Spectris Arcom VIPER Quick Start Manual page 24

VIPER RedBoot and AEL Quickstart
The cpufreq-set utility can be used to select a governor and to set the minimum and
maximum allowable frequency. This is illustrated by the following examples:
•
To select the performance governor and limit the maximum allowed speed to
200MHz:
# cpufreq-set --governor performance --max 199065
•
To select the powersave governor and set the minimum allowed frequency to
300MHz:
# modprobe cpufreq_powersave
# cpufreq-set –governor powersave --min 298598
•
To use the userspace governor and set a frequency of 400MHz:
# modprobe cpufreq_userspace
# cpufreq-set --governor userspace --freq 398131
Changing the clock frequency automatically regulates the core voltage in order
to maximize power savings.
The current settings can be examined using the cpufreq-info utility:
# cpufreq-info
...
analyzing CPU 0:
...
hardware limits: 99.5 MHz − 398 MHz
available cpufreq governors: performance
current policy: frequency should be within 398 MHz and 398 MHz.
current CPU frequency is 398 MHz (asserted by call to hardware).
Sleep and wake up support
The PXA255 processor supports a low power sleep mode in which the processor shuts
down entirely until some external or internal stimulus causes it to wake up. This
stimulus can be either an alarm generated by the processor's internal Real Time Clock
(RTC) or a change in the state of one of the processor's GPIO lines. For an explanation
of how to configure the wake up source and ensure that power consumption is reduced
to the minimum, see the VIPER Technical Manual.
Once a wake up source has been configured, the processor can be put into sleep mode
by writing to the special file /sys/power/state.
The following command, for example, would not appear to return until after the
processor has been woken up:
# echo –n "mem" > /sys/power/state
© 2005 Arcom Issue D
The governor "performance" may decide which speed to
use within this range.
Arcom Embedded Linux (AEL)
24