Overview

A PIC is a very simple microcontroller - everything on a single chip. This session uses a PIC to implement the simple Scissors, Paper, Stone (or Rock, Paper, Scissors or . . . ) game.

The PIC

This includes both hardware and software.

How much hardware you get pupils to do will depend on their age and skills. As described here it was run in a primary school, so, for example, getting pupils to do any soldering was not a realistic option, but getting them to plug wires into headers to connect everything up was not a problem.

The software element demonstrates the basic write code / compile / run cycle using an Integrated Development Environment (IDE).

Hardware

Microchip produce a development kit (PICDEM Lab Development Kit with PICkit 3) that includes the necessry PIC controller itself, a development board, programmer to connect the board / PIC to a PC and all the necessary software, tutorials and a bag of components (LEDs, resistors etc.) - available in the UK from RS Online at (currently) £94.50 plus VAT plus P&P.

PICDEM Lab Development Kit

For the game you will need additional components (the components in the kit aren't quite sufficient). The resources below include a component list from Rapid Online electronics - this list is complete and assumes you will use none of the bag of components from the PICDEM kit. Earlier in 2012, these cost 11.95 including VAT and delivery.

In addition you will need a supply of coloured wire (at least 7 colours).

The external components make up into 3 boxes - a master box, with a switch to start the game and 2 LEDs to indicate the winner - and two player boxes (red and green) each with 3 switches and 3 LEDs (switch + LED for each of rock, scissors and paper for the player to make their selection and indicate the selection made).

With hindsight, slightly larger boxes (in all 3 dimensions) would have made construction easier - so you may want to think about that when ordering components. The notes below also include a few other suggested improvements on my original design - all of these were included in later versions I built (isn't experience and testing wonderful . . . !!!).

For anyone comfortable using a soldering iron and with access to a pillar drill, construction should be straightforward.

• Draw up and print off a sheet (to scale) as a template for the top of each box showing where holes are required for LEDs and switches - the size holes and placing will depend on the components purchased.
• Temporarily attach the template to the top of the box and drill the relevant holes.
• Drill an additional hole in the back of the box as an exit point for the wires. If you want to use a rubber grommet to tidy this up, feel free - I didn't.
• Mount the switches and LEDs in the box. The switches I used had nuts on top to secure them and the LEDs were sufficiently secure just as a push fit, though they could have been glued.
• Now wire up the components - one I/O wire for each - with one wire as a common return to VSS - see the diagrams for orientation of the LED and resistor connections. No resistors are needed for the switches as we'll use the PIC's internal weak pullups. Pay attention to colour coding in the diagrams - adapt as necessary to suit whatever colour wires you are using.
• Consider using jumper cables (search for Arduino Jumper Cable on eBay - male to female or male to male as you only need the male end) with pre-attached jumper pins to make plugging into the headers easier (if you buy double-ended cables you may need to cut off the jumper pin on one end to be able to solder it onto the components in the box). If you don't use cables with jumper pins, strip and tin the end of the wire to be plugged into the headers - that's what I did, but it isn't brilliantly durable so if I was doing it again I'd use jumper pins.
• Small cable ties to keep the bundle of wires coming from each box tidy are a good idea - and putting the cable ties INSIDE the box can act as a cable restraint (protection against being pulled too hard and breaking internal connections).

Outside and inside the finished boxes

Software

The basic setup is that you do all the development of software (written in C) on a PC, compile it and then use the USB PICKit 3 programmer to connect to the PIC on the development board and copy the compiled program to the PIC. Then disconnect the PICKit and run the program on the PIC.

Download and install the IDE and C Compiler from the Microchip web site.

You have a choice here. The X version is the most recent / up to date. But the older v8 may be easier to get started with as (at least in the kit I bought) the tutorials were based on v8. Up to you. Or try both. I've included code downloads for both versions below.

If you are not familiar with PIC programming, working through at least the first few tutorials (LED outputs / simple switch inputs / weak pullups) is strongly recommended.

For power, I used connected a battery to the terminals on the development board rather than using a power adapter or taking power from the PICKit.

Don't forget to include the "#define _LEGACY_HEADERS" line (see the handout) - I wasted a lot of time before I stumbled on that one.

The kit comes with several PICs, but the one you need is the same one as is used in the tutorials - the PIC16F690.

Where you're ready, create a new project and add the C source file for the Scissors Paper Stone game. Check that it compiles correctly.

Putting it all together

With no power connected, plug the wires from the 3 boxes into the headers on either side of the PIC - by now you should be familiar with pin numbering and which way round the PIC is.

Then plug in the PICKit 3 and power, switch on, program the PIC, switch off, disconnect the PICKit and switch on again.

PICKit 3

After pressing the master start button, all LEDs should flash randomly for a few seconds then go off. Each player should then press and hold one button until the LEDs start to flash (end eventually go steady) to indicate the selections made and the winner. Press the start button again to start the next game.

Notes

The instructions in the main handout are based on how software and resources were set up on the network I was using. You may well have to make minor changes to the handout to reflect your network setup. In particular:
* I had a folder set up containing the C program code;
* The IDE and C Compiler were installed ready for use on computers.

C has a lot of similarities with JavaScript, so a lot of the code should feel fairly familiar to pupils who have worked with Code Academy.

However, the #define pre-processor directives will be new.

The structure of the 'main' routine and control loop should be fairly readily explainable.

Concepts of inputs and outputs can be explained, looking at both the relevant sections of the control loop and the physical components. Even if pupils have not been involved in soldering components together, the insides of the boxes can be shown, the types of component discussed, what they do and how they are shown on the circuit diagram.

This is also an opportunity to introduce binary - 1s and 0s in software - high and low voltages in hardware.

Depending on how much physics pupils have done, a hardware discussion could extend to looking at the voltage output from the PIC, voltage drops across the LED and resistor, maximum allowable current drain from the PIC and current through the LED/resistor to determine a suitable resistor value and consequent power output (light) from the LED.

How far you go into the detailed game logic / game state / code will, depend on time and skills of the pupils. One possible area is what happens if a player presses more than one button or no button and how the code checks for invalid inputs.

At a simpler level, making changes to the timing values demonstrates that the code really is doing the work - there aren't any tricks anywhere.

For this session with pupils aged 8 to 11, a group of six round a table was ample. The session started with simply giving them a PIC (not in a circuit) and asking them what they thought it was, continued with a discussion of inputs and outputs, voltages on pins and binary, looking at the circuit diagram sheet and then the insides of the boxes and talking about the components being used and what they did, before getting them to take it in turns to plug in wires. We then went through what an IDE was, saw the code being compiled and copied onto the PIC (they quickly got to grips with plugging / unplugging the PICKit and switching power on and off), followed by them playing the game and seeing how changes to the code altered it.

To emphasise that PICs aren't just toys, we also talked about where a PIC might be useful in practice. Without actually pulling gadgets apart to see what's in them one never knows, but things like household appliances (e.g. toaster with LEDs / buttons) are good candidates.

Handouts

Coding is Fun - Resources - 5 - PIC

Coding is Fun - Resources - 5 - PIC - Circuit Components

Coding is Fun - Resources - 5 - PIC - Component List

Coding is Fun - Resources - 5 - PIC - Wiring

C Source Code

For MPLab 8 : Coding is Fun - Resources - 5 - PIC - Scissors Paper Stone - MPLab 8

For MPLab X : Coding is Fun - Resources - 5 - PIC - Scissors Paper Stone - MPLab X

Links

MicroChip
   Producers of PIC microcontrollers
   Download of software - IDE, compiler etc.
      MicroChip - www.microchip.com

RS Online
   Possible supplier for PICDEM Lab Development Kit with PICkit 3
      RS Online - uk.rs-online.com

Rapid Online
   Possible supplier for other electronic components
      Rapid Online - www.rapidonline.com