Instruments4Chem

High Performance, Low Cost…

Test and Measurement - Some Prototyping Hints

Under this menu tab you’ll find a grab-bag of projects that I’ve placed under the generic heading “Test and Measurement”. Some of these projects have resulted in complete instruments, while others have simply been experiments I’ve done to evaluate chips that look promising for various measurements. Before looking at these, I’ll offer some general comments about how to go about prototyping your own instruments.

For starters, get yourself a good development board. Parallax offers several excellent, feature-packed, low cost development boards. The XMOS Startkit is also extremely good value for money for this same purpose. While there are numerous other choices - Arduino’s are extremely popular, Raspberry PI’s too - the ones I’m mentioning are my personal favourites and almost everything described on this site uses these.

The Quickstart board (below left) brings out the Propeller’s I/O pins to a female header at the top of the PCB. The Propeller Activity board (below right) makes 16 of the I/O pins available on a vertical header immediately to the left of the prototyping area. This latter board has many useful additional features.

The XMOS Startkit (below right) also has a wealth of I/O pins available on various headers, affording the possibility to design custom plug-in shields to add customized functionality.
Many interesting chips are available from the large semiconductor companies (Analog Devices, Texas Instruments, Freescale, and many others). After finding chips that you plan to use, carefully study their data sheets to see what I/O connections are needed to provide the interface. Also check out the timing diagram(s) as these give vital information that you'll need to understand before you can write some code.

If you are fortunate vendors like Sparkfun or Adafruit might already sell a breakout board for the chip(s) you want to use. When doing your I/O wiring, note that both the Propeller and XMOS processor’s I/O pins are only rated for 3.3V operation - ensure you don’t exceed that rating !

At this stage the fun begins - its time to wire up a prototype, write your code and start testing. Typical projects on Propeller-based products use a combination of Spin and assembler (PASM), and are developed in a custom (and free) Parallax IDE (integrated development environment).

If you are using the XMOS platform your software will be coded in XC and run from the XTime Composer IDE. Either way, your code will need to be downloaded onto the development board that is connected to your chip/sensor.

During the debugging phase a logic analyzer will be essential. I have two of these at my disposal, an old HP 54620A 16 channel, 500 MSa/s unit and a pod-style SALEAE logic unit. A logic analyzer, plus an oscilloscope and a digital multimeter are the three single most useful pieces of test equipment you'll need to assist you in getting your design to work. If you want to get serious about analog/digital design you can't do without these items of equipment !

Nearly all of my projects have a LabVIEWTM front end. This allows parameters and data to be downloaded and results to be uploaded and then displayed using a rich feature set of controls and indicators (both numerical and graphical). Why waste time re-inventing the wheel when such tools are available and are very easy to use ?

While LabVIEWTM is quite costly, National Instruments offer deep discounts if you qualify for an academic licence. If you are going to do a lot of instrument building its a really worthwhile piece of software to purchase - it will pay off handsomely in time saved down the track. Some very good news is that NI have recently made available a low cost Home Edition of LabVIEWTM; check here for details.

As you get to the pointy end of your project you’ll need to refine the hardware and software to your satisfaction. If you’ve created something worthwhile that you'd like others to be able to use, then why not lay out a printed circuit board and get it made and populated. It’s not as difficult as you might imagine and there are free tools out there for doing so.

That's a very brief summary of the workflow that I've adopted to complete the projects described on this website. I hope others will be inspired to follow a similar path and allow their own creativity to make useful things too.