1. Charles Moyes (cwm55) and Shentong Wang (sw477) Lab 5 Write-Up
NOTE: A YouTube video demo of our project can be viewed at www.youtube.com/watch?v=jYAYAhgnLdQ
Lab 5 Write-Up: MSP430 Microcontroller Craps Game
Project Overview
The idea was to implement a playable game of Craps (casino dice rolling game) using the MSP430 mi-
crocontroller board. External LEDs that were arranged in dice shaped format are used to show the
values of the dice rolls. A basic (yet detailed) description of the game rules can be found at http:
//en.wikipedia.org/wiki/Craps.
A more thorough explanation can be found in the How To Play Craps section. Current limiting
resistors are connected across the LEDs and the digital I/O pins in series to ground. The push button is
used to trigger dice rolls. The Charlie-plexing LED multiplexing scheme is used to connect lots of LED’s to
the micro-controller without running out of pins. Red and green LED’s on the breadboard indicate whether
the “Pass” (green LED lit) or the “Don’t Pass” (red LED lit) betters win the round. The built-in green
and red LEDs on the MSP430 board indicate whether the round is in the “come out” or “point” stage. The
Craps embedded system acts as dice for each player in the game, and as such, is passed around the table to
each player who wishes to be a “shooter.”
How To Play Craps
Craps is an easy-to-learn betting game played with two dice. The players bet “pass” or “don’t pass” (whether
the dice roller will win or lose). The player who rolls the dice is designated the “shooter.” The shooter begins
rolling. If the first roll is a 2, 3 or 12, the shooter loses (“craps”) and the “don’t pass” betters win the round.
On the other hand, if the first roll is a 7 or 11 (a “natural”) then the shooter wins and the “pass” betters
win the round. If the first roll is any other number (the “point”), then the shooter continually rolls the dice
until that number is rolled again (win!) or the shooter rolls a seven (lose!). The first roll is the “come out”
roll, while all others afterwards are “point” rolls.
Playing the game using the electronic device we designed is very easy. Simply press the push button on
the microcontroller and the dice roll will illuminate on the LEDs. If the dice roll wins, the green status LED
lights up and a happy song is played on the piezoelectric speaker. If the dice roll loses, then the red status
LED lights up and a sad song is played. If the round continues with another dice roll, then a chirp sound is
emitted, and the game waits for the player to press the push button again.
Game Logic Finite State Machine (FSM)
The game logic was easily represented by a finite state machine with several states, transitions, and variables
as shown in the following diagram:
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2. Charles Moyes (cwm55) and Shentong Wang (sw477) Lab 5 Write-Up
The game state variable stores 0 if the round is in the “come out” phase and 1 if the round is in the
“point” phase. The state led variable stores a 0 if nobody has won or lost, 1 if the shooter has won (pass),
and 2 if the shooter has lost (don’t pass). Lastly, the point variable stores the value of the point dice roll
that must be rolled again before a seven in order to win the round. Initially, state led = game state =
point = 0, and if an unknown state configuration comes up during gameplay, then the FSM resets to this
safe initial state.
The “come out” states (for game state == 0) are a first-roll lose state, a first-roll win state, and a point
state that sets game state to 1 and advances onto the “point” round. The “point states” are a win state
(for rolling the point again), a lose state (for rolling a seven), and an anything else state that allows the
round to continue in the “point” game state. Transitions between these states are governed by the value of
the dice roll, the point, and the game state.
Pseudo-Random Number Generator
A pseudo-random number generator will be used to determine the values of the dice rolls in software. The
Linear Congruential Generator algorithm based on modular arithmetic was selected due to its “randomness”
and its ease of implementation:
Xn+1 ≡ (aXn + c)(mod m)
where Xn is the sequence of values generated and m, a, c, and X0 (the so-called seed) are constants. Since
16-bit numbers are used, we chose values of m = 216 , a = 16, 807, and c = 33, where c and m are necessarily
relatively prime. Because the microcontroller board lacks a built-in real-time clock, an un-initialized garbage
value in memory is used instead to seed the random number generator (X0 ). Hardcoding a random number
generator seed into the code would make playing the game much less interesting since upon rebooting, all
games would have the same sequence of dice rolls and thus would not be random.
Project Construction
Project construction was accomplished using a breadboard and external electronic components that interface
with the digital I/O pins on the microcontroller breakout board. Bruce Land assisted us in soldering low-
profile headers onto the breakout board pins. From there, we wired a piezoelectric speaker across on of
the pins to the ground pin (pin 12). From reading its data sheet, we found that no limiting resistor was
necessary for the piezoelectric speaker. The LED’s were then wired to the board using a Charlie-plexing
LED multiplexing scheme described in the next section. Limiting resistors were calculated using Ohm’s Law
(180-220 Ohm resistors were used) for the specifications given in the LED data sheets. The overall allocation
and usage of I/O pins is shown in the following table:
Break-out Board Pin Microcontroller Pin Usage
5 P2.2 Pin 1 of Red Dice LED Charlie-plex Configuration
6 P2.3 Pin 3 of Red Dice LED Charlie-plex Configuration
7 P2.4 Pin 2 of Red Dice LED Charlie-plex Configuration
8 P4.3 Pin 1 of Yellow Dice LED Charlie-plex Configuration
9 P4.4 Pin 2 of Yellow Dice LED Charlie-plex Configuration
10 P4.5 Pin 3 of Yellow Dice LED Charlie-plex Configuration
11 P4.6 Middle LED of Red Dice Display
12 GND Ground for Piezoelectric Speaker
13 P2.6 Piezoelectric Speaker (+) Terminal
14 P2.7 Middle LED of Green Dice Display
15 Unused –
16 Unused –
17 P3.0 Green “Win/Pass” Status LED
18 P3.1 Red “Lose/Don’t Pass” Status LED
Note that break-out board pins 1-4 were inaccessible because the plastic enclosure of the USB programmer
dongle covered them up.
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3. Charles Moyes (cwm55) and Shentong Wang (sw477) Lab 5 Write-Up
Charlie-plexing LEDs
This project uses 16 external LEDs, but there are a limited number of I/O pins available on the microcon-
troller breakout board. This technique allows n pins to drive up to 2 · n = n(n − 1) LED’s. Moreover, only
2
n limiting resistors are needed. Each unique pair of I/O pins in the configuration supports two LED’s (one in
the forward direction and one in the reverse direction) in series. This pair is referred to as a “complementary
pair” and either LED in the pair can be lit by setting one I/O pin to Ground and the other to +5V. By
disconnecting all other I/O pins in the configuration, any one LED in any of the complementary pairs can
be selectively lit. Disconnecting an I/O pin can be accomplished by setting that pin to be an input pin using
the appropriate DIR register. This setting puts the pin in a high-impedance state. A schematic illustrating
the general idea follows:
In our project, two Charlie-Plexed configurations of 6 LED’s (using three I/O pins each) are used for
the two dice displays. Moreover, the seventh center LED is connected to a separate I/O pin across ground
without any multiplexing scheme. Each I/O pin has a limiting resistor connected in series to it. The
complementary pairs are connected in parallel on the breadboard. In order to light multiple LED’s in one
dice display at a time, cycling is used where each simultaneously-lit LED is toggled on and off one-by-one at
a high refresh rate. If the refresh rate is high enough, this technique gives the illusion that multiple LED’s
are being lit at the same time.
Piezoelectric Speaker
A piezoelectric speaker is used to play audio feedback during the game. Musical note frequencies are deter-
mined from the equal-tempered scale (tabulated at http://www.phy.mtu.edu/~suits/notefreqs.html).
A look-up table of compiler-time definitions stores these frequencies. Pulse width modulation (PWM) is
used to send a 5V amplitude square wave to the piezoelectric speaker from a digital I/O pin. By varying
the frequency of the square wave signal, different pitches can be played from the speaker. Songs are simply
sequences of notes (of varying time durations) with periods of silence in between. Sequential code can be
written to play notes at specific frequencies for certain durations and to add silences in between notes. As
an example, the code to play the “game win” song follows:
// P l a y s a happy song t h a t t e l l s t h e p l a y e r t h a t t h e y won
void playWin ( )
{
beep ( c , 3 5 0 ) ;
beep ( e , 3 5 0 ) ;
beep ( g , 3 5 0 ) ;
beep ( cH , 5 0 0 ) ;
}
Code Overview
A brief overview of the code written in the C programming language for this project follows. Note that the
code is heavily documented using lots of comments. The main.c file contains an intro() function that does
a light-up demo where the “Imperial March” song is played from Star Wars on the piezoeletric speaker. The
main() function sets up the I/O pins, interrupts, and enters the game loop. The Port 1 interrupt handler
catches the push button interrupts.
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4. Charles Moyes (cwm55) and Shentong Wang (sw477) Lab 5 Write-Up
The speaker.c file contains piezoelectric speaker code. The beep function plays a tone at a specified
frequency on the speaker using digital PWM. The playGoAgain(), playLose(), playWin(), and play()
functions all play various songs on the speaker using beep and appropriately spaced delays. Note that
some of the code here (particularly beep and the Imperial March song code) was borrowed from http:
//processors.wiki.ti.com/index.php/Playing_The_Imperial_March.
The util.c file contains code to delay the MSP430 using delay cycles and random(), a Linear
Congruential random number generator described in the earlier section.
The leds.c file contains code to light specific LEDs in the red and yellow Charlieplex configurations
(lightLED() and lightLED2() functions respectively). The lightDiceNum() function is used to show a
particular dice face pattern (for a die value of 1-6) on one of the dice displays by cycling through the LEDs
in the configuration at a high refresh rate. The lightStatusLEDs() function controls the red and green
status LED’s on the board and on the microcontroller (for communicating game state information to the
player). Lastly, the displayDice() is a utility function lights up the two dice displays with a given dice roll
by calling lightDiceNum() with the appropriate parameters.
The game.c file contains all of the game logic. The updateGameState() function implements the finite
state machine (FSM) described in an earlier section. This function encodes all of the game rules for Craps
and is heavily commented. The gameLoop() function updates the dice LED displays by cycling through them
once and then updates the status LED displays. It is continually invoked throughout program execution.
Lastly, the buttonPressHandler() push button handler invokes the game state FSM to update the game
state during a new dice roll. For more information about the code, feel free to look through it and read our
comments.
Conclusion/Photo/Demo Video
In conclusion, a fun and playable game was implemented using a microcontroller. The project was a rich and
worthwhile hands-on learning experience. We learned about Charlie-plexing, practiced soldering, assembled
a circuit using a breadboard, debugged a circuit using a multimeter, calculated appropriate resistances for
components such as LEDs, and got to plan, design, and create a tangible electronic gadget. Many hours of
work in the lab went into implementing and debugging this project, but the work was very fulfilling and the
end result was great!
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