Nerdy Texts of Analog and Embedded Systems Wizardry

Analog is sexy, we all agree, right? Embedded systems on the other hand, are full of lots of unglamorous problems. Filesystems, say. Inherently un-sexy.

But I think component variation is maybe the best, most un-sexy problem there ever was. The unsexy cherry on the diet sundae. Like, you HAVE to solve it if you are making lots of something or that thing as a population will suck, even though the one on your bench always ruled.

All the pots in WTPA2 are these custom Taiwan Alpha jobbies. There are two values, 10kA and 100kA. The VCO uses one of the 10kAs as a coarse control, and it sets the voltage into a current sink which in turn sets the frequency. I’d been messing with the op amps in this circuit to try and get some performance improvements and “all of a sudden” one of the DUTs didn’t work correctly. At first I figured it was the opamp change, but after a lot of measurement and desoldering and component testing, it turned out one of the 10k pots was really 11.4k. This was a greater than 10% variation!

I’d built a margin in for error, but this was above it, and the current sink was getting too high of a voltage. I tested a dozen pots or so from the bin, and all of them were much less off. Still, since one was off, probably another one could be as well. It could even have been a result of the soldering process. I actually bothered to do a DC simulation at this point (using qucs) and fiddled with the component values until they were all as off as I could imagine them possibly being, and then resized the scaling resistor that sets the upper range of the VCO. It was a really crappy annoying unsatisfying solution, because it means that MOST of the units will be operating at a slower maximum clock than they need to. But that one in twelve or one in 100 will work correctly. Serves me right for getting the cheap pots, but there you go. Margin. Component variation.

Least Sexy Problem Evar.

TB

Last week was hellish.
Srrsly, yo. I forgot how much work this is. I flew my buddy Nick out from Chicago to be in charge of kitting and assembly, and my job was to get the firmware rocking. We had from June 20 to June 24 to stuff 300 kits, 100 jack boards, 100 drilled and tapped enclosure kits, build and test 100 microSD daughterboards, build a dozen assembled units, and get ready for Bent and our Solid Sound panel talk with Moog!

Woes, take 1:
China called and were like, yo man, your main boards are gonna be late. I blame myself for letting it get so close to the wire, and to be fair they were totally sports about shipping the paste stencils and small boards early. Still, with no main boards, I would have nothing to show at the festivals. Eff that. So I called up Advanced Circuits and was like, hook a brother up in the meantime, and they were like BLING BLING. So, I got 27 “Limited Edition” green pcbs, and made some acrylic enclosures to match. Financially, it was retarded. But I have my pride.

Woes 2:
Joe at Prototope really nailed it cutting a ton of enclosures. T&T PlasticLand over by Canal also came through in the clutch with like 100 pounds of fluorescent acrylic with prices that McMaster can’t hang with. However, some dumbass specified that all these enclosures should be drilled and tapped, and those operations alone took DAYS, even with my fancy drill jig:

Here’s Nick hating life:

Woes 3:
That effing pulse shaper circuit (see the last couple posts) was wrong. Of course we didn’t figure this out until an hour before Bent. It was borderline such that it worked _a little_ even though the circuit had not changed since the prototype. The routing and components (though not the component values) had changed, and that was enough. Basically the LM358 had shitty rise times into whatever load the circuit presented, and the effective edge frequency (what the pulse shaper really looks at) was too low to work. I threw a handful of expensive TI opamps into some kits and dragged them out anyway, determined to have something to sell, but I only thought of this after Bent (but before we drove to North Adams for the festival). The new opamps slewed a lot faster and were an effective (if again, expensive, bandaid).

Woes 4:
The microSD card. I came up with new swears for these things:

Originally for this project I bought a crappy Kingston 2GB uSD card for testing from a pre-paid cellphone store near my house. FOR WHATEVER REASON, it turned out to be the fastest, most forgiving device ever. This week, on a whim, I ordered every crappy uSD card between 512MB and 2GB that I could find on Ebay. They all behaved differently. It took days to test my drivers to make sure that all the cards behaved correctly, and there are definitely exchanges in there that you have to do which have pretty much zero to do with the SD spec (or at least the free one). This sucked, to say nothing of then trying to make a filesystem and buffers to read audio in realtime. While card access was rock solid for all tested cards by Bent, I kinda though my sample read-write routines sucked. In the end I threw them out. The devices at Bent could format an SD to the WTPA filesystem (which is NOT FAT16, but a more real-timey system that I think makes more sense) and that’s about it.

Woes 5:
Driving to North Adams after Bent with a trunk full of expensive, lovely, VERY PROTOTYPE-EY WTPA2s was the worst experience ever. I’d been up for about 72 hours on about 4 total hours of sleep (none the night before) and I seriously saw animals that do not exist in this world. Anybody who can’t afford bad acid should try writing device drivers for three days while inhaling plastic fumes and then driving through a woods full of deer at midnight.

But then we got there, pounded a bunch of beers with our nerd friends, got pocket protectors from eminent wizard Cyril Lance of Moog and generally had a great time.

And, oh yeah, in the process we made THIS:

Bent looked like this:

Shop aftermathz:

WTPA2 is not ready to sell, but I have 300 of them and they’re pretty f’ing close. Expect to see the sales link by the end of July.
TB

Getting the PS2 data to playback correctly took some doing. The first problem was getting the controller synced to the PS2, which was necessary to handle the configuration streams. “Modern” Ps2 controllers can be set up in lots of different ways — as a standard digital controller or all flavors of analog. You can tell them (if you’re a PS2 or l33t haXX0r) to send stick data, analog button pressure, to configure the vibration motor in different ways, etc etc. So, when the PS2 does this, our doppleganger controller has to respond correctly. The big PITA there is that the PS2 will send any number of “don’t care” digital reads before this happens. In the terminal above I’m counting polls before we switch over to analog and begin a pre-determined configuration routine which is similar from game to game.

The next problem was buffer overflow. I used to use these fancy Atmel Dataflash AT45DB642D parts to record data, and they ruled. They had onboard RAM buffers and generally took care of erasing and writing flash memory at the same time. Then every distributor ran out of them. [2011-07-06T12:42:33+00:00 — they’ve been back for awhile now, thankfully] So I made a daughterboard and used a “peoples’ flash memory” IC by SST. It was a lot more like “normal” [read: annoying] flash in that you had to handle buffering and sector erasing yourself and it was painfully slow. This coupled with the fact that when this one bowling game used the analog button pressure (O GOD WHY?) the controller was spitting like 20 bytes a read led to trouble. Specifically, I needed to buffer all the data coming off the controller while the erase happened and that could take seconds. I ended up with a 3-way ping-ponged buffer AND compressing the data so that it would only log CHANGES in controller readings. This did the trick. I felt smug.

Here’s the pretty pretty result:

It’s official — I’m dropping cash on WTPA2. This is part of an order to my dudes at Taiwan Alpha for 1000 10kA pots, 1000 100kA pots, and 1000 very fancypants encoders. The cool thing about having the factory make this shiz for you is that you can get whatever specs you want on these guys, not just standard part numbers. All these components have long shafts to make it easier to project through a normal enclosure, and the encoders are both relatively high res and clickless. And most importantly, the pots cost like 20-25% of what they cost at Mouser!

Dealing with Taiwan Alpha has been good to me before. Shipping from China is a real bear, and there are always some weird exchanges, but generally they really want to make some business happen and this time they sent me some really complete spec sheets. Cross fingers!

A very exciting day! It’s like this:

…and like that:

and like this and uh:

These are the gerbers for the new WTPA2 and its Flash daughterboard, and the REAL THINGS fresh off the jet from Advanced Circuits! The new kind-of-questionable-but-kind-of-awesome through-hole SRAM got here also:

So these all can go together today!!! There’s a ton of new firmware that I’m still working on to support new features, so we may not hear any crusty samples, but I’m hopeful we’ll get to see some lights blinking at least before sundown. It’s da future!
Stay tuned,
xo
TB