Intel PXA26X - Manual (Page 76)

3-10 Intel® PXA26x Processor Family Developer’s Manual
Clocks and Power Manager
3.4.5.1 Entering Turbo Mode
The ratio between the run mode processor clock frequency and the turbo mode processor clock
frequency is programmed in CCCR[N]. The value in CCCR[N], and any other appropriate clock
configurations, must be programmed through the frequency change sequence. To simultaneously
change turbo mode and enter the frequency change sequence, use the steps to change the frequency
change sequence.
Turbo mode is invoked when software sets the TURBO bit in the Core Clock Configuration
Register (CCLKCFG) (See
Section 3.7.1 ). After software sets the TURBO bit, the CPU waits for
all instructions currently in the pipeline to complete. When the instructions are completed, the CPU
resumes operation at the higher turbo mode frequency.
Software can set or clear other bits in the CCLKCFG in the same write that sets the TURBO bit.
The other bits in the register take precedence over turbo mode, so, if another bit is set, that mode’s
sequence is followed before the CPU enters turbo mode. When the CPU exits the other mode, it
enters either run or turbo mode, based on the state of the CCLKCFG [TURBO] bit.
Do not confuse the CCLKCFG Register, which is in Coprocessor 14, with the CCCR (See
Section 3.6.1 ), which is in the processor’s clocks and power manager.
3.4.5.2 Behavior in Turbo Mode
The processor’s behavior in turbo mode is identical to its behavior in run mode, except that the
processor’s clock frequency relative to the memory and peripherals is increased by N, the value in
the CCCR (see
Section 3.6.1 ). Turbo mode is intended for use during peak processing, when there
are very few accesses to external memory. The higher core to external memory clock ratio
increases the relative delay for each external memory access. This increased delay lowers the
processor’s power efficiency. For optimum performance, software must load applications in the
caches in run mode and execute them in turbo mode.
3.4.5.3 Exiting Turbo Mode
To exit turbo mode, software clears the TURBO bit in the CCLKCFG Register. After software
clears the TURBO bit, the CPU waits for all instructions in the pipeline to complete. When the
instructions are completed, the CPU enters run mode.
Other bits in the CCLKCFG may be set or cleared in the write that clears CCLKCFG [TURBO].
All other bits in the register take precedence over turbo mode, so the new mode’s proper sequence
is followed.
Idle, sleep, frequency change sequence, and reset have precedence over turbo mode and cause the
processor to exit turbo mode. When the CPU exits of one of these modes, it enters either run or
turbo mode, based on the state of CCLKCFG [TURBO].
3.4.6 Idle Mode
Idle mode allows the user to stop the CPU core clock during periods of processor inactivity and
continue to monitor on- and off-chip interrupt service requests. Idle mode does not change clock
generation, so when an interrupt occurs the CPU is quickly reactivated in the state that preceded
idle mode.