October 2013 – Bitser

Nordson EFD Solder Paste

A very nice treat from the PCB Carolina show last week was this set of samples from Nordson EFD. With Freescale FXOS8700CQ magnetometer/accellerometer chips and a Sparkfun Heaterizer XL-3000 on the way I need to get with it, get a PC board design off to OSH Park and prepare to use small SMD components. The Heaterizer is a stop gap while my hot air tool budget catches up with desire, and I’m hoping to get some lessons from a friend with professional equipment. But with this top quality solder paste I think I’m approaching critical mass. Thanks to John Conti of Nordson EFD for the technical info and support for my modest goals.

Even if you’re not interested in the Sparkfun $10 hot air tool you might find the instruction manual fun to read. But seriously, folks, this surface mount tutorial at the Sparkfun site makes it clear that individuals inexpensive tools can join in the fun.

UPDATE FROM 2021: The SparkFun Heaterizer was incapable of surface mount soldering. I ended up using a cheapo Chinese oven until I could modify a convection oven with interior hi temp reflective material and thermocouple/microprocessor control for proper heating/cooling profiles to do surface mount soldering. The oven is inside a decent fume hood with exhaust air out a flue for safety and to avoid any odor problems. The proper hot air tool came along too, but I still find it best to apply paste with a stainless steel stencil from OSH Stencils and use a reflow oven for professional level results, with the hot air tool for rework or the occasional expedient operation. Finally, the FXOS8700CQ is EOL and last possible purchases will be at the end of 2021, so time to find a replacement. A client and I are investigating the ST LSM303AG as an alternative. It isn’t lacks the capability to interrupt on magnetic vector magnitude events but the price is right!

One Sneeze Equals Hours of Picking Up

This is a little plastic jar containing size “402” surface mount resistors. These resistors look like they belong in a pepper shaker. They were at a booth at PCB Carolina last week and there was a contest to guess how many resistors were in the jar. I guessed 10k, and that was good for a top ten finish, but the correct number was 10,863.

Wally Bot sweeps competition at Efland-Cheeks Elementary Robot Camp

Today I mentored Trey and Tyler, two elementary school children in a “robot camp.” They built a robot with Lego Mindstorm NXT 2.0 components and it swept the competition and then came out on top in a free for all battle between all seven robots. The robots fought “sumo style” within a roughly four foot diameter circle until all but one robot was forced outside the circle or disabled.Trey drew Wally’s face (his eyes are an ultrasonic rangefinder). The front ramp/scoop was very stiff and tended to make Wally’s wheels really dig in, frequently flipping the other robot if it could catch it from the side. In the final round of the main competition Wally and his opponent were locked together for almost the full two minutes (that would have made a tie), but Wally could push the other robot forward in a slowly wandering ellipse that eventually passed outside the ring, tricking the other robot into backing up (which put it completely outside the ring). Very exciting!

Right Thoracotomy Epicardial Ablation

Epicardial ablation is the technical term for the surgical procedure I had done at UNC Medical Center on September 19th. (A thoracotomy is just “cutting into the pleural space of the chest”) This was done by Dr Andy Kiser, a professor of surgery at the University and a pioneer of this technique for treating atrial fibrillation. It’s going to take a good while to properly account for everything that happened to me in conjunction with this procedure, but three weeks on I’m in normal sinus rhythm 90% of the time and the trend is definitely in the direction of further improvement. (This sinus rhythm is at least partly due to the antiarrythmic drug Tikosyn currently) Contrasting this state with the “24/7” afib I dealt with before the procedure is difficult, but is best shared by saying I was flabbergasted to realize that the feelings of well being that flooded over me in the minutes after I went into normal rhythm are called “normal” in other people. I’d forgotten what normal felt like, but I like it, and I’m in still in a state of pure celebration of living. The doubts about the future that have dogged me since I approached the age at which my grandfather died of heart ailments (57) has vanished, hopefully for at least thirty years!

The paper linked to below has a detailed description of the procedure written by doctors Kiser and Mounsey and an associate. This paper describes an initial approach to “convergent” treatment combining epicardial and endocardial ablation in a single session: a more recent refinement that applies to me involves a six to eight week gap between the epicardial and endocardial parts that enables the endocardial phase to be more effective. My endocardial procedure is scheduled for November 12th and there’s every reason to believe this will be a piece of cake compared to the surgical procedure. I confess to being very, VERY glad that I avoided reading these and other details before I had the surgery. Doctor Kiser’s 100+ epicardial procedure experience in combination with Doctor Mounsey’s 1300+ endocardial procedure experience allowed me to be confident and comfortable with going ahead with it, albeit mostly in an “ignorance is bliss” state in September. I think my subconscious was smart enough to know that foreknowledge of what was going to happen and what might happen would have only had a down side. Finally, “something” did happen to me such that, like the patient described in this paper, I required a dopamine infusion. I only jealized this was significant earlier today (i.e. I recall mention of dopamine being discontinued sometime after the surgery, but didn’t appreciate what use of that drug implied). I don’t know yet if I needed this for hypotension/bradycardia or for something else or as some kind of prophylactic.

If you’re considering having this procedure done my advice is to stop here, but otherwise, here are the details. WARNING: not for the squeamish.

http://www.eplabdigest.com/articles/Collaborative-Convergent-Epicardial-and-Endocardial-Atrial-Fibrillation-Ablation

OK, if you gotten this far you aren’t too squeamish, but here’s a bigger challenge.

This is a “central line”, the rightmost few inches of which sit inside the heart, allowing injection of drugs with fastest possible effect. This is the central line that was inside me for four days. By great good fortune, despite multiple nurses and doctors trying to explain the geometric reality of this gadget while I was in the hospital, I didn’t “get it” until a nurse pulled it out of me and I could see it as it is here. Pulling it out didn’t hurt. How on earth did they prevent internal bleeding with nothing more than the very firm pressure the nurse held on my chest for a few minutes? I don’t know, and I don’t want to know!

Trouble in Arduino Paradise

(This is another article I wrote a year or more ago but never got around to publishing until now)

I’ve been helping an ecologist make a “compass bearing data logger” using an Arduino Uno. Actually, I’ve been doing most of the implementation while Erik has defined the requirements based on his many years doing field work with other logging tools. (It is pure joy to have crisp requirements so you know your solution happens to match the problem at hand!) Erik picked the Uno and it seemed like an excellent choice because it was very easy for him to combine bits and pieces from Adafruit to create a solution. We quickly became aware of off the shelf software libraries that either come with the Arduino IDE or that are available as add-ons from the main Arduino repositories to support the hardware. At the start of the project it seemed unimaginable to exhaust the Uno’s memory capacity with such a simple application. Does that sound familiar?

After getting about 98% of the functionality in place the IDE still reported only about 24 kilobytes of text usage along side the Uno’s 32 kilobyte capacity figure. But after the addition of the last hundred or so lines of C++ the system became unstable. It wasn’t unstable in the usual sense that the new code didn’t work right the first time. (My batting average up to that point had been excellent, but there had been a number of surprises.) The system was unstable in the sense that only the first sliver of initialization code was executing, but it was executing over and over forever. The CPU was reseting after just a little bit of the application code had executed.

When I was much younger I might have thrashed with this a long time, struggling to determine what broken code fragment I’d added somehow explained the failure. Instead I got out the machete and gutted the bodies of several functions until the overall code size was similar to what it had been the last time the system had run properly. Sure enough, it ran properly again. Replacing stub code with full function bodies in various combinations proved that it was simply the amount of code involved that caused the instability. I should point out that this program has very little “variable” storage in relation to the Uno’s 2kb of RAM. That is, it has maybe a dozen scalar variables, one small character array for building file pathnames, and a couple of objects to do with the clock/calendar and compass chips and the SPI interface to the SD card used for the actual data logging. Also, there are no recursive routines and very few local variables and very shallow call nesting, so stack demands are trivial too. In short, the bad magic was to do with undiagnosed overflow of “something” to do with the amount of text (machine instructions produced by the C++ compiler).

Except that C semantics require initialized string constants to be put into the data segment, and this has to be mutable, and therefore in RAM vs flash memory. Duh. So I was overflowing RAM, causing the stack to walk over the top of variable storage as it nested during routine calls.

The trouble in paradise is that in my world it’s just not acceptable that overflow of a statically allocated memory segment would go unnoticed by the tool chain. In my world this kind of misbehavior forces the Arduino IDE into the “piece of sh*t” bucket and I’m only persevering with this tool chain now for the sake of Erik’s target user group being able to make this logger with user-friendly tools. The Arduino IDE is fantastically user-friendly for making an Arduino blink LEDs. Going much beyond that in my experience has given appreciation for the “get what you paid for” adage.

But the other trouble is that it appears that some combination of Linux, the USB library “RXTX”, and the Arduino IDE are conspiring to ruin my system’s uptime record. If I had a nickle for every time a failure to do with the USB connection between the IDE and my Uno has forced a reboot I could buy another several TI MSP430 Launchpads. More on this here.

Hand soldering TSSOP packages with MSP430G2955 example

While researching an alternative to the Atmel 328p for the Pathfinder (compass data logger) project I decided I just had to try out the latest, greatest chip in the Texas Instruments “value line” G series, which is the MSP430G2955. This chip features 56k of flash, 4k of RAM, and more I/O than you can shake a stick at. This chip was only released a few months ago and none of the sources I can access had any stock yet. I’d been using the TI developer web site a lot the past couple years and decided to order a sample part for the first time. TI sent one immediately by FedEx. Hopefully this article will serve as a small, initial thank you. I’ve loved the folks at TI for 35 years since I was part of a company that OEM’d their computer systems. I still have fond memories of visiting the R&D sites in Dallas and Austin.

The problem, however, is that the least impractical package this device comes in is a 38 pin TSSOP package with .025″ lead spacing. Notice in the picture above there are four leads in the .1″ spacing between protoboard holes. Notice the bypass cap leads look like the Alaskan pipeline. (I used wire wrap wire to connect to the power and ground pins.) The most pointy soldering iron tip I own looks like the end of an unsharpened pencil in relation to this pin spacing. What to do?

I found a Youtube video (if I can find the URL easily I’ll paste it back into this blurb). It held the big secret: you solder a chip like this and then unsolder the spaces between the pins. Physics won’t allow the solder directly under the pins to be slurped out by braid type solder wick. Wow.

Here’s how to solder a chip like this. I assume you’re very experienced with soldering except for these itty bitty components and that you’re using a temperature controlled, low power iron with a tip so sharp that it’s painful to put your finger on it (when cold). A regular, small chisel tip iron might get the job done with the right (very small) amount of solder on the tip.

Meticulously clean the board with alcohol
Apply liquid flux to the pads
Put the chip in place and slop more flux across two diagonal corners
Hold the chip EXACTLY right on the pads and touch the two corner pins with an iron holding the merest dab of solder
Put flux across both rows of pins
Put a length of very small gauge (e.g. 26 gauge) solder lengthwise across one row of pins, stopping short of the corner you tacked on.
Run the soldering iron across the pins, melting the solder and going at a speed such that you can see the solder wicking under the pins. IGNORE the fact that you’re soldering multiple pins together into unrecognizable blobs.
Put flux on some very small solder wick and lay it across a row of pins at a time
Put the iron across the flux and let it slurp excess solder off the pins a group at a time
Inspect the soldering with a decent magnifier and convince yourself there’s a nice solder joint under each pad
Ohm out the connections to confirm everything is right and there are no shorts or opens

That’s it. I couldn’t believe that I could do this right the first time and when my health recovery allows it I’m going to have a blast with this chip. Some back of the envelope calculations suggest I can get the total quiescent current of the logger under a milliampere, so one large (nonmagnetic) Lithium coin cell may get the job done and run the logger for an entire summer of butterfly tracking, with the big flash memory standing in for the SD card used by the current logger.

By the way, the excellent quality DIP adapter came from a firm with a presence in nearby Morrisville, NC called EZ Prototypes. They had to ship my part from Boston but it got here in two days at a very reasonable price. Update September, 2019: This link is no good any more: the domain is no longer registered. Too bad. But anybody unable to find a 38 position adapter and interested could ping me at pete at apexprotofactory dot com about making one, as this is a simple exercise for me now.

Logic Analyzers Rule

A few months ago I was using an Open Bench Logic Sniffer v1.04 from Dangerous Prototypes to decipher a problem with an SD card controller. It sure was nice to have a $50 device that works at least as well as the $5k devices I used in the 80s, and the performance is perhaps a hundred times greater than the tools I used with Intel and Motorola processors in the datacomm devices we created at Network Products.

Do it yourself Printed Circuit Boards

Last year I taught myself how to use the Eagle computer assisted design (CAD) program, which supports design of electronic circuit schematics and corresponding printed circuit board (PCB) designs.

Then last winter I researched affordable sources of small quantities of PCBs made from Eagle files. I settled on OSH Park, a small firm in the Portland Oregon area that converts a given Eagle board description (“.brd”) file into a complete set of Gerber photoplot and CNC drill files necessary for commercial PCB production. OSH Park combines designs sent to them with surrounding “slot” specifications and joins them all together into a large “panel” design. They send the completed panel description files to their supplier. The supplier creates three finished panels of PCBs corresponding to the panel design and sends them to OSH Park, which breaks the panels up into the individual boards and mails three copies of each board design out by US mail. The turnaround time has been gradually improving and now stands at around 10 days from hitting ‘submit’ to pulling boards out of my mail box. Three boards roughly 2×2″ costs $20. The quality is astonishing. If you look closely at an Arduino Uno, for example, you’ll see very fine detail, precise solder masks and silk screening, etc. Add to this a gold finish making soldering a breeze and you can appreciate the fine quality of boards from OSH Park. They are in every respect professional quality if the design involved follows best practices for layout.

Five Days in Heaven

Sunday, September 29th, at 4:10am a drug called tikosyn converted my heart from atrial fibrillation to normal sinus rhythm after I was admitted to UNC Medical Center with low blood pressure and way too fast pulse rate. I’d had 24/7 afib for a long time, and a heavy afib “burden” (ratio of afib time/normal rhythm time) before that had kept me dragged down for years. So the switch back to normal rhythm was the very definition of pleasant surprise.

The five days in normal rhythm this week were blissful. I’d forgotten how totally wonderful it feels to be in normal health. I walked, rode my bicycle, and did the other things allowed by the doctor as my wounds heal from a right thoracotomy epicardial ablation procedure my surgeon performed on September 19th.

But now I’m back in the “oh oh” state, and if I’m loaded with digoxin and more carvedilol I’ll be in the precurser state I was in almost exactly a week ago when I suddenly had to prop myself against a wall while I determined just how dizzy I was going to get. Ugh.

I have no choice but to “hang in there.” I’ll upload the detailed EKG data being captured by a Medtronic “loop recorder” implanted near my left collar bone, send an email to one of the specialists, and be caller number 1 when the electrophysiologist’s office opens at 8:30. And this time, unlike last Friday morning, I won’t eat any breakfast. So if they want to do a cardioversion I won’t have to lay on a gurney for eight hours while my stomach empties.

But I’m almost entirely the same guy that went to bed with sinus rhythm, and my new found enthusiasm for life is still clear in my memory. Hopefully I’ll still be back to work on two embedded computer projects soon as I “stay stable” until I’m ready for the final step of my afib cure, an endocardial ablation procedure.