EEEN20011 Microcontroller Engineering II
Dr L A Marsh and Dr P N Green
Lab Task 4 (40 Marks)
In this lab task you are expected to use the joystick, LCD screen, speaker, both potentiometers and
the RGB LED on the Application shield. These are shown in the diagram below:
This task is designed to integrate many of the peripherals and programming techniques that you
have learned about so far on the course. It also requires you to implement a state machine in C++,
for which there are two example programs available that you may wish to study.
In this task you are required to make use of the LED, and Potentiometer Classes provided in
“Example Program 5: Creating a finite state machine and using TimeOut APIs”. You should not
modify the Potentiometer class. EITHER: The LED class should be used unchanged, OR you may use
your RGBLED class from task 3.
You should create a class called SamplingPotentiometer which inherits from the class Potentiometer.
This class should make use of a Ticker object to regularly execute the member function sample() in
the class Potentiometer. The constructor for this class should accept the pin name, the VDD for the
analogue input and the sampling frequency as parameters, as shown in Listing 1. The constructor
should configure and attach the Ticker callback to trigger the sampling process at the specified
frequency.
class SamplingPotentiometer : public Potentiometer {
private:
float samplingFrequency, samplingPeriod;
Ticker sampler;
public:
SamplingPotentiometer(PinName p, float v, float fs);
};
Listing 1: SamplingPotentiometer Class declaration for Task 4
You are required to implement a simple state machine using an enum as a means of defining a
program state. There are examples of this type of operation in “Example Program 4: Example of a
Finite State Machine to control program states”, and “Example Program 5: Creating a finite state
machine and using TimeOut APIs”. For this task you should implement the state machine within the
main() function, as shown in Example Program 4. The format should be similar to that shown below
in Listing 2.
typedef enum {state1, state2, state3, … staten} ProgramState;
//Definition of the enum that refers to the states of the program
ProgramState state;
int main()
{
while(1) {
switch (state) {
case (state1) :
//Code for this state
break;
case(state2) :
//Code for this state
break;
default :
//Default if not matching other state
}
}
}
Listing 2: Architecture of a simple state machine using an enum
In this type of operation your program can do different tasks according to the current state of the
program. You can, and should, use more memorable names for each of your states. Some
suggestions that you might wish to use for your program are:
initialisation, set_time, current_time, world_time, stopwatch, countdown_timer,
You are likely to need more states than this, but the above list should give you some idea as to how
you can implement a state machine to fulfil the requirements of the task. Bear in mind that you will
want to loop in some states, but may want others to only do something once, change state and
move on in the next iteration of the loop.
You should implement a Clock class that represents the functionality that your clock requires. A
suggested format is shown below, though you may wish to expand on this to include the world clock
functionality should you find this easier.
class Clock
{
private:
int time_hour, time_min, time_secs
public:
Clock(){}
void reset()
void tick()
void setClock()
int getHours()
int getMins()
int getSecs()
};
Listing 3: Example Clock class structure.
Your Speaker class from Task 1 should be updated to include member functions for toggling the
output, and returning the status of the output, if this has not already been implemented. The
Speaker class should have the same format of that shown in Listing 4 below.
class Speaker {
private:
DigitalOut outputSignal;
bool state; // Can be set to either 1 or 0 to record output value
public:
Speaker(PinName pin);
void on(void);
void off(void);
void toggle(void);
bool getStatus(void);
};
Listing 4: Speaker class format including toggle()
The toggle member function should be called from a Ticker API, where the time period should be
provided as part of the attach function. You should consult lecture notes and Mbed examples for
attaching member functions as a callback (ISR). You are free to choose an appropriate audible
output frequency.
You will need to implement some of the functionality from previous tasks, such as interrupts to
enable you use any required buttons on the joystick. Your ISRs should all be simple functions that
refrain from calls to computationally intensive operations, or blocking functions such as wait(). Your
ISRs should be used to switch the program on to the next state, or to update simple variables.
Some variables or objects may need to be declared outside the main function as global variables.
This should be avoided unless strictly necessary, and the use of all global variables should be
carefully justified.
You should use the LCD screen to display different data depending on which state the program is in.
You should avoid calling lcd.cls() in a loop – which can cause flickering, I’d recommend implementing
a filled rectangle of no colour to overwrite any data on the screen, or overwriting existing text as this
will be flicker-free. There will no doubt be other ways to implement this functionality, and as long as
your screen is updating smoothly then this won’t be a problem.
You should also use the RGB LED as a means of indicating the state some aspects of the program. It
should show the following:
Off: No timers running
Flashing Green: Countdown timer running
Blue: Stopwatch running
Please note that as per the video, both LEDs can be on simultaneously, with corresponding colours
visible.
You should be able to use the potentiometers to set the time during the relevant states; minimum
and maximum values should be contained to be within permitted ranges dictated in the video. The
potentiometers should also be used to navigate time zones with the minimum and maximum values
representing the cities furthest to the west and east respectively. A list of time zones will be
provided on Blackboard for you to choose from, and more details are available in ‘Marking
Requirements’ overleaf.
Where you are required to use the buttons on the joystick you should use the InterruptIn API and
attach appropriate callbacks. You are likely to need to attach different callbacks depending on the
required functionality of the button, which will change depending on the program state;
alternatively, you may have a single callback which checks the existing state of the program in order
to determine what the next state should be. Each approach is equally valid.
Your final program should display the same functionality as that shown in the example video on
Blackboard. Marks will be awarded for smooth operation of the program. So please ensure that
updates to the LCD screen are free from flickering, LEDs illuminate correctly, and button actions
perform the correct actions in each state.
Marking Requirements
The program required for this task demonstrates a significant amount of functionality, including user
input and a variety of outputs including LEDs and the LCD screen. As such the marking requirements
dictate that in order for full marks the final program should show all of the functionality described in
the video, as well as the essential criteria documented below:
The program should contain:
• The Class SamplingPotentiometer which should inherit from the Potentiometer class. This
class should automatically sample the analog inputs at periodic intervals and store sampled
values as data members of the class. The latest samples should be accessed by member
functions, as described above. You should take care to select an appropriate sampling
frequency.
• An object of type Ticker in the SamplingPotentiometer class.
• A class Clock which represents the functionality of a 24-hour clock in the format HH:MM:SS.
• A world clock which implements at least 20 time zones from -11 to +12 hours relative to
GMT. The ‘home’ city must be in the GMT time zone and can be a city of your choosing –
either ‘Manchester’ or ‘London’ are suggested but you are free to select another city in this
time zone. At least one city must have a non-integer offset e.g. +5h30 mins. You may choose
your cities from the list available on Blackboard.
• Objects of type InterruptIn for buttons of the joystick
• An object of type C128**. This will need to be imported – please see the guide on
Blackboard for using the LCD screen.
• Objects of type LED to allow for the illumination of LEDs that matches those described in this
document. You may also use your RGBLED class from task 3 if you wish.
• A number of ISRs/Callbacks which are called when your interrupt events occur (e.g. button
presses or timer events). These ISRs should be as efficient as possible and should be used to
change program state, or update global variables.
• You will need to declare some of the above objects outside of the main() function so that
they are in scope for your ISRs. You will need to carefully think about which of these will be
needed, and to appropriately justify all global variables that are used in your program.
• Implementation of a state machine that is able to represent the various states that your
program is in e.g. Initialisation, ClockSet, WorldTime etc.
Your main function, int main(), should contain code capable of:
• Declaring all necessary objects and variables which are only needed in the main function
• Implementing a state machine which can be used to control the flow of the program
• Setting the program in a suitable initial state
• Recovering from undefined states that might occur through the implementation of a
‘default’ case e.g. returning to the default state
Important notes (continued on the next page):
You will be expected to fully explain how your program works in order for marks to be awarded.
There will be marks available for your ability to answer questions relating to your program’s
implementation.
Your program must compile to be assessed. If you cannot implement the entire functionality without
compilation errors then consider if you can implement some of it in a stable version of the program
so that you can gain some marks.
Important Notes (continued):
The implementation of this program is expected to differ significantly between students. As such all
students must upload their source code via Turnitin for plagiarism checking. Lab marks will be
provisional pending this plagiarism check, and any suspected cases of academic malpractice will be
referred to the Department for investigation. There is a guide available on Blackboard which details
this process; please ensure that you are familiar with it before the lab session, and check with staff in
the assessment labs if there are any problems.
This is an in-lab assessment, and no late submission will be permitted. If you do not submit your
work for assessment in your scheduled lab, AND correctly upload your code for plagiarism
checking before the end of the session you will receive a mark of zero for this task.
A suggested workflow is provided below which you may wish to follow to ensure that this program is
implemented in logical stages. Though you may wish to undertake this task in a different way that
makes best sense to you:
1. Implement the class for a SamplingPotentiometer and verify that this works in its own
program.
2. Implement a basic clock that starts from 00:00:00 and counts sequentially.
3. Add code for setting this to arbitrary values and test the clock counts correctly from there.
4. Implement either the countdown timer or stopwatch timer in a program on its own.
5. Experiment with a simple state machine e.g. that uses a button to transit between
different states with a clear output. For example, illuminating a different LED for each
state or displaying a different message on the screen.
6. At this point you should write a brief plan of what your program needs to do. Usually for
state machines a flowchart is appropriate, but any alternative method would be suitable.
This plan will make it much easier to implement the program in stages and test more
gradually.
7. Bring together the clock, and timing functionalities into distinct states.
8. Add the more advanced features.
9. This is a complicated program, and you are strongly recommended to build it up in stages
and debug each stage separately. You are likely to find this task very hard to accomplish if
you develop large parts of it at once and don’t test in between, as it will be hard to track
down where errors are occurring.
This task is the most difficult of the four tasks. The main advice is to take things in stages,
and to remember that you can get marks for achieving some of the criteria, without having
to achieve them all. If you can’t do the full program perfectly, then do as much of it as you
can, as well as you can.
Application Shield Schematic
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