In November 2008, EPE (Everyday Practical Electronics) magazine had an amazing offer. For around £10, you could purchase a Microchip PICKit 2 programmer, software, cable AND they included a 44-Pin DemoBoard.

Figure 1 – Microchip’s 44-Pin demo board.

The demo board alone is usually around £24 in the UK, so the fact that EPE could do the whole kit for less than £10 including postage and packing was simply amazing, and contained a switch, variable resistor, some LED’s and of course a PIC. In the documentation that was provided there were some code examples to make use of the board’s features and help get you started in the world of PICs.

Now we all need goals in life and to that end when some of my colleagues also made use of the above offer and then, like a bunch of children on a wet and windy day announced “we’ve flashed the LED’s... now what”, it was time to set them a simple challenge and that was to write a simple OXO game.

The OXO board for this project is a fairly simple PCB that plugs onto the 44-Pin demo board. The new PCB contains nine bi-colour LED’s arranged in a traditional OXO board layout of 3 x 3, and contains two push button switches. Some enhancements to the Microchip Demo board are needed to make use of its expansion capabilities; namely the soldering of 4 x 11 pin headers. Also of course the OXO PCB’s needed to be constructed.

To save on costs I’d opted to use SMT (Surface Mount) bi-colour LED’s as I already had a stock of them thanks to a lucky find on Ebay and over an evening designed, etched and soldered a set of boards for everybody to use.

Below, you will find basic constructional details for the OXO board. I've also included some very simple test software to help you check that you've assembled the board correctly, but as for the actual game, you will have to write your own software. There's no fun in me giving you that now is there, but for the record, yes it can be done and yes, I've done it.

 
Figure 2 – Demo board and the OXO prototype board.

The above picture shows the completed OXO board next to the 44 Pin Demo Board and as you can see they are approximately the same size and later in the article you will find a parts list with part numbers where known, and possible suppliers.

Ok, let’s get started on building this masterpiece of modern technology.

First, you have to "enhance" your 44-Pin demo board. Microchip thankfully provided lots of expansion pads on the board including a set of gold through-plated holes around the PIC that gives us access to every pin on the PIC. However, they set the spacing of these holes at the none-standard pitch of 2mm so you probably won't have any of these header pins kicking around.

You will need 44 pins if you are to fit a pin to every hole and for stability of the OXO board and future expansion, I would suggest that you do so. You can at a push get away with just 14 pins.

Figure 3 – Close-up of the 44 pin connectors soldered around the PIC.

I'm hoping you can see from the above picture where the header pins need to be soldered. You will need 4 strips of 11 pins if you are to use every hole. Soldering is quite simple however the pitch (distance between the pins) is smaller than you may be use to so be careful that you don't make any short circuits.

The other “enhancement” to complete is the bending of the Demo Boards LED power jumper; you can see this jumper in figure 3 on the left hand side. This jumper when fitted allows power to be applied to the 8 surface mount LED's that are pre-soldered to the demo board, but it needs to be bent over at 90deg so that the plug-on board can sit correctly. Using a pair of pliers gently bend each of the 2 pins over as shown. You can then fit the jumper link to ONE of the pins so you don't lose it. Do NOT fit the jumper across both the pins as the PIC I/O signals that drive those LED's are also used by the OXO board.

That's it... the board is now suitable “enhanced”.

Now to build the plug-in OXO board.

I'm not going to tell you how to make PCB's here but there are a couple of things worth noting. The OXO board has 88 holes arranged as 2 squares. You only need to drill out the inner 44 holes. Originally, this board started out as a simple way to convert from the 2mm pitch that Microchip used to the more traditional 2.54mm (0.1”) pitch that we all know and love so well. I left those pads there in case anybody wants to "hack around" my PCB art work and make their own boards.

Drill the inner 44 holes using a 0.8mm drill bit, and take your time.

After this, there are 12 holes across the centre of the board to be drilled. Again, you can drill them using a 0.8mm bit. These holes are for the jumper wires (more on those in a moment).

Lastly, there are 4 holes to be drilled for the 2 x PCB switches. I used a 1.5mm bit for these holes.

Figure 4- The bottom side of the OXO PCB.

The above image shows how the board’s underside is assembled. You can see that I’ve drilled some of the outer holes around the square just so you can see them, but you don't need to do this. This was the prototype board and I was actually short of 2mm pitch PCB sockets so didn't fill the right hand column totally. For stability you should if you can.

You can clearly see the wire jumpers fitted to the left of the board. There are six of them.

The bottom left of the board shows the 4 holes that allow the PCB switch's to be pushed through from the other side. In the very bottom left corner of the picture you can see a stick on rubber foot pad. This provides stability to the board when it's fitted to the demo board for when the player presses down on the two push switches.

Assembly is quite straight forward but I would solder the components in the following sequence. First off we will solder the resistors, then the LED's then finally the wire jumpers, sockets and then the two switches.

If this is your first outing with surface mount (SMT) components, don't panic. If you take your time, use the smallest soldering iron bit you can find and keep it clean, you won't have too many problems. See here on SMT soldering for more information.

First off, we need to solder 2 x 2.2K resistors that are used as pull-ups for the two switches, see figure 5 below.

 
Figure 5 – Two SMT 2.2K pull-up resistors

Once those two 2.2K SMT resistors are in place, you can then solder the remaining 6 x 330 ohm SMT resistors.


Figure 6 – 6 x 330 ohm SMT resistors on the right, and shows some of the bi-colour SMT LED’s. Notice there is a green dot on each LED (top green circle). This shows you which way around the LED should be positioned.

Next, solder the 9 x bi-colour SMT LED's.

These LED's contain 2 LED's in one package; one red and one green.

Figure 7 – The SMT LEDs as supplied in a plastic peel back strip.

The picture above shows the LED's that I purchased from Ebay. Now, if you look at the top of the LED you can see a green bar or a dot. When you solder these LED's to the PCB, make sure that this green dot faces the bottom edge of the board. Check out figure 6 to better orientate yourself.

Position the LEDs very carefully making sure that they are centred correctly else you will short out the pads when you solder them in place.

Once you've soldered on the resistors and LED's, that's the hard bit done. Next, solder in the 6 wire links. Now solder the two switches. Remember to push the switches through from the copper side!!!

Finally, you can solder in the sockets. These are supplied in strips and you can cut them with a pair of wire cutters but this unfortunately sacrifices a single socket for the cut, but I don't see you have any choice unless you can get them pre-cut to the right size.

I would suggest you solder just the end pins of each row first and then make sure that the sockets are aligned correctly before soldering the rest of the pins.

Perform a quick visual inspection of the board, especially around the SMT components. A x10 small magnifier is great for this.

Last but not least, fix a rubber foot to the board as shown in figure 4 above, and that's it, you’re done.

Plug the OXO board into the Demo board making sure that you align up the pins correctly, and push it down firmly. If you press the two push button switches, the rubber foot you attached to the board should help take all the strain and stop the board flexing and moving around too much.

I've supplied two software files for you:
       Proton BASIC source code for OXO1
       OXO1.HEX - Suitable for the PICKit2

Download the OXO1.HEX file and use it to program the PIC using the PICKit2.

As soon as you power up the Demo board by putting a tick in the ON check box from within the PICKit software, the board should start to cycle through a test pattern. It will quickly light each red and green section of each LED in turn. Pressing the two push switches will either speed up, or slow down the pattern.

Now, this raises an interesting point. If you slow the pattern right down, you will see that only one LED is ever illuminated at any one time. If you speed the pattern up you will slowly see that it starts to appear like all the LED's are constantly on. This is an optical illusion and one that you will need to exploit to make this project work.

To keep the hardware as simple as possible, I've used a matrix display. This means that the PIC has to scan through every LED on the board and decide if it should be lit or not. I've also supplied the Proton Basic source code for you to look at. It's got plenty of comments in and should make things quite clear on how to drive the board.

The reason I’ve supplied the sample code in BASIC, is that just about anybody can read and understand BASIC. You are of course free to write your software in any programming language you like.

Parts List

Just as an afterthought there's no reason why you can't use only 330 ohm resistors and dispense with the two 2.2K ones. I used 2.2K for the two pull-up's because I have them and that's what I would normally use. But for this project, just using 330 ohms will be fine.

If there's an interest, I'll look at supplying a kit of parts including an etched PCB. I may even drill it for you :)
Contact me if this is something you’re interested in.

Happy OXOing