Datasheet

VIPER / VIPER-Lite Embedded Linux Development Kit Quickstart Arcom Embedded Linux (AEL)

The current settings can be examined using the cpufreq-info utility:

# cpufreq-info

...

analyzing CPU 0:

...

hardware limits: 99.5MHz − 398MHz

available cpufreq governors: performance

current policy: frequency should be within 398MHz

and 398MHz. The governor "performance" may decide

which speed to use within this range.

current CPU frequency is 398MHz (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 / VIPER-Lite 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

If you intend to use the PXA255’s internal RTC as a wake up source, you must unload

the driver for the external RTC (which cannot perform the wake up function) and load

the driver for the internal RTC, as follows:

1 Remove the external DS1307 RTC driver:

# rmmod ds1307

2 Install the SA1100/PXA255 RTC driver:

# modprobe sa1100-rtc

3 Resynchronize the PXA255 RTC to the correct time from the system clock:

# hwclock –systohc

The source for the arcom-utils package (which is on the Development Kit CD, in the

/packages/arcom-utils folder) contains the script viper-sleep.sh, which is an example of

how to put a VIPER / VIPER-Lite board to sleep and wake it up again.