Raspberry Pi gets 'Turbo Mode' support

September 20, 2012 // 11:16 a.m.

Tags: #arm #arm11 #armv6 #bcm2835 #broadcom #overclocking #overvolting #raspberry-pi

The Raspberry Pi Foundation has announced a new feature for its low-cost credit-card-sized ARM-powered microcomputer: overclocking up to 1GHz without voiding the warranty.

The Broadcom BCM2835 system-on-chip (SoC) processor found at the heart of the Raspberry Pi was originally designed for use in multimedia-centric applications like set-top boxes and smart TVs. The result is a design which pairs a massively powerful graphics processor with a relatively weedy CPU based on the outdated ARMv6 instruction set architecture.

To boost the Pi's performance in general-purpose computing tasks, some users choose to edit the config.txt file found on the Pi's SD card to boost the clockspeed above the stock 700MHz. The result, as our own testing proved, is a near-linear improvement in CPU-based performance - but one that is limited by the capabilities of the SoC itself.

During testing, we were able to get our Raspberry Pi to 900MHz, after which it would fail to boot. The config.txt file, which replaces the CMOS setup of a desktop or laptop PC, provides a helpful tool for fixing this: overvolting. Trouble is, increasing the voltage sets a 'sticky bit' in the processor and renders the warranty null and void.

At least, it used to. In efforts to appease those who find the performance of the sub-£30 Raspberry Pi disappointing, the Foundation has announced Turbo Mode - a software-driven dynamic overclocking system which includes integral overvoltage support without sacrificing the warranty.

Putting the SoC's performance under the control of the cpufreq driver - the same Linux daemon that provides control over dynamic clock speed on processors from Intel and AMD - the Turbo Mode, unlike editing config.txt, is able to modify the speed of the processor on the fly and includes safety valves like clocking the processor back down again should temperatures raise above the safe point of 85°C and only overvolting and overclocking when the system is under load.

The Turbo Mode is accessible using the raspi-config tool on the latest Raspbian Debian variant, and provides access to five pre-set overclocking modes. The highest of these increase the clock speed of the processor to 1GHz under load - something we were unable to achieve without overvolting - and combines with other performance-boosting tweaks to offer around 50 per cent better performance on CPU-driven tasks than the stock 700MHz clockspeed.

Additional changes to the Raspbian Linux distribution add out-of-the-box support for Wi-Fi dongles based on the RTL8188CUS chipset, boosted analogue audio quality and some additional software, along with a bugfixed USB driver that boosts overall system performance by around 10 per cent without overclocking.

Full details of the changes can be found on the official website. Sadly, there is still no news yet of accelerated graphics support for the GUI - one of the biggest performance-sappers for general-purpose usage.

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