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% Design Decisions for "Quiz-Board" % Joerg Desch ( % Revision R.1

About this document

This text documents some of the design decisions I've made. Remember that one of the main goals of the project is the KISS principle!

Hardware decisions

Our "Quiz Board" contains 8 questions with 4 possible answers. The result is shown with 2 LEDs. We need a button to start the game and a LED to visualize the state.

Note: all pin numbers mentioned in this document are Arduino pin numbers! See the Arduino Pin Mapping table for details.

Keypad Inputs

Each button is realized as a closing switch. If pressed, this switch is closed and pulls down the input level to GND. If not pressed, the internal pull-up resistor is used to hold the level up to Vcc.

The "Start" button is a special input. Since this button must be able to wake up the device, the input must be capable of the "pin change interrupt" feature of the AVR2560.

The 32 answer buttons (8x4) are connected to the pins 22 to 53. The "Start" button is connected to pin 12. This pin is "PCINT6" of the MCU.

LED Outputs

The anode pins of all the LEDs are connected to Vcc. The cathode pins are connected to an 200 ohm resistor. The other end is routed to an output pin of the MCU. A low level at this output lets the LED glow.

We don't use the analog inputs. So we connect the 16 (2x8) LEDs to the analog inputs A0 to A15. The pins are used as GPIO instead. The yellow status LED is connected to pin 11.


Software decisions

Usage of the IDE

The Arduino IDE comes with several limitations. Since this is my first Arduino project, I possibly have not found the best solutions to these limitations. So this section may change some time. ;-)

The source code of C++ is split into a header and a code file. The header includes declarations and the code file the implementation. To be included into the project, those files must be opened as a tab in the corresponding sketch file. To avoid a cluttered IDE, we put both, the declaration and the implementation into the header file.

In the end, we need one place where all the implementations are included in code files. The solution is to declare a C define in the main file (.ino).

The only other solution would be a library which must be stored in the Arduino toolchain path. In my opinion, this solution should only be chosen for independent code.

Sample Header File: the sample shows the class declaration and the code implementation with the ifdef.

// file QuizBoard.h
class QuizBoard
    static int ListOfLEDs[];
    void setLED(int nr, bool state);

int QuizBoard::ListOfLEDs [] = {..};
void QuizBoard::setLED ( int nr, bool state )
{ ... }

Sample INO file: the sample shows the usage of the class. The define of __ALLOC_STATICS_HERE__ enables the code implementation inside this project.

// file Sample.ino
#define __ALLOC_STATICS_HERE__ 1
#include "QuizBoard.h"

QuizBoard BoardTest;

C++ design issues

Even if we use C++ to implement this project, we are running it on an microcontroller. This means, not all feature of C++ are usefull if the target is a embdedd 8-bit system.

We avoid things like C++ callbacks (functors) or listener classes with virtual methods. If a normal API doesn't the job, normal C callback should be prefered. Shure, this is not as nice as the variants mentioned before, but it is much easier to understand and is costs less ressources.

One example of such a "work arond" is the class SimpleTimer. Insteas of using a listener interface, the methods are designed to query the occured events.