[Note: in response to this story, Compaq sent LinuxDevices.com a letter to the editor summarizing the history and accomplishments of Compaq's sleep mode support for the StrongARM processor.]

Applied Data Systems (ADS) will demonstrate what is being described as the first “full sleep mode” support for Embedded Linux on Intel's StrongARM SA-1110 processor at LinuxWorld Expo in New York next week. ADS CEO Robert Olsen claims that “previous power management solutions for the SA-1110 were interim steps toward this more complete, 'full sleep mode' support.” The hardware platform being used in the demonstration is ADS' Graphics Master Plus StrongARM-based single board computer (SBC).

The new support, which reduces the SA-1110's power drain from half a watt (full on) to less than ten milliwatts, is of major potential significance to battery operated systems and handheld devices. One important beneficiary of the new power management support is likely to be the handhelds.org project, which has developed an Embedded Linux implementation for Compaq's iPAQ PDA. Additionally, the SA-1110 lies at the basis of numerous other Linux-based mobile or deeply embedded projects — environments where enhanced power management is always a plus.

“Idle” vs. “sleep”

ADS says previous power management schemes for Linux on StrongARM have been limited to the “on”, “off”, and “idle” states. According to ADS, the idle and sleep power management modes differ from each other as follows . . .

• Idle mode — a non-operational state initiated by the kernel due to a lack of system activity, where the CPU is running at a slower rate. Peripherals are typically unaffected. The system continues to process interrupts.

• Sleep mode — all system peripherals are powered off. Only the system's SDRAM memory, the processor's internal real time clock, and certain of the processor's I/O inputs remain powered on. This state is initiated by a call to a sleep function by the application or by an inactivity timeout. Device drivers are notified of transition to this state by a call to their respective power management functions.

In both of these modes, the kernel remains resident in RAM and can “wake up” in a timely fashion without having to be reloaded and uncompressed from non-volatile memory. However, the two modes have greatly differing power requirements due to the fact that nearly all subsystems are powered down in sleep mode but remain on (though idle) in idle mode.

Actual power comparisons

ADS has characterized the new sleep mode support on its Graphics Client Plus SBC, with the following results . . .

• Full on — the board consumed 2W of power, of which 0.5W was attributed to the StrongARM SA-1110

• Sleep mode — the entire board, including the StrongARM processor, required less than 10 milliwatts (0.01W) total power.

ADS will make the new StrongARM sleep mode support available to the Open Source community as soon as it is fully tested and has been incorporated into the company's SBC Embedded Linux support software, which is expected to occur by March, 2001.

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