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PXA300 Processor and PXA310 Processor
Vol. I: System and Timer Configuration Developers Manual
Doc. No. MV-TBD-00 Rev. A
CONFIDENTIAL
Copyright © 12/13/06 Marvell
Page 156
Document Classification: Proprietary Information
December 13, 2006
Not approved by Document Control. For review only.
Warning:
Writing different values to XL and XN bits in ACCR register must be followed by writing 1 to
F-bit in “Application Core Clock Configuration Register (XCLKCFG)” register.
7.2.3
System Phase-Locked Loop (624 MHz)
The system PLL creates the fixed-frequency clock used for generating the following:
•
High-speed and low-speed I/O bus
•
Peripheral units
•
Core clock when selected
The system PLL outputs a fixed 624-MHz clock. The system PLL may be enabled when the processor is in D0
power mode and is disabled in D1, D2, D3, and D4 power modes. During these low-power modes, the peripheral
clocks are then either derived from the ring oscillator or disabled.
Because many of the peripherals require specific fixed frequencies that can not be achieved exactly by the
system PLL output frequency, many of the clocks are derived to the closest possible frequency.
The selection of frequencies for most clocks is performed automatically by hardware in a given mode of
operation. For programmable clocks, such as the HSIO and DDR, the selection is performed by writing to the
appropriate fields in the
“Application Subsystem Clock Configuration Register (ACCR)”
.
7.2.4
Ring Oscillator (120 MHz ± 15%)
The ring oscillator outputs a nominal 120-MHz clock (±15%) and can be the clock source for several peripherals
(if enabled) in the following conditions:
•
The processor during power-mode transitions to D0 power mode
•
The processor clocks in D0 mode
•
The processor during S3 low-power reset exit
The 120MHz ring oscillator output passes through a divide by 2 circuit before going to any internal device or
peripheral block input clock. The 60MHz output from the divider circuit is the main clock source and frequency
of all the devices while in D0CS, making the actual ring oscillator frequency 60MHz. However, when the
processor is in D1 mode, the RO output goes through a divide by 3 circuit. Refer to
for details.
Note 1: The ring oscillator is enabled only when it is selected as a clock source for the processor. The
ring oscillator is not enabled when it is not selected. See
Section 7.2.4.2, “Ring Oscillator
for more information on selecting a clock source for the processor. See
and
for information and warnings associated with entering and
exiting D0CS (ring oscillator) mode.
Note 2: The output frequency of the ring oscillator is 120 MHz ± 15%. The output frequency is not
consistent (has a ± 15% tolerance) between different processor devices and cannot be used for
peripherals that require a stable, consistent clock source. The ring oscillator frequency drift for a
given PXA300 processor device is estimated to be a few percent and the clock-to-clock jitter is
also expected to be very small. Consequently, many peripherals do not function when running