Build your own Arduino retro handheld game console

This project is maintained by sy2002

TeleBall - Software


The newsest stable release version is TeleBall Version 1.4, you will also find the release notes here.

TeleBall is implemented in a C / C++ mixture (more C than C++) as this is rather typical for Arduino projects. You need the Arduino IDE to get started.

Folder Structure

All sources are located below the src folder, which contains the following file and folder structure:

archive         Historic TeleBall software releases
TeleBall        As per Arduino convention, the "Sketch" that contains
                the source code is located inside an own folder that
                has the same name as the Sketch itself: TeleBall.
                Compile TeleBall.ino and upload it to create TeleBall's
                firmware.   This ZIP contains the 3rd party libraries that
                TeleBall.ino needs to compile in a proven-to-work


TeleBall is using two 3rd party libraries. As mentioned above, the GIT repository contains a ZIP with versions of these libraries that are known to work with TeleBall. We strongly recommend that you use these versions. If you prefer to use newer versions, you can find them here:

Instructions for how-to install these libraries are here.

Flashing the Firmware

  1. Install the Arduino IDE and the libraries (as described above)
  2. Connect TeleBall via USB
  3. Double-click TeleBall.ino or open it manually in the Arduino IDE
  4. Select the right Serial Port in the IDE, this can be a bit tricky also depending on the actual board you are using (original or clone): You need to find and install the right USB-to-serial driver.
  5. Choose File/Upload or press the “=>” icon: The upload should start
  6. After a successful upload, TeleBall starts in BreakOut mode.

Hint: USB is not strong enough to power TeleBall. Particularly the analog inputs are not calibrated correctly when running on USB. That means: USB power is good enough to check, if flashing the firmware (uploading the firmware) worked, but not for much more.

Main Ideas

The software implementation of TeleBall relies on three main ideas:

TeleBall works a lot with global variables and constants as we are in a single instance scenario and therefore encapsulation is not necessary.

Heartbeat of TeleBall

The whole TeleBall application is written in a way, that in (nearly) no function or situation we have a blocking state. That means, that the main loop (Arduino’s loop() function) is always running, i.e. called again and again, like a heartbeat. This leads to:

The main loop is essentially executing the following steps:

  1. Check if a reset is to be performed and execute it, if necessary
  2. Check if another player/device is there for Tennis for Two, if yes: initiate the pairing process
  3. Read and handle input from the potentiometer and the button (called Universal Button)
  4. Call a Game Mode handler, which itself is a small main loop on its own
  5. Check and handle timeout
void loop()
    if (perform_reset)
    //check for other player/device and set the variable RadioMode,
    //i.e. no radio connection, act as master or act as slave
    //handle various input ports and the orientation
    switch (game_mode)
        case gmBreakOut:    playBreakOut();        break;
        case gmTennis:      playTennis();          break;
        case gmSpeed:       adjustSpeed();         break;
        case gmBrightness:  adjustBrightness();    break;   
        case gmPaddleLeft:     
        case gmPaddleRight:

        case gmEEPROM:      manageEEPROM();        break;            
    //if radio is active (aka if tennis), time out, e.g. because the other
    //device was switched off or it was moved out of transmission range, etc.
    if (RadioMode > rmNone && millis() > RadioTimedOut)
        game_mode = gmTennis;           //make sure, that tennis is reset, when calling reset() ...
        reset();                        //... game_mode could be config modes like gmSpeed, etc.
        tennisSwitchToBreakOut(false);  //switch to BreakOut but continue to try to find another device for tennis

Game Mode State Machine

The GameMode models the macro state the device is currently in: Are we playing BreakOut or Tennis for Two? Are we currently in any configuration menu?

enum GameMode
    //standard configuration menu
    //advanced configuration menu
    //EEPROM: store current settings or reset to default

There is a global variable called game_mode which reflects the current state of the game. GameMode is modeled as a classical state machine (FSM), whose state progressing is like described in the following; b(x) means button pressed as long as x:

[gmBreakOut | gmTennis] => b(1sec)    => [gmSpeed]
[gmSpeed]               => b(quickly) => [gmBreakOut | gmTennis]
[gmSpeed]               => b(4sec)    => [gmBrightness]
[gmBrightness]          => b(quickly) => [gmPaddleLeft]
[gmPaddleRight]         => b(quickly) => [gmEEPROM]
[gmEEPROM]              => b(quickly) => [gmBreakOut | gmTennis]

[gmBreakOut]            => successful pairing   => [gmTennis]
[gmBreakOut]            => unsuccessful pairing => [gmBreakOut]
[gmTennis]              => timeout              => [gmBreakOut]

The button-press related state changes are done in the function readUniversalButton(). All other state changes are scattered all over the code, because changing the Game Mode is an essential thing to do that can be necessary in a lot of situations.

Master/Slave Concept in Tennis for Two

Tennis for Two is implemented using a Master/slave concept. That means, that as soon as the Pairing finished successfully (see below), one device will become the master and one the slave:

This approach has the big advantage that no additional device synchronization is necessary - it is implicitly achieved due to the fact that the slave is doing nothing without the master. The master/slave approach also fits best to the radio chip’s architecture: The nRF24L01+ can not act as a real bi-directional transceiver, instead the recipient of a data package (slave) is able to send payload inside the acknowledge package to the sender (master).

Common Mechanisms

“Ball Physics”

As we only have 8x8 pixels, implementing real ball physics is near to impossible. This is why TeleBall is doing it according to the following rules:

The current state of the ball is comprised of the X and Y position stored in BallX and BallY as well as the relative ball speed in both directions, stored in BalLDX and BallDY. All of the he above mentioned mechanics are implemeted by the following two functions, that are used within BreakOut as well as within Tennis for Two.


There are multiple situations in BreakOut as well as in Tennis for Two, where the ball is stopped for a while and “waiting”, before the game continues. Some examples are: at the beginning of the game; when loosing a ball before the new game begins; after switching to Tennis; after switching back from a configuration menu to the game; etc.

The waiting is implemented by setting the velocity vector (BallDX | BallDY) to zero. Obviously calculateBallMovement(...) is then only adding zeros to the ball’s position and the ball is waiting / standing still.

At each playBreakOut() and playTennis() game loop iteration, the function respawn_management() is called. It checks, if a certain timeout period set in the variable respawn_timer is over and if yes, it sets the velocity vector (BallDX | BallDY) to (BallDX_tbs | BallDY_tbs); the suffix _tbs stands for “to-be-set”. This “respawn management” mechanism is not only used at actual respawns (i.e. a ball was lost), but also to pause the game, as shown in pauseGame(...).

Mirrored X-Axis

Due to the way how we wired the MAX7221 to the KWM-R30881CBB, where the rows are the cathodes, we need to mirror the x-axis. This is why putPixel(...) is doing the following: `Matrix.setLed(0, 7 - x, y, on)

Saving SRAM

The Arduino Nano only has about 2kB of SRAM, so it makes sense to use the program memory for storing constant values. This is done by the PROGMEM keyword. Data retrieval is done by specialized functions having the _from_PROGMEM suffix.

Hint: Declaring a global variable const is not enough to move it to the program memory because the ATmega328 CPU needs special machine commands to read data from there.

EEPROM Memory Layout

Upon each device start, the TeleBall device fingerprint is checked. If it is there, then TeleBall assumes valid data for the other variables. Otherwise it assumes a factory new device and initializes the EEPROM memory with factory default values.

Bytes Type Value
00..07 chars TeleBall device fingerprint:
{‘T’, ‘e’, ‘l’, ‘e’, ‘B’, ‘a’, ‘l’, ‘l’};
08..09 unsigned int Ball speed
10 byte Display intensity
11..12 unsigned int Leftmost poti position
(calibrate paddle user experience)
13..14 unsigned int Rightmost poti position
(calibrate paddle user experience)


Game Loop

BreakOut’s game loop is part of the main loop() as it is called from the inside the switchstatement there. playBreakOut() must not disturb the “heartbeat”, therefore also no function there shall be blocking. An excpetion is the case, when a player wins or looses the game. Here, the game loop comes to a halt and the programm is looping in a tight while loop.

void playBreakOut()
    //draw all playfield elements
    drawPattern(bricks, bricks_levelheight[Level]);
    handlePaddle(Paddle, Paddle_Old);


    //calculate ball movement
    //the ball movement is de-coupled from the paddle movement, i.e. a lower ball speed
    //does NOT lower the paddle movement speed    
    if (!Timer)
        Timer = millis();
    else if (millis() - Timer >= Speed)
        Timer = 0;
        calculatePaddleImpact(Paddle, LastPaddlePos, PaddleHit_Bottom);    
    if (Balls != Balls_Old)
        Balls_Old = Balls;

It is worth mentioning, that the movement of the paddle is asynchronously decoupled from the movement of the ball: handlePaddle(...) is called with the maximum speed that the Arduino can run through the game loop. On the other hand, calculateBallMovement(...) and calculatePaddleImpact(...) is only called, when an amount of milliseconds specified by the variable Speed has passed since the last ball movement. This mechanism ensures, that the paddle movements feel natural to the user, independent from the game’s current speed.

Adding or Modifing BreakOut Levels

  1. Adjust the const byte Levels to the actual amount of levels that you have.
  2. For each level, add a structure with 1 and 0 as shown below. Each 1 represents a brick and each 0 is an empty space.
  3. For each level, adjust the bricks_levelheight array to reflect the amount of lines / rows that the actual level is having.
  4. Currently, 4 is the hardcoded maximum height of a level. If your level has a smaller amount of rows, then fill it with 0 as shown in the below-mentioned code snippet's first level. We do not reccomend to have a larger amount of rows than 4`.
const byte Levels = 3;
const byte bricks_levelheight[Levels] = {3, 4, 4};
const byte bricks_reset[Levels][4][8] PROGMEM =
        {1, 1, 1, 1, 1, 1, 1, 1},
        {1, 1, 1, 1, 1, 1, 1, 1},
        {1, 1, 1, 1, 1, 1, 1, 1},
        {0, 0, 0, 0, 0, 0, 0, 0}
        {1, 0, 1, 1, 1, 1, 0, 1},
        {1, 1, 0, 1, 1, 0, 1, 1},
        {0, 1, 1, 0, 0, 1, 1, 0},
        {0, 0, 1, 1, 1, 1, 0, 0}
        {1, 1, 1, 1, 1, 1, 1, 1},
        {1, 1, 1, 1, 1, 1, 1, 1},
        {1, 1, 1, 1, 1, 1, 1, 1},
        {1, 1, 1, 1, 1, 1, 1, 1}

Tennis for Two

Device Pairing

In the first two minutes after turning the device on - or to be more precise: until the milliseconds counter of the Arduino reaches the value specified by the global constant RadioPowerSaveTime, TeleBall is scanning for another device / another player, that could be a potential partner for Tennis for Two. After that time period, for saving battery power, the radio device is put into a low-power mode (switched-off).

The scanning itself has two functions: Find another device and determine, who is master and who is slave. It works like this:

  1. Wait a random amount of time as specified by RadioWait_Min and RadioWait_Max; currently this is between 500 and 850 milliseconds.

  2. Check if in the meantime - between the last check and the random wait time - a package with the Master Token arrived. If yes, then there is another device: We acknowledge (ACK) the Master Token with the Slave Token, which means that we are now the slave and the other device is the master and we continue with step (4).

  3. If no Master Token was recieved, the radio chip is switched into sending mode and we are sending a Master Token. If we receive a Slave Token as ACK then we are now the master and the other device is the slave and we can continue with step (4). Otherwise we switch back the radio chip into receive mode and go back to step (1).

  4. Switch the global game_mode to gmTennis, which means that from now on the Tennis game loop will run and that the master / slave state machine will run.

  5. Ask each user, if he wants to play Tennis for Two, by showing a “?” and let the user answer the question by turning the paddle. Right means yes and shows a “!” and left means no and shows a “?”. If no: Stop scanning for other TeleBall devices by setting RadioMode to rmIgnore and go back to BreakOut. This functionality is performed in tennisHandleMultiplayerQuestion().

  6. As soon as a player answers “!”, i.e. yes, the state machine takes care, that the loop executes tennisWaitForOtherPartyToJoin(), which does what the name suggests. Additionally, it implements a timeout, so that after ~9 seconds of no answer, the device goes back to BreakOut mode and also stops scanning via rmIgnore.

  7. If both players accept, the Tennis for Two game is started.

The above mentioned master and slave tokens are technically 32bit numbers that are unique within the TeleBall radio protocol.

Note, that this whole Device Pairing process happens kind of asynchronously, i.e. the whole operation is not blocking but part of the main loop() via the radioScanAndDetermineMode() function and part of the playTennis() game loop via “Radio Modes” (see below).

This algorithm, as well as the way the current whole TeleBall firmware talks with the radio device is not capable of handling more then two TeleBall devices in radio range; the behaviour would be undefined.

Radio Specifics

Radio Configuration

TeleBall is currently hard coded to “channel 76”. The nRF24L01+ has a channel width of 1MHz on the 2.4GHz band, i.e. we are sending on 2.476GHz. Due to the fact, that the 2.4 GHz band is pretty congested (WLAN, BlueTooth, microwave ovens, etc.), we should implement some smarter behaviour and also in general there is definitly room for improvement, when it comes to the current shortcomings of TeleBall’s radio range as described in Play: We could reduce the transmission speed, increase retries on failed sends, use a better CRC mode and so on. Future versions of TeleBall will/should implement this.

For establishing links between two radios, nRF24L01+ works with so called “pipes”. Pipes have addresses and only, if two pipes are having the same address, the sender and the recipient can communicate with each other. Also this property is hardcoded; in TeleBall’s current firmware, everybody is sending into and listening to a pipe called TELEB, i.e. having the 5-byte address that corresponds to the ASCII code of TELEB.

The payload size as well as the ACK payload size (see below) is set to 4 bytes length. In the code, we map this to unsigned long int values that we send and receive.

FIFO Buffers

The radio hardware is working fully asynchronously from the Arduino by using a send FIFO buffer (TX FIFO) and a recieve FIFO buffer (RX FIFO). When the connection is established, the FIFOs will be filled in the background and TeleBall’s firmware code can check, if for example a new package arrived in the RX FIFO or if there is still something in the RX FIFO which means, that the recepient did not ackknowledge, yet for a multitude of reasons.

The TeleBall implementation of the receive function radioReceive(...) is currently always emptying the RX FIFO which means, that messages from the master can get lost not only due to radio problems but also due to timing problems. Also, there is no TX FIFO for ACK messages built into radioReceive(...), so also messages from the slave to the master can get lost. This opens room for improving the communication / radio performance and robustness of TeleBall.

Unidirectional Communication

As described above, our radio chip, the nRF24L01+, is not offering a real bidirectional communication mode. But it offers an advanced unidirectional communication called Enhanced Shockburst. The most important feature of it for our purposes is the fact, that the sender gets not only an acknowledge (ACK) from the recipient, that his package was well received, but he also can get payload within the ACK packet.

In our master / slave model, the master is always the sender and the slave always the recipient. In other words: The slave can never directly send something to the master, he can only wait for receiving something and then use the ACK payload to send to the master. So the master always needs to poll for the slave’s data and the slave always needs to be listening if a new package from the master arrives.

In normal game operation, we are using two global 32bit (4 byte) unsigned long int values to implement this: RadioGameDataFromMaster is sent from the master to the slave and RadioGameDataFromSlave is the ACK payload package that is sent from the slave to the master. Both variables are implemented as bitfields, so that we can conveniently treat these 32bits / 4 bytes like a struct.

In other operation modes, e.g. during Pairing or during configuration, the nature of the 4 byte packages changes to “Tokens” (as described above) or to simple unsigned ints like in Speed Set mode.


The two main functions for implementing our pseudo-bidirectional communication aka sender pools and recipient answers via ACK payload are: radioSend(...) and radioReceive(...).

Both functions are resetting the timeout counter on success, that means that they need to be called very regularly, otherwise the timeout function of the main loop will be activated. Therefore, even when “nothing is happening”, master and slave need to communicate with each other. This is why for example in Tennis for Two even the “won or lost smileys” are not leaving the main game loop: Communication is kept alive and therefore the timeout will not happen.

Implementing the Master / Slave Concept

Radio Mode State Machine

The master / slave mechanism is implemented using a second state machine on top of the Game Mode state machine: Radio Mode. The basic Radio Mode states are handled in the main game loop of tennis called playTennis(). Other Radio Modes are handled at adjustSpeed() and at places scattered all over the code, similarly to the Game Mode.

    //some checks are done via "> rmNone", i.e. it is important that the modes are in order
    rmIgnore                    = -2,   //do not scan any more, but ignore other TeleBall devices
    rmNone                      = -1,   //no other TeleBall device found
    //initial find devices and ask the question
    rmMaster_init               = 0,    //about to enter Master mode
    rmSlave_init                = 1,    //about to enter Slave mode
    rmMaster_wait               = 2,    //waiting for Slave to start the game
    rmSlave_wait                = 3,    //waiting for Master to start the game
    //standard run mode
    rmMaster_run                = 4,    //running in Master mode    
    rmSlave_run                 = 5,    //running in Slave mode
    //special modes for reset and speed adjustments
    rmMaster_reset              = 6,    //Master device resetting
    rmSlave_reset               = 7,    //Slave device resetting
    rmMaster_speedset_by_Master = 8,    //Master device in speed set mode
    rmMaster_speedset_by_Slave  = 9,    //Master device listening to Slave's speed set
    rmSlave_speedset_by_Master  = 10,   //Slave device listening to Master's speed set
    rmSlave_speedset_by_Slave   = 11    //Slave device in speed set mode
} RadioMode = rmNone;

The following state changes are for the sake of simplicity shown by only using the master as an example. They work the analogical at the slave’s side:

[rmNone]         =>  found slave     => [rmMaster_init]
[rmMaster_init]  =>  answered: yes   => [rmMaster_wait]
[rmMaster_init]  =>  answered: no    => [rmIgnore] & game_mode = gmBreakOut
[rmMaster_wait]  =>  other joins     => [rmMaster_run]
[rmMaster_wait]  =>  timeout: user   => [rmIgnore] & game_mode = gmBreakOut
[rmMaster_run]   =>  b(quickly)      => [rmMaster_reset]
[rmMaster_run]   =>  b(1sec)         => [rmMaster_speedset_by_Master]
[rmMaster_run]   =>  slave: SpeedSet => [rmMaster_speedset_by_Slave]
[rmMaster_reset] =>  master: reset   => [rmMaster_run]
[<any state>]    =>  timeout: radio  => [rmNone] & game_mode = gmBreakOut

[rmMaster_speedset_by_Master] => b(quickly) => [rmMaster_run]

[rmMaster_speedset_by_Master] => b(4sec)
                              => [rmMaster_speedset_by_Master] & 
                                 game_mode cycles through:
                [gmBrightness]          => b(quickly) => [gmPaddleLeft]
                [gmPaddleRight]         => b(quickly) => [gmEEPROM]
                [gmEEPROM]              => b(quickly) => [gmTennis]

[rmMaster_speedset_by_Slave] => slave: Flag_Leave => [rmMaster_run]

Explanation of state changes:

Different Game Loops for Master and Slave

The master / slave concept implies that both devices are functioning completely differently, as already described above. In short: The master “does / calculates everything” and the slave is just a “dumb display device”. This is reflected in different game loops for master and slave.

Pseudocode for Master Game Loop tennisPlayMaster():
  1. Transform own coordinates into the coordinate space of the slave
  2. Transmit control structure, including mirrored coordinates, current score of slave, flags (won or lost? reset? speed set?), sound commands (wall, paddle, lost)
  3. Send RadioGameDataFromMaster and recieve RadioGameDataFromSlave
  4. Handle won or lost state
  5. Draw: ball, local paddle, remote paddle
  6. Handle special requests from the slave: reset, speed set mode
  7. Calculate the ball movement
  8. Calculate the paddle impact: for the master and for the slave paddle
  9. Check for lost ball
  10. Issue sound commands during (7), (8) and (9)
  11. Handle ball respawn
Pseudocode for Slave Game Loop tennisPlaySlave():
  1. Draw own paddle
  2. Transform paddle coordinates into the coordinate space of the master and fill the ACK data structure that is used to transmit these coordinates during receiving
  3. Receive RadioGameDataFromMaster and send RadioGameDataFromSlave via ACK
  4. Copy the ball’s coordinates and movement vector plus other data into local variables
  5. Handle won or lost state
  6. Draw the ball and the remote paddle and play sounds
  7. Handle special requests from the master: reset, speed set mode

Identifying the Master and the Slave Device

The current version 1.3 of the TeleBall firmware shows during the first two seconds of the “question mode” (in Device Pairing), while the “?” displayed statically, if a device is a master or a slave device:

Adding More Games

TeleBall’s current firmware is only using about half of the program memory of the Arduino Nano. That means there is plenty of room left for software extensions. As an inspiration for you, here are simple games, whose gameplay can be kind of adapted to the 8x8 LED display:

If you want to add more games we suggest, that you do it like described here:

  1. Use the concept of Multi File Sketches to stay updatable: If you put the main code of your new games to separate files with no file extension, e.g. you add the file MyNewGame (no file extension) to the folder src/TeleBall, then you are able to use everything from this file inside the main file TeleBall.ino. Even though you will need to touch TeleBall.ino, this way of working minimizes the code merging in future. You might need to declare function headers (one liners) in each of the files so that the compiler finds everything.

  2. Add a new mode to the GameMode enum as shown above and extend the main loop’s switchstatement by a new case, something like gmMyNewGame. Be mindful, that no function should “block”, so if something takes longer, implement it asynchronously. Furtheron, add a custom reset routine and add it to the switch statement in reset(). Last but not least, write some routines that are able to backup and to restore your game state and call them inside backupGameState() and restoreGameState().

  3. Exending the behaviour of readUniversalButton() seems to be a good place to add the entry point for your new game. Alternately or additionally, you can change game_mode_default to let TeleBall start with your new game instead of with BreakOut.

  4. There is plenty of room left in the EEPROM, too, so it is safe to create new loc... constants and to extend eepromWriteFingerprintAndDefaults(), eepromReadSettings() and eepromWriteSettings() accordingly. We suggest you keep to the convention and use the suffix _defaultfor factory default const variables.