7. Power-saving, Benchmarks And Battery Life

As we mentioned at the beginning of the article, all PXA27x processors support the new power-saving Wireless SpeedStep technology. It enables the CPU to automatically adjust both the clock speed and the supply voltage to the processing power required by whatever applications are running. In other words, should more computing power be needed, the CPU clock speed and voltage are automatically ramped up. If the processing workload should be reduced again, the voltage and processor performance are ramped down again. That means that the PXA270 inside the A730 can reduce its core speed from the maximum 520 MHz (system bus 208 MHz) to a mere 104 MHz (system bus 104 MHz), which then requires a supply voltage of 0.9 volts instead of 1.45 volts at maximum capacity. As a result, the power input plummets from 747 mW at full load to less than 64 mW in idle mode.

The theoretically possible low-power modes of the PXA270 CPU

But since a PXA27X processor is not just a simple CPU but rather a System on a Chip (SoC), other, non-essential functional features like the numerous interfaces and controllers can be shut off to save power.

System on a Chip (SoC): block diagram of the PXA270

If all of the energy-saving functions are fully utilized, the SoC uses only 0.1 mW in deep-sleep mode.

As is the case with the typical SpeedStep notebook, PDA users likewise have the option of either turning off the energy-saver or adjusting it to their requirements. This can be done under Settings | Power | Processor.

These settings let the user tweak the energy-saving function of the Mypal A730 processor

The Mypal A730 lets you set the CPU speed to turbo, standard, power-saving or automatic mode. Depending on the mode, the core speed is 520, 312 or 102 MHz.

In order to determine how fast the 520 MHz PXA270 Xscale CPU really is in practice, we brought in a PDA (Dell Axim X30) with a 312 MHz PXA270 CPU for comparison. We also thought a head-on comparison with the 400 MHz predecessor PXA255 would be interesting (Dell Axim X3). Remember: this processor supports neither wireless MMX commands nor Wireless SpeedStep to save energy. However, the CPU speed can be manually reduced to 200 MHz using software.