The $10 Raspberry Pi Zero W

Roger Thornton of the Raspberry Pi foundation talked about designing their latest computer, the Raspberry Pi Zero W, which is a tiny single-board computer even smaller than the original Raspberry Pi but it also has WiFi and Bluetooth...and instead of costing $35 costs just $10. The Raspberry Pi Foundation just sold their 13 millionth computer (they originally hoped to sell 10,000). A couple went up to the International Space Station so that some kids could have the experience of writing programs that ran in space. It just won the accolade of being the most successful British computer ever. The organization has just 15 people and only two of them are hardware engineers, one of which is Roger.

For background on Raspberry Pi, see The Amazing Raspberry Pi Story and James Adams Talks About How Raspberry Pi Was Designed.

The main chip in Raspberry Pi's computers comes from Broadcom, like Roger himself, who worked there before joining the Raspberry Pi Foundation. The board design was done using Cadence's Allegro PCB tools. 

It's not very big. But it is a real computer. It has an HDMI output for display, it is powered off the USB connection, and, as I already said, it has full WiFi and Bluetooth. It runs a Raspberry-flavored version of Linux (Raspbian), and can run applications like YouTube. However, at a $10 price point, people use it for all sorts of situations where it is complete overkill. For example, with two of them, you can build a wireless doorbell, with a duty cycle of a few seconds per week.

Here's a photo of it. I didn't have any raspberries to show the scale, so I had to use blueberries. The Broadcom chip is just below the middle, and the radio is just above it. There are interfaces for USB, HDMI, and a microSD card. The philosophy with all the Raspberry Pis (Pies?) is to make them usable with stuff that is lying around. Most people have an old SD card from an old phone, an old phone charger, and a TV with an HDMI interface. That's all you need to get it up and running.

The previous version of Raspberry Pi, the Zero, was on a same-sized board, but had no radios (and only cost $5). So designing this board had some severe constraints:

• Board size 30mm x 65mm
• Squeeze the WiFi and BT interfaces onto a board that was already busy
• Add an antenna
• Cost as low as possible, meaning a single-sided board
• Drilled vias, no blind vias, no laser vias
• 6-layer board
• Wide track width and pitch
• Same test points as the previous board to keep manufacturing costs down

The above picture shows the Raspberry Pi Zero ($5, no wireless) at the top and the Zero W ($10, with WiFi and BT) at the bottom. Perhaps the most obvious change is that there is no longer any room for a Raspberry Pi logo, but if you look closer you can see how things have been moved around and larger components replaced with smaller ones.

The result of these constraints was that every component except the connectors moved a little bit. Apart from the radio, the connectivity did not change, so a key feature of Allegro was that it was possible to move one (or a group) of components a few millimeters, and have all the routing get adjusted automatically. For example, to get the radio circuitry onto the board required the whole processor getting pushed to the left to make some space, and the power supply stuff moved out of the way and miniaturized, too. They took advantage of Allegro's subdrawings since the Raspberry Pi 3 (the big version with wireless) was "known good."

A six-layer board sounds like quite a lot but the top of the board is all components, so there is no room for routing. There are two internal power planes since the radios, in particular, need a good power deliver network (PDN). So there were two signal layers left. The track and gap width kept cost down but lost a lot of "space" just due to the size of the traces. Being restricted to drilled vias had two issues. The vias are relatively large. And every via requires space on every layer. There are a lot of traces, too, since there are two SDIO buses, four GPIO, HDMI, USB, CSI, and all the control signals. Another unusual requirement was aesthetic. Since Raspberry Pi sells bare PCBs, they need to look good, too.

The methodology was very iterative, each time a new way was found to remove a few cents of cost. In fact, the first prototype was two-sided, with the radio on the back, but they realized they might be able to get it all on a single-sided board. Roger said that the Zero W is a true testament to what can be achieved with powerful tools to design around the constraints. The iteration is not just at the layout level. Even though the schematic is relatively simple, they had to go over every component and throw out any that were not essential to make the board work. The Cadence tools really excel in iteration both at schematic and layout, and moving schematic changes into the layout.

To go with the product, there is a new case (which, ironically, costs $6, almost as much as the computer). They could export a DXF of the board and hand it to the case designer. There is also a separate camera board ($35 gasp!) which connects with a flex cable in the case. It is connected to a two-lane high-speed CSI (that's the MIPI camera standard interface). It was quite hard to route the differential pairs the normal way. Instead they routed the signals as best they could and then fixed the propagation delays at the end.

The antenna on the Zero W is built into the board and replaces the component one used on the Pi 3. That reduces cost but also the pick and place time. It is a resonant cavity designed by Proamp in Sweden. There is a ground cutout through all six layers that creates a cavity and the resonance is tuned to 2.4GHz. In fact, it performs so well that it is better than the chip antenna they were previously using. There is a blowup picture of the antenna to the right, and you can see it just below the Broadcom chip on the diagram up above.

There are no through hole components on the board, so the manufacturing is just pick and place on one side and a single reflow. Then reuse the Zero test jig. It is manufactured by Sony in Wales so it is "Made in UK". One goal of the design is to keep the time of touch down so it can be manufactured as fast as possible. The bigger Raspberry Pi is fully automated with robots, only touched manually to be put in the bag. It ships in volumes that justified buying the robots but the Zero W does not.

Roger offered a free computer for every question. There were a lot of questions. I already incorporated the Q&A in the post where most appropriate. His final comment was "Without Cadence we would not be able to achieve these impressive price points."

Europe: Roger will be giving what I assume is pretty much the same presentation at CDNLive EMEA in Munich on May 15-17.