resulting turbo-clock frequency is the same as the run-clock frequency.
The run-frequency clock defines the operating frequency of the core bus interface. Setting and clearing the T-bit
does not affect the run-frequency clock used by this block as it affects only the frequency of the clock used by the
core itself.
The penalty for entering and exiting turbo mode is an interruption in execution while current instructions and
loads are completed and stores are sent to memory. Interrupt requests to the core are held until the new frequency
is enacted, resulting in slightly longer and less predictable interrupt latency.
Because entering and exiting turbo mode does not stop the HSIO or LSIO bus clocks or the peripheral clocks,
additional steps are not required before switching frequencies with respect to any modules using these clocks.
7.2.2.2
Core Frequency Change
The core can change frequency by changing the core PLL frequency. The core PLL frequency can be adjusted by
writing XL and/or XN in the
“Application Subsystem Clock Configuration Register (ACCR)”
prior to writing
the F-bit in the “Application Core Clock Configuration Register (XCLKCFG)”. When the frequency change is
enacted, the core is switched to use the system PLL output clock selected by the XSPCLK bit in the
Subsystem Clock Configuration Register (ACCR)”
.
A short interruption in the core clocks occurs while the clock source is being changed. The core clocks remain on
while the core PLL is locking, and are switched back to the core PLL clocks after the frequency change sequence
has completed. Use the frequency change interrupt bits in the
“Application Subsystem Interrupt Control/Status
to determine when the clocks are switched back to the core PLL following a frequency
change.
If XSPCLK is set to 0b11, the core clock is halted while the core PLL is re-locked and is not re-enabled until the
core PLL has locked to the new frequency. In this case, the clocks are stopped during the entire core PLL lock
time, which results in a longer interruption of the clocks.
Setting the F-bit initiates the frequency-change operation in the “Application Core Clock Configuration Register
(XCLKCFG)” register. When this bit is set to 1, the values of XL and XN in the
Configuration Register (ACCR)”
are applied to the core PLL. If the values of XL and XN are not changed before
setting the F-bit in the “Application Core Clock Configuration Register (XCLKCFG)” register, the “before” and
“after” frequencies are identical, although the sequence goes through the same steps with the same latency.
Software may also set or clear other bits in the ACCR during the same same write operation. Upon exit of the
frequency-change sequence, the indicated frequency change is enacted as well, and updates the status bits in the
“Application Subsystem Clock Status Register (ACSR)”
appropriately.
Note:
A core frequency change may be executed automatically due to a high temperature Frequency
and Voltage change, if the PVCR[TVE] bit is set. Refer to the Services Power Management Unit
Chapter fo details.