Picking a Cortex A8 processor for an embedded Linux board in 2010

ARM has several different processor families in production at present. The newest releases are the Cortex processors, which come in three series-- A, R, and M. (See what they did there?) The M processors are the weakest and cheapest, below $10 even in low quantities. The R processors, intended for real-time applications like anti-lock brakes in cars, are the next step up. The top of the heap are the Cortex A series. So far, the A8 and A9 have been released, and the A5 is scheduled for release in 2011. The A8 is the processor at the heart of some recent smartphones, like the Iphone 3GS, the Nexus One and the Droid, for example.

Of the A8 and A9, the A8 is the cheaper, slower one, running in the 500 MHz to 1 GHz range; the A9 can have multiple cores and run up to 2 GHz. To me, the A8 crosses the threshold where we can build embedded systems that connect to the internet, have decent performance without requiring tuning to make applications run fast, and have a price in the $100-200 range. There are certainly many processors that can handle internet traffic in that price zone, like every decent consumer-grade Ethernet switch built in the last 10 years, but those are machines optimized for niche tasks. What's new is that we're finally getting enough clearance above the minimum requirements that cheap, general-purpose systems can function on the internet.

So if you want to make an embedded device that uses a Cortex A8, what chips can you buy? ARM is an unusual company in that they don't produce chips themselves; they license their designs to manufacturers in return for royalties on each chip sold. Right now, ARM lists 7 public licensees of the A8 design; in addition to those listed, Alchip and Ziilabs are claiming to have licensed the design. Several of the licensees, such as Ziilabs and Broadcom, are targeting niche multimedia applications and will likely only sell to large manufacturers of stuff like DVD players and video cameras. Of the remaining companies, two have released general-purpose A8 processors: Texas Instruments and Freescale.

Freescale has released the i.MX5x series, with two subfamilies: i.MX50 and i.MX51. They cost $30-40 in low quantities but the sole distributor listed (Avnet) reports a lead time of 26 weeks for all parts.

TI has two series of A8 processors: OMAP3 and the not-quite-released-yet Sitara AM35xx series. The OMAP3 series has been around since 2008, and there are several embedded Linux boards (Gumstix Overo, Beagleboard based around the OMAP35xx series, though none have a Ethernet port in their stock configuration. The first Sitara, probably the AM3517, will likely be the cheapest of the lot, but assuming we want to limit ourselves to chips that we can actually order, that leaves us with four choices (before we consider packaging): OMAP3503, OMAP3515, OMAP3525, and OMAP3530.

The two higher-end OMAP35xx chips, the OMAP3525 and OMAP3530, include a TMS320 DSP onboard, which is not worth the cost in a general purpose tool. This leaves us with the OMAP3503 or OMAP3515. The major difference between the two is the PowerVR SGX graphics accelerator in the OMAP3515, which, like the DSP, isn't worth the cost. Until the AM3517 or AM3505 hit the distributors, I think the OMAP3503 is the winner. We'd have to choose between the three different packages the chip comes in, but Digikey only has the CBB package (a 515-pin ball-grid array, distinguished from the CBC package by the pin spacing of 0.40 mm rather than 0.50 mm).

In the words of Captain Stillman, "Load and fire the weapon, soldier!"

Estimating air changes per hour with a blower door

When I was trying to figure out how big a gas boiler we needed for our house a few months ago, I tried to account for both the insulation in our walls as well as the air leaks that let warm air escape as cold air sneaks in. I had read that an old, drafty house turns over its volume in air once per hour. That seemed high to me, but I was looking for a conservative estimate, so that's what I used in my calculations. Since then, I've been hoping to find a way to make a better estimate.

Solution: Colin's blower door

A friend of mine from Stanford, Colin Fay, runs a company with his dad, David Fay, called Energy Metrics. Last weekend, Colin and his nearly homonymic associate Cullen were kind enough to bring Colin's blower door over to our house to run a test to see how drafty our house is.

The basic idea of a blower door is this: you fill your front door with a curtain and a massive fan that forces air out of the house. While it's doing that, a small sensor measures the air pressure difference between the inside and outside of the house. There are a few different tests you can run, but the standard test that the fan controller runs is to automatically adjust the fan speed until the pressure inside is 50 Pa lower than outside. For comparison: 50 Pa is roughly equivalent to the pressure from a windspeed of 20 mph, but blowing at your house uniformly from all directions. Atmospheric pressure is around 100,000 Pa.

When the fan reaches a steady state, air is whistling in through all the gaps around your windows, doors and foundation, and you can tell where the problems are. For us, the largest draft was coming under the basement door. The next worst were the gaps between the sashes in our larger, older double-hung windows. In real life, I suspect that the gap under the basement door is not so bad-- the thermal gradient keeps the colder, denser air sunk down in the basement. I didn't realize it at the time, but most of the draft was probably coming down through our vestigial chimney.

Colin's blower door, a Retrotec 2000 with a DM-2 Mark II controller, pulled air through our house at 3900-4000 ft³ per minute to generate a pressure difference of 50 Pa. Our house has a volume of around 18000 ft³, so with the fan blowing, we were replacing all the air in our house every 4.5 minutes, or 13.3 times per hour.

Assembling the blower door frame

The blower door from the inside

Once you know how drafty your house is with a fan pulling the heavens through it, you need to scale that to match the typical conditions for your house. As a rough rule of thumb: just divide by 20. With the fan, we had 13.3 air changes per hour, so that's about 0.7 air changes per hour without the fan.

But if you want to ascend to the peak of Mount Energygeek, and you're willing to do it unashamedly, you can use the empirical corrections of Max Sherman of the Energy Performance of Buildings Group at Lawrence Berkeley National Lab, who completed his thesis on modeling building air infiltration in 1980, when oil rolled down like waters and righteousness like acid rain. You look up correction factors for climate (~18 for Boston), building height (0.7 for our house), wind shielding (1) and leakiness (1), multiply them together, and you've got a better correction factor than the rough guess of 20. For our house, we end up with 13.3/(18 x 0.7 x 1 x 1) = 1.06 air changes per hour.

With that knowledge, you can calculate the power required to offset the drafts cooling or heating your house. Our house, nominally a 1900 ft² Victorian, has an internal volume of 18000 ft³, or 510 m³, so when it's 0 C outside, we're heating about 1.06 x 510 m³ of air per hour by around 20 C. The heat capacity of air is around 1200 J/m³C. That means we need to pour in 1200 J/m³C x 540 m³ x 20 C every 3600 seconds. By my calculation, that's about 3.6 kW.

The conductive heat loss model I developed for our house a few months ago when we were installing the boiler predicts that the conductive heat loss at the same temperature will be around 18 kW, so we lose about 1/6th of our heat from air infiltration.

Colin suggested we could reduce our draftiness by around 2x before we'd have to worry about the effects of too little fresh air (farts, basically). He suggested picking up a tube of transparent silicone caulk in the fall to fill the gap between the window sashes, as that's where our worst leaks are. In the spring, when it's time to open the windows again, the silicone peels off.

After seeing fellow energy geek Holly's sexy basement windows last weekend, I think I might look into replacing those as well.

Vise resident

My brother was kind enough to give me a woodworking vise for my birthday. In related news, my dad gave me a beefy old vise from his basement the next day (which was Christmas). As a result, there are now two heavy-duty vises residing in our basement.

The vise from my brother arrived first, so I mounted it up first. My dad brought the other vise over a few days later; with the arrival of some bolts from McMaster-Carr, I now have enough clamping force for three men.

Thanks, family!

Counterbores to keep the bolt heads below the table surface

One vise mounted

Enough vises to conquer Afghanistan

The bench with both vises (and a pile of pallets turned into firewood)

Measuring insulation with an IR camera

I got my hands on a thermal imaging camera for a few hours recently and took a look around the house to see what I could learn. The camera detects infrared radiation, which is proportional to surface temperature. When you're inside a house in the winter, the poorly insulated bits look blue, because they're colder. From the outside, the poorly insulated parts look red, because they're hotter than the surroundings. (This is assuming you have the camera set to adjust the spectrum to cover the temperatures in the field of view. The camera I was using, a Flir i60, could either adjust automatically or stay fixed so you could compare temperatures across multiple pictures.)

I learned some interesting stuff. The gables of our attic appear to be completely uninsulated. There is one stud bay missing insulation next to our front stairs. I hadn't noticed it, but once I knew to check, I could feel the temperature change with my hand.

Our windows are generally our most poorly insulating surface, but in the picture below, you can see the panels in the front door are a close second. (The large box of fire is a radiator.)

I was thinking I should insulate the garage roof, but the thermal camera revealed that the door was leaking more heat.

I had noticed that our cats preferred lounging in certains parts of our kitchen floor, but I hadn't noticed the large cold stripe down the middle.

Here, you can see that there are two rooms we aren't heating-- their windows are blue.

Hard Times athletics

(See my previous post for the background to this post.)

There aren't many activities in our modern society where it makes economic sense for me to engage in the kind of physical labor that will keep me healthy, but there are a few. From what I can tell, they tend to cluster at the margins of fossil fuel consumption. I'll explain what I mean by that shortly.

The first good example I've come up with involves the woodstove we had installed recently. (Barry John Chimney did a great job, by the way.) We can buy a cord of wood for around $250, but I can collect pallets from around Somerville pretty easily as well. We get some pallets at work, but I can also pick up pallets around Davis Square; I guess they're a waste stream diffuse and intermittent enough that the only collectors are amateurs such as myself.

Before I can burn the pallets, I have to cut them up. I'm not strong enough to break them into 13" lengths without the help of steel, at least stone, tools. Cutting them up with only a handsaw is possible, but grueling. A better combination for casualties of the modern workplace like me is to use a circular saw with the blade set to cut just shy of the full depth of the cross planks. If you cut all the way through, the planks sag and bind the blade. Once the planks are 95% cut, you can stomp on them, and then cut up the remaining stringers wih a handsaw. Between the two types of sawing, the stomping, and the lugging of pallets, it's a fair bit of work.

The pattern I've noticed is that I can substitute labor for fossil fuels at margins of our consumption. Heating our house entirely with wood would take a lot of effort; I've spent enough time with a splitting maul (10 hours, maybe) to know that I don't want to do it all winter. But, dragging home some pallets and cutting them up piecemeal in the basement is pretty satisfying.

One of the other large fossil fuel sinks in our lives is commuting. When I worked out in Lexington, I commuted 22 miles a day on a bike, rain or shine, all winter long. But, as we've moved and I've changed jobs, I switched to biking 16 miles per day, then 5 miles on foot. (Last year, we moved just a few blocks from my office, so I had to take up running, but you won't hear me complaining about that.)

I've been casting about for a name for these activities, and Hard Times Athletics is the best I can do.

(If you called it Fake Athletics for the Hard Times, or FAHT, you could say, "Have you been FAHTing in the basement again? It smells terrible down there." In Boston, that's funny.)

Below are some snapshots of my Hard Times gymnasium. Further suggestions of new Hard Times Athletics events are welcome in the comments.

The input

The tools of transformation

The output