3-6 Intel® PXA255 Processor Developer’s Manual
Clocks and Power Manager

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 configur a tion bits are used when a module is not being
used. After Hardware Re set, any module that is n ot being used must have its clock disabled. If a
module is temporarily quiescent but does not have clock gating functionality, the CKEN re gister
can be used to disable the unit’s clock.
When a module’s clock is disabled, the regi st ers 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 differe ntly d ur ing op era tio n a nd has s p ecif ic entr y a nd e xit s equen ces .
The resets and power modes are:
Hardware Reset
Watchdog Reset
GPIO Reset
Run Mode
Turbo Mode
Idle Mode
Frequency Change Sequence
Sleep Mode

3.4.1 Hardware Reset

To invoke the Hardware Reset and reset all units in the processor to a known state, assert the
nRESET pin. Hardware Reset is only intended to be used for power up and c om plete resets.
3.4.1.1 Invoking Hardware Reset
Hardware Reset is invoked when the nRESET pin is pulled low by an external source. The
processor does not provide a method of masking or dis a bling the propagation of the external pin
value. When the nRESET pin is asserted, Hardware Reset is invoked, regardless of the mode of
operation. The nRESET_OUT pin is asserted when the nRESET pin is asserted. To enter Hardware
Table 3-3. 147.46 MHz Peripheral PLL Output Frequencies fo r 3.6864 MHz Crystal
Unit Name Nominal Frequency Actual Frequency
UARTs 14.746MHz 14.746MHz
AC97 12.288MHz 12.288MHz
I2S 146.76MHz 147.46MHz