DIY CPLD board

The project has been sleeping for ages now but it has not died (yet ^^). A few simple thoughts came to mind recently on how to improve it.

I was thinking, why not improve the board design directly from the start and add a few components:

- I have a few ULN2003A darlington driver arrays lying around: each chip has 7 outputs that can each drive about 500mA. Useful for stepper motors, LED arrays and other higher current (and inductive load) devices and the outputs can be tied together for even higher currents. An extra voltage supply would be needed (external or onboard, probably external) that I could hook to the board and let the ULN's (or TIPs) do the switching

- add a few TIP120's for switching even higher loads as some sort of electric relay similar to the ULN's, I probably would only need either TIP's or ULN's

- add a few EEPROMS for extra storage (data values, settings, what have you, possibly even new CPLD configurations, i. e. store several sets of CPLD programs in the EEPROMS and let the MCU (PIC or Attiny) do the programming if that is possible)

- and if I'm totally nuts add a 2.7V/500F capacitor (about 0.5Ah worth of energy) + converter to the board to power the board from xD (only bad: the cap is pretty big, about 5x2.5cm and would add huge bulk to the board).

Another thought that came to mind: I am thinking of replacing the Attiny with a PIC16F. I know that they are more difficult to program than the Atmels but as a learning exercise it would do. So far just going a bit nuts on the specs for the fun of it. We'll see what the first version will include.

The box with the parts from reichelt.de arrived today at the same time as my eBay order of a breadboard (came from the UK). Nice timing.

Searching my DIY stock I found a 600g bin of sodium persulfate that will come handy when it is time to etch my PCBs. 600g will be largely enough for the next few boards (it is mixed at a ratio of 1:5 with water). Since sodium persulfate works best at around 40-50°C (104-122°F), I have to find a solution for heating up the etching bath. Some people simply use an electric kettle and some tap water to mix water to >40°C which seems to be hot long enough for about one or two PCBs and will probably be my method of choice for the first few tries.

I would like to have a permanent heating option though with some kind of double-boiler to heat the etching solution to avoid the hassle of calculating and mixing hot fluids. I still have a few peltier elements lying around that I could throw into a design. Peltier elements are small pads that convert electricity into heat/cold - on eBay.com they start at 20$, on eBay.de they start at around 3€ =~ 5$. 

Since there is a large hardware store nearby that has a 3D printing facility, this sparks up a few ideas for a new project (read: 3D printed µC operated adjustable temperature controlled double-boiler etching bath using peltier elements - could probably be made for under 25$, at least with the cheap peltiers on eBay.de).

But lets not overdo it project-wise. Should I really go that far, it will certainly appear as a project here on hackaday ;) with STL files and the like so that other people can have their own one printed.

For now I'll do a bit of Arduino hacking and see to the CPLD boards in the next few days.

So yesternight I went component hunting for my two beginning projects (this CPLD board and the parallel port JTAG programmer necessary to program the CPLD). 

At first I searched at www.farnell.com or rather http://hbe-shop.de/. HBE sells the complete range of products Farnell does at a very slight markup but they also ship to individuals in contrast to Farnell.

For component search the main Farnell site is a real trove though: they have free datasheets to just about any product they have on sale (I did not come across one they had no sheet for), their search is much more detailed and they list more product specifics per listing. So all in all a very good start.

I had some difficulties finding some of the parts though (in particular the 2x7 JTAG header, 9 pin 8 resistor network, etc.) so after a cursory search on www.eBay.de I switched to www.reichelt.de. In Germany (where I live and come from), reichelt.de is pretty cherished among DIY electronics enthusiasts because it offers a very large range of products at a very decent price point.

One or two hours later, I had all the products in my shopping cart with one exception: the tutorial board uses a PLCC socket for the main chip (as in the first example picture below - the socket is the brown thing around the chip) but Reichelt only had the VQFP-44pin version. They seem to be stocked on pretty much everything (including the 2x7 JTAG header and the resistor network) but CPLDs are a bit niche it seems (more so than FPGAs).

(nevermind the image placing / sizing - it is not quite working yet - all images from Wikipedia under Creative Commons)

A socket gives you the advantage of being able to switch the chip (second pic is an example socketable chip) in case you want to program another CPLD. The VQFP-44 version I got (reading: Very small Quad Flat Package with 44 pins - example TQFP 80-pin: third pic above) has to be soldered onto the board.

That means:

a) should I fry the chip when soldering it or by a wiring mistake or similar I'll have to (re)solder a new one - bummer! Only good thing: they cost 1.80 € a pop (about 2.50 US$) so I ordered three of them ;). I don't think I will send them all to chip nirvana if any, so I'll have one or two spare ones for other pet projects.

and

b) I'll have to learn how to change the PCB schematic provided by the tutorial. I'll have to switch the PLCC socket diagram for a 44-pin TQFP SMD part diagram and wire it correctly in KiCad.

and

c) I'll have to learn to solder small pitch SMD parts which is not one of the easiest things to do.

Sounds challenging to me but (hopefully) offers some worthwhile learning.

All in all I spent about 13-17$ per board plus components (CPLD board and JTAG programmer board) plus a parallel port PCI-Express card needed for the programmer (at about 15-20$) - so all in all not that very expensive for a start :).