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Archive for September, 2010

Monthly Aphorism: On Choosing Seafood

  • Thou shalt not eat siphon feeders, bottom feeders, things without eyes, or bugs

Someone famous once observed that being a gourmet consists almost entirely of being able to make approving noises and say “That was very good!” after eating a morsel that would cause ordinary folks to throw up.

The lab tech who coined that aphorism, obviously a man with an earthy sense of humor, also experimentally determined that the women he dated couldn’t tell the difference between fancy wine in ornate bottles and cheap wine in screw-top gallon jugs. So he kept a couple of ornate bottles around which he refilled as needed from the jugs. He simply pushed the corks back in and did a credible job of re-sealing the top with paper and wax.

We worked together on the IBM Video Disk project, then served time together in the East Fishkill Factory. Lost touch over the years and I think I just saw his obituary go by… sic transit, etc.

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Broken Tap Removal: The CNC Way

Having successfully drilled and tapped eight 4-40 holes for the MOSFETs and two 8-32 holes for the heatsink clamps, I needed four more holes for the 6-32 standoffs that will mount the heat spreader to the base. As is always the case, the tap broke in the next-to-last hole…

Broken tap

Broken tap

This is a three-flute tap, the break is recessed below the surface, and it looks like it’s cracked along one of the flutes. Bleh! I don’t have any tap extractors, mostly because I don’t do that much tapping, and I doubt the extractors work all that well on tiny taps.

I tried something I’d never done before: slit the top of the tap with an abrasive wheel and unscrew it. That didn’t work, of course, but it’s a useful trick to keep in mind. I think the tap was cracked lengthwise and, in any event, a three-flute tap doesn’t have the proper symmetry for a slot. Better luck with larger four-flute taps.

Slotted tap

Slotted tap

So I must dig the mumble thing out…

Starting the moat

Starting the moat

The overall plan:

  • Clamp the heat spreader to the Sherline tooling plate
  • Helix-mill a trench around the tap
  • Grab the stub with Vise-Grips
  • Unscrew it
  • Repair the damage

The clearance hole for a 6-32 screw is 0.1405 inch and that’s a 3/16-inch end mill: 70 + 93 = 163 mil radius, call it 0.170 inch. You really don’t want to kiss the tap flutes with the end mill, so you could make that the ID a bit larger.

Manual CNC, feeding commands into Axis and using the history list to chew downward 20 mils on each pass. With the origin in the middle of the broken tap and the cutter starting at (-0.170,0), the code looks like:

G2 I+0.170 Z=-0.020
G2 I+0.170 Z=-0.040
... and so on ...

About 3000 rpm and 2 inches per minute feed; the feed was too slow, because the aluminum chips were much too fine. I actually used cutting lube for this job: the heat spreader got nice and warm.

Coolant

Coolant

I stopped at Z=-0.100 and made a final pass around the bottom of the hole to clean out the ramp. Then, try unscrewing the tap…

Tap stub - first attempt

Tap stub - first attempt

Of course, the stub broke off more or less flush with the bottom of the hole, so I continued milling downward to Z=-0.260, a bit more than halfway through the plate. This time, the needle-nose Vise-Grips got a good grip on an uncracked section and the remains twisted out with very little effort.

Grabbing the stub

Grabbing the stub

Although the central pillar is outside the tap’s OD, leaving a solid aluminum shell, there’s not much meat to it. The shell broke off with the first twist and came out with the tap.

Those are not, by the way, gold-plated Vise-Grips. It’s a flash picture and the worklight is a warm-white compact fluorescent: the color correction that makes the aluminum look neutral gray turns the reflected CFL into gold.

Aligning replacement nuts

Aligning replacement nuts

I milled off the remains of the shell around the tapped hole, leaving a more-or-less flat bottom. If I cared enough, I’d machine a snug-fitting replacement aluminum plug, epoxy it into place, then (attempt to) drill-and-tap the hole again.

Instead, because the hole was deep enough for a pair of 6-32 nuts and a washer, I simply aligned those on a screw and filled the hole with JB Weld epoxy.

It doesn’t show in the picture, but the screw is well-lubricated with silicone grease to prevent it from becoming one with the nuts.

I eased epoxy into the recess, chasing out the inevitable air bubbles, and then scraped off most of the excess.

Epoxy fill

Epoxy fill

Let it cure overnight, scrub it on some sandpaper atop the sacrificial side of the surface plate, and it’s all good again…

Sanded flat

Sanded flat

The little finger of epoxy sticking out to the front fills the end of the slit I carved into the top of the tap, which is visible in the other pictures if you look closely. The area around the hole isn’t stained; that’s smooth epoxy.

Of course, the thermal conductivity of epoxy is a lot less than that of solid aluminum. I’m not really pushing the limits of TO-220 packages, so this kludge will work fine in this application. It’s also nice that the repair is on the bottom of the heat spreader, where nobody will ever know I screwed up…

Now, to return to the project at hand, with even more motivation to avoid tapping holes in the future!

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Arduino Mega: Showstopper Workaround

The discussion following that post gave me enough impetus to figure this out. What I have here is not a complete solution, but it seems to solve the immediate problem.

Downside: this will not survive the next regular system update that touches the gcc-avr package (yes, it’s the avr-gcc compiler and the gcc-avr package). Hence, I must write down the details so I can do it all over again…

To review:

The problem is that the avr-gcc cross-compiler produces incorrect code for Atmega1280-class chips with more than 64 KB of Flash space: a register isn’t saved-and-restored around a runtime routine that alters it. Simple sketches (seem to) run without problems, but sketches that instantiate objects crash unpredictably. Because Arduino sketches depend heavily on various objects (like, oh, the Serial routines), nontrivial sketches don’t work.

The workaround is to patch the library routine that invokes the constructors, as detailed in that gcc bug report, to push / pop r20 around the offending constructors. The patch tweaks two spots in the libgcc.S source file, which then gets built into an assortment of chip-specific libgcc.a files during the compile.

I was highly reluctant to do that, simply I’ve already installed the various gcc packages using pacman (the Arch Linux package manager) and really didn’t want to screw anything up by recompiling & reinstalling gcc from source. It’s certainly possible to update just the avr portion, but I don’t know exactly how to do that and doubt that I could get it right the first time… and the consequences of that catastrophe I don’t have time to deal with.

So I elected to build the avr cross-compiler from source, verify that the as-built libgcc.a file was identical to the failing one, apply the patch, recompile, then manually insert the modified file in the right spot(s) in my existing installation. This is less manly than doing everything automagically, but has a very, very limited downside: I can easily back out the changes.

Here’s how that went down…

The instructions there (see the GCC for the AVR target section) give the overview of what to do. The introduction says:

The default behaviour for most of these tools is to install every thing under the /usr/local directory. In order to keep the AVR tools separate from the base system, it is usually better to install everything into /usr/local/avr.

Arch Linux has the tools installed directly in /usr, not /usr/local or /usr/local/avr, so $PREFIX=/usr. Currently, they’re at version 4.5.1, which is typical for Arch: you always get the most recent upstream packages, warts and all.

Download the gcc-g++ (not gcc-c++ as in the directions) and gcc-core tarballs (from there or, better, the gnu mirrors) into, say, /tmp and unpack them. They’ll both unpack into /tmp/gcc-4.5.1, wherein you create and cd into obj-avr per the directions.

I opted to feed in the same parameters as the Arch Build System used while installing the original package, rather than what’s suggested in the directions. That’s found in this file:

/var/abs/community/gcc-avr/PKGBUILD

Which contains, among other useful things, this lump of command-line invocation:

../configure --disable-libssp \
               --disable-nls \
               --enable-languages=c,c++ \
               --infodir=/usr/share/info \
               --libdir=/usr/lib \
               --libexecdir=/usr/lib \
               --mandir=/usr/share/man \
               --prefix=/usr \
               --target=avr \
               --with-gnu-as \
               --with-gnu-ld \
               --with-as=/usr/bin/avr-as \
               --with-ld=/usr/bin/avr-ld

Yes, indeed, $PREFIX will wind up as /usr

Feeding that into ./configure produces the usual torrent of output, ending in success after a minute or two. Firing off the make step is good for 15+ minutes of diversion, even on an 11-BogoMIPS dual-core box. I didn’t attempt to fire up threads for both cores, although I believe that’s a simple option.

The existing compiler installation has several libgcc.a files, each apparently set for a specific avr chip:

[ed@shiitake tmp]$ find /usr/lib/gcc/avr/4.5.1/ -name libgcc.a
/usr/lib/gcc/avr/4.5.1/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr35/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr3/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr51/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr4/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr6/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr5/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr31/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr25/libgcc.a

The key to figuring out which of those files need tweaking lies there, which says (I think) that the Atmega1280 is an avr5 or avr51. Because I have an Arduino Mega that’s affected by this bug, I planned to tweak only these files:

/usr/lib/gcc/avr/4.5.1/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr51/libgcc.a
/usr/lib/gcc/avr/4.5.1/avr5/libgcc.a

I have no idea what the top-level file is used for, but … it seemed like a good idea.

Now, I innocently expected that the libgcc.a files for a 4.5.1 installation would match the freshly compiled files for a 4.5.1-from-source build, but that wasn’t the case. I don’t know what the difference might be; perhaps there’s an embedded path or timestamp or whatever that makes a difference?

The Arch Linux standard installation of gcc 4.5.1 has these files:

$ find /usr/lib/gcc/avr/4.5.1/ -iname libgcc.a -print0 | xargs -0 ls -l
-rw-r--r-- 1 root root 2251078 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr25/libgcc.a
-rw-r--r-- 1 root root 2256618 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr31/libgcc.a
-rw-r--r-- 1 root root 2252506 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr35/libgcc.a
-rw-r--r-- 1 root root 2256310 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr3/libgcc.a
-rw-r--r-- 1 root root 2250930 Sep  4 16:26 /usr/lib/gcc/avr/4.5.1/avr4/libgcc.a
-rw-r--r-- 1 root root 2251846 Sep 27 12:58 /usr/lib/gcc/avr/4.5.1/avr51/libgcc.a
-rw-r--r-- 1 root root 2251550 Sep 27 12:58 /usr/lib/gcc/avr/4.5.1/avr5/libgcc.a
-rw-r--r-- 1 root root 2252458 Sep  4 16:27 /usr/lib/gcc/avr/4.5.1/avr6/libgcc.a
-rw-r--r-- 1 root root 2251474 Sep 27 12:57 /usr/lib/gcc/avr/4.5.1/libgcc.a

The compilation-from-source using the gcc 4.5.1 tarballs has these files:

$ pwd
/tmp/gcc-4.5.1/obj-avr
$ find -iname libgcc.a -print0 | xargs -0 ls -l
-rw-r--r-- 1 ed ed 2250258 Sep 27 15:51 ./avr/avr25/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2255798 Sep 27 15:51 ./avr/avr31/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251686 Sep 27 15:51 ./avr/avr35/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2255490 Sep 27 15:51 ./avr/avr3/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2250110 Sep 27 15:51 ./avr/avr4/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251838 Sep 27 15:51 ./avr/avr51/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251550 Sep 27 15:51 ./avr/avr5/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251638 Sep 27 15:52 ./avr/avr6/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2251474 Sep 27 15:52 ./avr/libgcc/libgcc.a
-rw-r--r-- 1 ed ed 2250258 Sep 27 15:51 ./gcc/avr25/libgcc.a
-rw-r--r-- 1 ed ed 2255798 Sep 27 15:51 ./gcc/avr31/libgcc.a
-rw-r--r-- 1 ed ed 2251686 Sep 27 15:51 ./gcc/avr35/libgcc.a
-rw-r--r-- 1 ed ed 2255490 Sep 27 15:51 ./gcc/avr3/libgcc.a
-rw-r--r-- 1 ed ed 2250110 Sep 27 15:51 ./gcc/avr4/libgcc.a
-rw-r--r-- 1 ed ed 2251838 Sep 27 15:51 ./gcc/avr51/libgcc.a
-rw-r--r-- 1 ed ed 2251550 Sep 27 15:51 ./gcc/avr5/libgcc.a
-rw-r--r-- 1 ed ed 2251638 Sep 27 15:52 ./gcc/avr6/libgcc.a
-rw-r--r-- 1 ed ed 2251474 Sep 27 15:52 ./gcc/libgcc.a

The top-level files have the same size, but are not identical:

$ diff ./avr/libgcc/libgcc.a ./gcc/libgcc.a
Binary files ./avr/libgcc/libgcc.a and ./gcc/libgcc.a differ

Haven’t a clue what’s going on with different files in different spots, but I saved the existing files in the installed tree as *.base and copied the new ones from ./gcc/avr* into place. While there are many ways to crash a program, the AnalogInOutSerial demo program ran correctly on a Duemilanova (presumably with the existing libgcc.a) and failed on the Mega (with the recompiled libgcc.a). Save those files as *.rebuild just in case they come in handy.

Manually change the libgcc.S source file (it’s only four lines, I can do this), recompile, and the build process recompiles only the affected files; that’s comforting. Copy those into the installed tree and, lo and behold, the demo program now runs on both the Duemilanova and the Mega.

While it’s too soon to declare victory, the hardware bringup program I’m writing also works, so the initial signs are good.

Thanks to Mark Stanley for blasting me off dead center on this. I didn’t do a complete install, but he got me thinking how to make the least disruptive change…

And a tip o’ the cycling helmet to the whole Free Software collective for making a mid-flight patch like this both feasible and possible: Use The Source!

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Tour Easy: Underseat Pack Repair Finished

So, after a bit more than a year, I replaced the cracked backing plate in the other ERRC underseat pack on my Tour Easy. The first plate held up much better than I expected: hasn’t cracked or poked through the pack fabric.

This repair followed the same outline, including cutting off the ripped netting on the outside of the pack and marching the pack into the clothes washer for a spin with a few shop rags. Reassembled everything, put it back on the bike, and … the new aluminum extrusion across top¬† of the plate smacked firmly into the water bottle holder clamped to the rear of the seat frame for the amateur radio.

Underseat pack vs radio holder

Underseat pack vs radio holder

The extrusion is the lump running horizontally, just under the seat cushion. The corner of the pack extended rearward (left) of the water bottle holder’s black plastic body.

The original flexy plastic pack plate simply bent out of the way, but that’s not going to work now.

So I loosened the clamp, moved it a bit more to the right, and tightened it up again. I’d originally located it at the far right end of the straight part of the seat frame, so it’s now edging into the curved part that eventually forms the right side of the frame, but it’s good enough.

My shop assistant says she wants another water bottle holder for an actual water bottle on her bike. I say she should just go to the shop and make whatever she wants, then install it. Negotiations continue…

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Copper Tape for PCB Edge Binding

I’m laying out a PCB with ampere load currents, millivolt sense voltages, and PWM drive, all connected to an Arduino’s strictly digital ground layout through the usual headers. While I’ve laid the board out with the high-current stuff over there, the sense inputs here, and the PWM as far off in its own corner as possible, I fear this will get ugly.

One step to reducing the noise involves a decent ground system. The Arduino pretty much eliminates the whole single-point ground concept, so I’m using a double-sided ground plane with plenty of Z-wire stitching , plus copper tape around the edge binding the top and bottom planes.

Copper tape on PCB edge

Copper tape on PCB edge

The PCB is 60 mils thick, so I cut four copper foil strips about 3/16-inch wide, folded them around the board edges, then burnished the surfaces flat.

Although the tape has adhesive on one side which is allegedly conductive, I figured running a solder bead along the edges couldn’t possibly hurt. That worked out reasonably well, if you don’t mind blobular solder along the edge of your board.

Copper tape solder joint

Copper tape solder joint

The joint along the bottom edge shows one problem: some adhesive oozed out while soldering and formed a barrier. I think that happened along the tape edges from the outside of the roll, because it’s most prominent along two board edges.

Memo to Self: Slice off and discard the outer few millimeters. Mask the outer board edge for a solid pour, not a hatch.

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Sharpie as PCB Etch Resist

Because my hombrew circuit boards don’t have plated-through holes, I solder Z-wires from top to bottom. This entails little more than a solder blob around the wire on each side, but this time I wondered if having a slightly larger solid-copper area on each surface would be an improvement. Regrettably, I wondered this after masking the board.

Because I use an Ultra-fine-point Sharpie to touch up pinholes & suchlike, I decided to try it on larger areas by simply coloring in a few of the openings in the ground-plane grid.

Sharpie etch mask - Results 1

Sharpie etch mask - Results 1

Short answer: doesn’t work so well.

However, I’m using direct etching: rubbing ferric chloride on the masked PCB with a sponge. The abrasion probably wears the Sharpie ink off the surface and then the copper begins etching as usual. If I were doing this with normal agitation / aeration, perhaps a Sharpie mask would work better.

This is also 1-ounce copper, so there’s twice as much etching going on. Perhaps half-ounce copper would vanish fast enough that the Sharpie mask would remain effective.

A bit more detail, with another Z-wire pointed right at you.

Sharpie etch mask - Results 2

Sharpie etch mask - Results 2

The grid is 20-mil wide on 50-mil centers, with 25-mil isolation to other signals. The “via” holes use a 24-mil drill.

The row of dents just below the wire came from tiny openings in the mask that happen when Eagle poured the ground plane against the isolation surrounding the trace at the bottom. The toner-transfer resolution isn’t quite good enough to leave a clean opening and the etchant can’t quite reach the bottom to dig out the copper.

Memo to Self: Next time, try a 100-mil square pad around the via, centered on a grid intersection to fill in four adjacent openings.

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Arduino Mega: Showstopper!

I planned to use an Arduino Mega for an upcoming Circuit Cellar project, but … it doesn’t work. Well, it works, but under very limited circumstances.

The problem manifests itself as a complete crash / lockup under very straightforward conditions: attempting to use the serial output will suffice. This unmodified example sketch fails: AnalogInOutSerial.

After considerable Googling, there’s the showstopper: the gcc-avr compiler fails to save-and-restore a register that gets clobbered by the object constructors. Simple code doesn’t instantiate any objects, so it works fine. The serial failure is just a symptom, which means the various workarounds suggested in the forums don’t fix the general case.

The patch offered for gcc-avr is basically four lines (a pair of save / restores on R20), but requires recompiling what seems to be the entire AVR toolchain from source. That, alas, lies far beyond my capabilities… I could probably figure out enough to recompile it, but I’m very uncertain I could accomplish that without screwing up the main gcc compiler or the setup thereof.

It is not clear to me that the many claims of “it works on this version” are correct. From the nature of the problem, the failures depend critically on addresses occupied, final layout of the program / data in Flash, and (most likely) the execution path. The “working” configurations / systems may simply not fail using the sample programs.

This is on Arch Linux, for what it’s worth, with gcc-avr 4.5.1.

If anybody can walk me through the process of rebuilding whatever must be rebuilt, preferably in a safe place, perhaps I can manually stuff the new file(s) into the proper spots(s) to replace the incorrect ones…

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