James Adams Talks About How Raspberry Pi Was Designed

I wrote recently about Eben Upton's presentation about the genesis of business-card-sized Raspberry Pi, and how they went from optimistically hoping to sell 10,000 to surpassing 10 million three months ago. If you don't know what Raspberry Pi is, then read The Amazing Raspberry Pi Story because in this article I'm going to assume you already do.

Raspberry Pi 3

Raspberry Pi has gone through several generations. The latest is called Raspberry Pi 3, released on February 29 of this year, and has WiFi and Bluetooth, Ethernet, 1GB RAM, a full HDMI port, (wired) Ethernet, and more. And, yes, it still has a 3.5mm audio jack, something maybe missing from your smartphone! The processor is a 1.2GHz 64-bit quad-core ARM^® Cortex^®-A53. It's just $35, although you will need some other stuff to get it up and running, depending on what you have lying around at home. It doesn't come with a power supply, keyboard, mouse, display, cables, nor an SD card to be its "disk". If you decide you want to try playing around with one, you'll be amazed how much stuff your local Goodwill store has for extremely low prices.

The Raspberrry Pi Foundation doesn't do their own chip design. The two chips on the board are a Broadcom BCM2837 system chip. Networking is provided by a SMSC LAN9514 chip (Standard Microsystems, now part of Microchip).

The credit-card-sized computer is on a six-layer circuit board with through-wires and impedance control. Given the foundation's goals, they want the boards to be low cost and designed right the first time. They also need to be easy to test. Another goal is reuse, being able to reuse functions from one design to another while keeping the board tiny. Their design cycle spans seven months up to a year. As James Adams, Chief Operating Officer there, put it, "We needed something we knew was going to be reliable, something that would scale and integrate with other design tools."

PCB Design Tools

They selected Cadence's OrCAD and Allegro PCB design tools. Since both families inter-operate, they can choose which tool to use based on the complexity of the design.

 Adams again: "Picking up the Cadence OrCAD tools is very easy. You can quickly make cost optimizations and spot errors in the design. And, most importantly, the OrCAD solution scales very easily to the Allegro environment.”

Everything is done in-house at Raspberry Pi, with a single engineer respon­sible for both schematic capture and board layout. This setup allows a greater degree of control from the engineer. Since the engineer knows in detail what to expect from the board layout when designing the schematic, the engineer can make adjustments to either the schematic and/or the board layout as layout progresses. For example, changes can be made to pinouts, packaging choices, and circuit structure to optimize the layout. In this case, having tools that seamlessly work together is very valuable.

One of the key benefits that the Raspberry Pi Foundation gained from using OrCAD and Allegro tools is the ability to meet its aggressive schedules even while designing more complex boards. Doing things manually would extend the cycle. For example, Adams noted that the copying feature in OrCAD Capture enabled the team to easily re-use a proven WiFi layout from one board on another. Done by hand, this would have taken many extra hours of work, plus the risk that things were not copied 100% exactly.

“It’s about productivity. It makes a big, big difference,” said Adams. “Now, we can concentrate on actual design work.”

Also beneficial is the high-speed signal routing/matching feature in OrCAD PCB Designer. Said Adams, “We have a lot of differential pairs, so that’s a critical part of the tool. The ability to take interfaces and extract all of the stuff required to do high-speed interfaces is also fairly easy. And, of course, there’s reliability—we’ve never seen the tool produce internal errors or do silly things.”

Adams final recommendation: it’s worth doing PCB CAD work in-house versus outsourcing it. “It’s not that difficult with the Cadence tools.," he said.

The Raspberry Pi Foundation is evaluating Cadence’s Sigrity tools, which integrate seamlessly with the Allegro tools, for in-house high-speed simulation of their designs’ memory inter­faces. The team also plans to use OrCAD CIS (component information system) for component data management. This should both increase their productivity and ensure correct designs that do not have lurking signal and power integrity problems.

The Results

More than 50,000 Raspberry Pi 3s are manufactured per week. You can buy one for $35 from Amazon or Fry's (or pretty much anywhere else).

More information on the Raspberry Pi Foundation. More information on OrCAD and Allegro PCB solutions. You want to try OrCAD out? You can download OrCAD Lite for free.

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