Intel PXA26X - Manual (Page 72)

3-6 Intel® PXA26x Processor Family Developer’s Manual
Clocks and Power Manager
3.3.5 147.46-MHz Peripheral Phase Locked Loop
The 147.46-MHz PLL is the clock source for many of the peripheral blocks’ external interfaces.
These interfaces require: ~14.75 MHz for the UARTs, 12.288 MHz for the AC97, and variable
frequencies for I
2
S. The generated frequency may not exactly match the required frequency due to
the choice of crystal and the lack of a perfect least common multiple between the units. The chosen
frequencies keep each unit’s clock frequency within the unit’s clock tolerance. If a crystal other
than 3.6864 MHz is used, the clock frequencies to the peripheral blocks’ interfaces may not yield
the desired baud rates (or other protocol’s rate)
3.3.6 Clock Gating
The clocks manager contains the CKEN register. This register contains configuration bits that can
disable the clocks to individual units. The configuration bits are used when a module is not being
used. After a hardware reset, any module that is not being used must have its clock disabled. If a
module is temporarily quiescent but does not have clock gating functionality, use the CKEN
register to disable the unit’s clock.
When a module’s clock is disabled, the registers in that module are still readable and writable. The
AC97 is an exception and is completely inaccessible if the clock is disabled.
3.4 Resets and Power Modes
The clocks and power manager unit determines the processor’s resets, power sequences and power
modes. Each behaves differently during operation and has specific entry and exit sequences. The
resets and power modes are:
• Hardware reset
• Watchdog reset
• GPIO reset
• Run mode
• Turbo mode
• Idle mode
• Frequency change sequence
• Sleep mode
Table 3-3. 147.46-MHz Peripheral PLL Output Frequencies for 3.6864-MHz Crystal
Unit Name Nominal Frequency Actual Frequency
UARTs 14.746 MHz 14.746 MHz
AC97 12.288 MHz 12.288 MHz
I 2 S 146.76 MHz 147.46 MHz