A SPARE ATOM

Still no indications from Tilde.club that I've been granted an 
account there, and the web page did suggest it would only take 
about a day, almost four days ago. Am I being too patient again? 
Well I'll give it until Thursday this week and then try somewhere 
else. I probably won't get around to setting myself up there until 
next weekend now anyway.

Instead today I've been busy trying to get a new computer of my own 
to work. Long time readers (who've somehow managed to find this 
post wherever it ends up) might remember me writing about the 
Atomic Pi single-board-computer that I've been using as my 
"internet client" PC (I wrote a sort-of review in 
2020-12-06Me_and_My_Xcellent_Ideas.txt). That's been working great, 
except that if I ever make the mistake of powering it up without 
the SD card inserted the stupid UEFI BIOS needs me to connect a 
keyboard and monitor, then muck around trying to make it show the 
SD boot option in its settings again (give me a 'dumb' 
old-fashioned BIOS any day!). But that aside I've had no issues 
since setting it up.

My problems actually came in the form of a spare that I bought a 
few weeks ago. I've been looking for someone selling one in 
Australia for ages, to get around the high postage costs that all 
the US sellers charge (and actually there are fewer of them now 
too, as stocks are obviously depleting). For years I've had Ebay 
search alerts set (as it happens these seem to have stopped working 
as of about a week ago, Ebay seem to be working full time to annoy 
me lately), but I ended up finding this one on Gumtree (classified 
ads website). $50 + $10 postage was a bit more than I'd hoped to 
pay, but by now it's looking like I might soon have trouble even 
getting them from overseas if my first one breaks, and by now I've 
also discovered that Intel released an errata document describing 
how various parts on this generation of their Atom SoCs might wear 
out.
https://www.servethehome.com/another-atom-bomb-intel-e3800-bay-trail-atom-vli89-bug/

So I decided a ~30% cost saving, and without more international 
postage uncertainty, was good enough. Bought it, got it, dead. The 
description said that it was unused, but being a Gumtree thing it 
wasn't really refundable. I guess maybe it was unused because it 
never worked?

Anyway the problem was that it was drawing down the power supply to 
about 4.7v. These boards are known for being demanding on their 5v 
power source, but this was the same plugpack that's been powering 
my other one for over two years. The vague smell of very hot 
electronics also suggested a more drastic issue - something was 
shorting out.

As it happens I'm really using very few of the peripheral chips, so 
yesterday I had a go at it with the old trick of poking chips with 
a hot glue stick to see which one melts it (richer electronics guys 
may have traded this technique in for a thermal vision camera, but 
for me it's still better than a burnt finger). Conveniently it 
turned out to be U30, the 'XMOS' USB audio chip on the under-side 
of the board that feeds the onboard audio amplifier, one of those 
functions that I've never used.

Unlike certain Pis made of Raspberry, these Atomic creations have 
schematics available which actually do seem to show everything, so 
on the "AUDIO1" page I identified pin 23 as U30's only connection 
to 5V. After a fair bit of messing about because I'm really much 
better at through-hole electronics than this super-tiny 
surface-mount stuff, I got that pin lifted off the board, and sure 
enough when power was applied it now booted up to log-in screen of 
the Lubuntu OS that comes pre-installed on its onboard flash!

But just to be sure I gave the chip another fingering with the hot 
glue stick and... it melted again. OK there's also a 3V3 and 1V0 
supply to that chip, the latter unfortunately connected to a whole 
bunch of pins. I can't really desolder a 64pin surface-mount chip 
because I don't have a hot-air rework station, but the schematics 
showed that these power lines were only used by the audio chips, so 
I could just disconnect them entirely.

The 3V3 line (AU_P3V3) comes from U64, and AU_P1V0 from U39. U39 is 
hooked up so that its "EN" (enable?) input is pulled up by the 
AU_P3V3 voltage, so I actually only have to disable AU_P3V3. U64 
also has an "EN" input that's pulled up to 5V by a resistor, and 
it's right next to its GND pin. So just short those pins and no 
more AU_P3V3 or AU_P1V0. But it didn't work, U64 kept pumping out 
the power even with its "EN" pin grounded. Could "EN" on a voltage 
regulator chip really be for something else? Had the over-load 
damaged it so that now it's always on? Either way that wasn't going 
to work, but U64 was small enough that I could desolder it, so off 
it went. Without any voltage from U64, the "EN" input on U39 did 
behave itself and turn off the AU_P1V0 supply and the 
audio-chip-turned-heating-element was finally cut off.

Booting up the board again, I tested it booting to the 2GB SD card 
that my Internet Client system is installed to, and all worked 
fine. The Ethernet interface works, along with HDMI and USB. So 
that's all that I've used on the other board. Success! I've got 
myself a spare Atom.

Interestingly the Atomic Pi that I've been using doesn't show an 
audio interface in the lsusb output either, so maybe it's something 
that I'm doing wrong that's killing these chips, or was that part 
just very unreliable? Anyway it might be worth checking on my other 
board seeing as I did house it in a box made of card, which is 
potentially flammable...

 - The Free Thinker