Thursday, September 1, 2011

Final Hardware Design

In early 2008 Atmel announced a whole new line of microcontrollers called XMEGA that sounded pretty interesting. Compared to the existing ATmega line, they offer much higher performance and much more capability, all at around the same price or less.

As a new product, availability is an issue. I originally placed an order when they first showed up in my distributor’s catalog in September 2008, and just let it ride on backorder. The order was eventually cancelled because that specific part (ATxmega64A4) was never produced. A year later I did get my hands on one of the top-of-the-line XMEGA chips, the ATxmega256A3. This was not the chip I would use for my final hardware design, but it did give me a chance to test it out.

The XMEGA family is only available in surface mount (SMD) packages, so I mounted mine onto a DIP adapter. Then I put it on my solderless breadboard in place of the ATmega324P, wired up to an SD memory card socket and serial port. By the end of September 2009 I had my “mini-DOS” FAT/SD test program working on it.

SD Card Breadboard with XMEGA. Parts include a small 3.3V regulator, reset button, and simple RS-232 serial interface just using a 74AC04 inverter.

Also in September I started work on the hardware design for the “production” WebCell, using the ATxmega128A3. I had never used either of the radio circuits on the engineering prototype, so I decided to leave them out. In the home automation application I envisioned, the WebCell would have to connect to some other device. I had experimented with Insteon products so I knew the WebCell could be hooked up to control their devices, for example.

In November I finished the layout of the WebCell PCB. Next I started design of the “Development Kit”. This is another PCB that the WebCell can plug into that makes it easy to try out. For example, it powers the WebCell from USB, provides a serial connector, and includes a reset button.

I also designed a variation of the WebCell that I call the FileCell. This variation removes the network interface and replaces it with a USB port and battery backup. So instead of being a web server, it is more of a general-purpose controller module. It could be particularly useful for data logging because of the high storage capacity available on the microSD card. I wasn’t sure if I would ever build this variation, but I wanted to explore how it would be different from the WebCell so I could make sure the Development Kit would work with both. Because of the battery backup, I did end up changing some of the interface connector pins on the WebCell.

I had to order samples of various parts to decide which ones I wanted to use and how they would fit. I finally placed orders for production prototype PCBs on January 21, 2010. I had been planning to get PCBs for all three designs, but in the end I saw no reason to build the FileCell variation yet.

The PCBs arrived on February 8. I took them and a kit of parts to my local electronics assembly company, Printed Circuits Assembly. Both boards use chips in leadless (QFN) packages, which are really hard to solder by hand. I got the assembled units back about a week later.

WebCell on DevKit. Blue jumper wires correct some PCB layout errors, and provide test points to internal circuitry. Long tails on board-to-board connector were useful as additional test points. Micro USB connector for power is in lower right corner.

It took a few weeks to get the software ready for the production prototype and to get the prototype working. The final hitch was an error in the wiring of the Ethernet jack, requiring cuts and jumpers of the PCB traces. Once that was fixed, on February 28, 2010, the prototype began serving up web pages.

During the wait for assembly, my work on the WebCell software started bumping up against the 64K program limit of the engineering prototype. The production prototype with its 128K program size came just in time to keep me working.

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