Techy Tuesday - Digital Design Primer - Part II

There have been some unfortunate delays since Part I due to things popping up around the house that just had to be done, and sucking up all the time I'd use to write this.  So let's just dive in, shall we?

I wrote a lot about combining signals with logic; the area called combinatorial logic.  How is this used?  When an electronic system is being developed, the subsystems and lower level assemblies pass a lot of information back and forth between themselves.  These things are undoubtedly hard for someone who has never built electronics to envision, but the individual signals might be commands to do something, or time signals that say it's time (or it isn't) for an event to happen.  If you're operating a handheld transceiver, for example, you will press a "Push to Talk" button and that signal will often need to do many things.  It may turn on an audio amplifier at the microphone, turn on voltages that power the transmitter, turn off the receiver (or just mute its audio), and other things.  It may be combined with other signals; for example, the radio may tune a wide spectrum but only be authorized to transmit in ham bands.  "Something, somewhere" in the radio says transmission is authorized, and the signals go through an AND gate.  That's the equivalent of saying "If PTT is pressed, AND I can use this frequency, then I transmit". 

In the days of lots of discrete logic, designers would map out the logic required to implement functions.  Some of it was done with graphical design techniques, Karnaugh or K Maps; some of it was done by hand (experience).  Compared to today's consumer products, these were big and bulky circuits.  Miniaturization of packages and more integration helps, but not as much as you might think; the more signals going in and out of the logic portion of the design, the more pins you need and connections limit the size far more than the logic. 

The world, though, is full of cheap consumer devices like your GPS or cycling computer or dive computer or heart rate monitor (pictured).  This monitor is sold with a transmitter you wear, together for just over $100.  How do they make them so cheaply that they can be manufactured, put in a package and shipped halfway around the world for that price?  No, it's not by going to China (at least, not exclusively).

 

They do it by turning everything in this package into a the smallest number of parts possible, replacing the many individual counters and other components with a custom ASIC - Application Specific Integrated Circuit.  Similarly, the display is a custom Liquid Crystal Display -LCD.  Together in an injection molded plastic case, they can be popped off the assembly line with the least effort and cost.  Designing for low cost is done in the design group, not by taking any approach and finding cheap labor to build it.

Design is all about trades, and the trade here is that developing ASICs is expensive.  The last time I was around a project that wanted to develop one, the first batch of parts was a quarter million dollars, and I'll bet it's not significantly cheaper today.  If your parts don't work, you pretty much have thrown that $250k out.

While you can't make an ASIC, you can get most of the way there with a Field Programmable Gate Array, an FPGA.  An FPGA allows the user to minimize the number of digital gates and parts in your design, which minimizes the size - and usually the power consumed, too.  It's programmed on power-up by a small programmable memory, and may be reprogrammed over and over.  Long the domain of the digital design wizards, they are coming onto the hobby market with the emphasis of learning how to design the FPGA.  This isn't done with any of those old techniques, like the Karnaugh  map; it's done by programming what you want the FPGA to do in a high level language that looks like Pascal.  Why Pascal?  It's sort of the default software training language these days; most enginerds get exposed to it in school.  It also bears a strong resemblance to a language called ADA that the DOD uses.   

Check out The Gadget Factory and Papilio - a board for experimenting with programming your own FPGA.  They have a page full of free downloads to help you learn the programming language, called VHDL, an acronym of acronyms if you listen to that Wikipedia page that says it stands for VHSIC Hardware Description Language (VHSIC was military acronym or program; it stands for Very High Speed Integrated Circuit) .  Adafruit features the Mojo FPGA Development Board also intended for learning FPGA programming.  I expect more of these systems to start showing up.  While top end FPGAs can be very expensive, there are many which can be used in small group designs.  

Make no mistake: this isn't beginners' stuff.  But the tools are out there, the open source community is out there and the potential rewards are great, even for small patriot groups.