Spectris Arcom VIPER Quick Start Manual page 22

VIPER RedBoot and AEL Quickstart
For more information, see the VIPER Technical Manual and PXA255 Developer's
Manual.
CPU clock frequency scaling
The PXA255 processor used on the VIPER can scale the core clock frequency from
100 to 400MHz, allowing an application to dynamically trade off performance against
power consumption. The functionality is exposed in Linux via special files in the /sys
system. These can be manipulated directly (e.g. using cat and echo), but are better
controlled using the cpufreq-info and cpufreq-set utilities. Both utilities are described
below.
The processor's clock and power manager, including the available Core Clock Control
Register (CCCR) multiplier configurations, are described in detail in the PXA255
Processor Developer's Manual, which is in the /references/ folder on the Development
Kit CD. The CCCR consists of 3 multipliers - L, M and N:
•
The L multiplier converts the basic 3.6864MHz crystal frequency into the memory
clock frequency. Typical values of L are 27, 36, and 45.
•
The M multiplier converts the memory clock frequency into the run-mode frequency.
Typical values of M are 1, 2 and 4.
•
The N multiplier converts the run-mode frequency into the turbo-mode frequency.
Typical values of N are 1.0, 1.5, 2.0 and 3.0.
Not all combinations of L, M and N are valid. Some valid combinations are
described below, and more information is available in the PXA255 Processor
Developer's Manual.
The Linux kernel divides the available clock configurations into two complementary sets
of available frequencies, corresponding to the run-mode and turbo-mode multipliers.
The first set is the 'performance' set. These use the run-mode multiplier (CCCR[M]) to
scale the clock frequency. Increasing the run-mode multiplier in this way gives the best
performance but at the cost of increased power consumption.
The second set of frequencies are the 'powersave' frequencies. These use the turbo-
mode multiplier (CCCR[N]) to scale the clock frequency. This uses less power than the
run-mode multiplier but with a performance cost (especially if frequent memory
accesses are required), since the core must wait for these while running in turbo mode.
The Linux kernel manages the current clock configuration using a software governor
combined with user-specified minimum and maximum desired clock frequencies.
In the default VIPER kernel three governors are provided:
•
The 'performance' governor selects the maximum allowed frequency from the
processor's available performance frequencies.
© 2005 Arcom Issue D
Arcom Embedded Linux (AEL)
22