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Raspberry Pi opts for microUSB power supply

HardwareNews
, 21 Oct 2011News

The Raspberry Pi $25 computer has received one of its final design tweaks prior to launch: a move to using a 5V microUSB connector for power input, in order to make the device as universally accessible as possible.

The Raspberry Pi project, founded by Eben Upton and David Braben, has a clear goal: to produce a low-cost, compact, ARM-based computing system that can be connected to a TV or monitor to teach children how to code.

Currently in a closed alpha programme, the hardware includes a powerful ARM-based processor, up to 256MB of RAM, and USB interfaces for peripheral devices.

Previous board designs have relied upon a coaxial power jack which accepts between six and 20V input, from which switch-mode power supplies on the board itself generate 5V and 3.3V rails while LDOs generate 2.5V and 1.8V rails.

It's a flexible design, but suffers from two key drawbacks: the extra components required add around $2 to the cost of the project - and on such a low-price device, that's around eight per cent of the overall cost - and there's no support for an input voltage of 5V.

"After a lot of experiments and spreadsheet work, we finally settled on an LDO-only power supply design, with a fixed 5V input, and the 1V2 core voltage generated directly from the input using the internal switch-mode supply on the BCM2835 die," writes Upton on the official project blog.

The 5V input will come courtesy of a microUSB jack on the board, which Upton explains was chosen due to the ubiquity of the connector thanks to the GSM Association's decision to pick microUSB as the universal charging standard.

There are a couple of caveats, however: despite being amazingly low power, the Raspberry Pi hardware draws more current than the standard USB specs allow. "Model B owners using networking and high-current USB peripherals will require a supply which can source 700mA," admits Upton. "Model A owners with powered USB devices will be able to get away with a much lower current capacity - 300mA feels like a reasonable safety margin."

With the USB specs stating a 500mA maximum, the Model B board isn't something that will run from a computer's USB port - but that's an unlikely usage scenario, given that the Raspberry Pi is a computer in and of itself.

With many USB phone and tablet chargers - including cheap aftermarket models - easily reaching 1A or more, the 700mA draw shouldn't be a problem for the project.

The Raspberry Pi team is still hoping for a launch towards the end of this year, but a firm date is not yet available.

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