Tuesday, June 13, 2017

Low Power MCU Fetishes

So, I am pretty familiar with the STM32L4xx low power Cortex-M4 MCU.
It has some insanely low power consumption profiles including a  0.65 µA standby mode with RTC (and just 14 µs wakeup time).

This thing is a beast to program (1600+ reference manual to start with and copious app notes).  But, I work with this chip for a living.

This current consumption is specified (in the documentation) at 1.8V, so a more typical 3-3.3V actual power supply will likely cause it to consume more current.   I am assuming direct battery hookup otherwise the current consumption of an LDO regulator has to be considered.

Still... this is insanely low.

I am looking to play around a bit with some "old" 8-core Parallax Propeller Chips (P8X32A) I have laying around and I read some forum discussion where you could likely get it to consume as little as 7- 10 µA  when running just one COG doing not much (maybe as a timer?).

To these jaded ears that sounds like a lot of power, but... honestly... really?

With a couple of 1200 mAh  AAA batteries (3V) the P8X32A would run around 11 years.  That is greater than the shelf life of AAA batteries.

Most of the current consumption these days aren't from the MCU but from the peripherals and sensors. If I wanted to beacon some temperature measurements via BLE (connection-less -- just as a "tag") maybe every minute, the battery life drops to around 1 year.  So, it would be more reasonable to beacon every 10 minutes.  Then I could get maybe 5-6 years.

We get lured into thinking too much about how low an MCU can go, when in the world of IoT, it is the RF that is killing us.

Just food for thought.

Monday, June 05, 2017

My Forth Story (Part 1)

This is just a few collected thoughts on my 30+ years of using Forth. So, if you are expecting high quality technical content, please move on.  Yes, nothing to see here, move on...

This past weekend I was going through old books, trying to clear out some bookshelf space, when I came upon a yellowing Forth Dimensions from 1986.  It got me thinking about when I first became enamored with Forth and how it is has popped up now and then throughout my career.

Back in 1982, armed with my first computer: a Commodore VIC-20, I started my first year in college (I was 16 years old -- I skipped a year in grade school) infatuated by the possibilities offered by computer programming.  I wasn't really college material (I was planning on going into TV repair or maybe an Art school), but I had just (to everyone's surprise) won the Engineering division of the DC Science Fair and was offered a 4 year scholarship to the University of the District of Columbia.  I had prototyped an LED display based oscilloscope using some op-amps and 555 timers.  It was inspired by a design I saw in Popular Electronics.  But I digress...

So, here I was starting college (and a job as a TA in the computer lab!) and I knew it was time to "up" my skills (I was proficient in BASIC and some 6502 assembly).  We had a lab full of the newly purchased Commodore 64s (C64) and a terminal room (oooh, remember green phosphorous terminals?) remotely connected via 1200 baud modems to the school's DEC2060 (more on that later). I would split most of my day time between the C64 and DEC260 and my nights were spent hacking on my venerable Commodore VIC-20.

Suffice it to say, my VIC-20 wasn't cutting it to get me kick started into  the highly competitive CS department.   I saved up money to get a Commodore 64 so I could continue my hacking education from the comfort of home.

On the DEC2060 we didn't have BASIC.  We had a sophisticated Macro assembler, Rutger University Pascal and Fortran IV and 77.  None of this would work on the C64, and BASIC was quickly running out of steam.

It was either through BYTE (or maybe it was Compute!) magazine that I stumbled upon this language developed by this guy named Chuck Moore.  It was Forth and there were a couple of implementations available on the C64.  An implementation that intrigued me, in particular, came in cartridge form and booted (nearly) instantaneously.  This wouldn't require me to fiddle with the painfully slow floppy drive.

I became obsessed with Forth. The interactivity and the power (to lock up the C64) was addictive.

But, my CS (well EE, I started as an EE student and defected to CS) courses were on a DEC2060. The DEC20 was a lovely 36-bit word "mainframe" (shhhh! DEC wasn't allowed to call them mainframe as they didn't want to face the wrath of IBM and their patents). The 36-bit word size happened to be perfect for a Lisp cons cell.  I found Lisp quite lovely, powerful and intriguing but I was still in the midst of my Forth obsession.

This obsession became even more all consuming, around 1985, when I read about Chuck Moore's  amazing Novix NC4016. I even ordered a fact sheet from Novix Inc so I could pour over as much detail as I could -- knowing I would never likely touch one.

In late 1985, my C64 Forth obsession hit a wall.   This wall was my obsession with Fractals, particularly the Mandelbrot Set, of which I first heard about in the August 1987 issue of Scientific American.  The C64 just didn't seem to have enough processing power to execute my naive implementation of Mandelbrot's algorithm.

Eventually, I found a Forth that ran on a DEC20, converted the algorithm to fixed point and managed to get the set rendered on a graphics terminal (over a 1200 baud modem!).  If my memory serves me correctly, the terminal was a fancy Tektronix 4150.  It took a lot of false starts and missed classes, but a couple of days later I had a color fragment of the famous fractal.

As I got further along in my CS curriculum, I discovered that technologies like home computers (C64, etc) and languages like Forth were not really encouraged as tools of study.  So, I learned TOPS-20 assembly, Pascal, Fortran (IV and 7), TeX and Lisp.  I fell in love with Emacs (the original written in TECO!) and generally was happy, but I was missing some of the hands on immediacy of having my own personal computer and personal happy-to-crash-it-language like Forth.  There was a driving need, brewing inside me, to do something low level -- something dangerous.

So when a secretly procured  copy of AT&T's UNIX Version 7 arrived at the University Computer Center (where I worked, distracting me from my scholarship and short circuiting the pursuit of my degree), I worked with a couple of my friends to boot it on the DEC20, play around with it and then remove it,  before the next day's classes began.  This was no trivial task as it required hand entering the bootloader on the front panel. Fun stuff.

I soon fell for the spartan language that accompanied the UNIX tape: C.

Over the years, I would continue to play with (and implement my own) Forths, but it wouldn't be until 20 years later that I would get a chance to program in it extensively (around 2006), when I revisited my low level hardware past in the form of embedded development on Microcontrollers.

To be continued...