gba-link-connection

A set of Game Boy Advance (GBA) C++ libraries to interact with the Serial Port. Its main purpose is providing multiplayer support to homebrew games.

• 👾 LinkCable.h: The classic 16-bit Multi-Play mode (up to 4 players) using a GBA Link Cable!
• 💻 LinkCableMultiboot.h: ‍Send Multiboot software (small 256KiB ROMs) to other GBAs with no cartridge!
• 🔌 LinkGPIO.h: Use the Link Port however you want to control any device (like LEDs, rumble motors, and that kind of stuff)!
• 🔗 LinkSPI.h: Connect with a PC (like a Raspberry Pi) or another GBA (with a GBC Link Cable) using this mode. Transfer up to 2Mbit/s!
• 📻 LinkWireless.h: Connect up to 5 consoles with the Wireless Adapter!
• 🌎 LinkUniversal.h: Add multiplayer support to you game, both with 👾 Link Cables and 📻 Wireless Adapters, using the same API.

(click on the emojis for documentation)

Usage

• Include the library you want (e.g. LinkCable.h) in your game code, and read its comment for instructions.
• Check out the examples folder
  • Builds are available in Releases.
  • They can be tested on real GBAs or using emulators (NO$GBA, mGBA, or VBA-M).

Makefile actions (for all examples)

make [ clean | build | start | rebuild | restart ]

👾 LinkCable

(aka Multi-Play Mode)

This is the Link Port mode that games use for multiplayer.

The library uses message queues to send/receive data and transmits when it's possible. As it uses CPU interrupts, the connection is alive even if a console drops a frame or gets stucked in a long iteration loop. After such event, all nodes end up receiving all the pending messages, so a lockstep communication protocol can be used.

Constructor

new LinkCable(...) accepts these optional parameters:

Name	Type	Default	Description
baudRate	BaudRate	BAUD_RATE_1	Sets a specific baud rate.
timeout	u32	3	Number of frames without an II_SERIAL IRQ to reset the connection.
remoteTimeout	u32	5	Number of messages with 0xFFFF to mark a player as disconnected.
bufferSize	u32	30	Number of messages that the queues will be able to store.
interval	u16	50	Number of 1024cycles (61.04μs) ticks between messages (50 = 3.052ms). It's the interval of Timer #sendTimerId.
sendTimerId	u8 (0~3)	3	GBA Timer to use for sending.

Methods

Name	Return type	Description
isActive()	bool	Returns whether the library is active or not.
activate()	-	Activates the library.
deactivate()	-	Deactivates the library.
isConnected()	bool	Returns true if there are at least 2 connected players.
playerCount()	u8 (0~4)	Returns the number of connected players.
currentPlayerId()	u8 (0~3)	Returns the current player id.
canRead(playerId)	bool	Returns true if there are pending messages from player #playerId.
read(playerId)	u16	Returns one message from player #playerId.
consume()	-	Marks the current data as processed, enabling the library to fetch more.
send(data)	-	Sends data to all connected players.

⚠️ 0xFFFF and 0x0 are reserved values, so don't send them!

💻 LinkCableMultiboot

(aka Multiboot through Multi-Play mode)

This tool allows sending Multiboot ROMs (small 256KiB programs that fit in EWRAM) from one GBA to up to 3 slaves, using a single cartridge.

Methods

Name	Return type	Description
sendRom(rom, romSize, cancel)	LinkCableMultiboot::Result	Sends the rom. During the handshake process, the library will continuously invoke cancel, and abort the transfer if it returns true. The romSize must be a number between 448 and 262144, and a multiple of 16. Once completed, the return value should be LinkCableMultiboot::Result::SUCCESS.

⚠️ for better results, turn on the GBAs after calling the sendRom method!

🔌 LinkGPIO

(aka General Purpose Mode)

This is the default Link Port mode, and it allows users to manipulate pins SI, SO, SD and SC directly.

Methods

Name	Return type	Description
reset()	-	Resets communication mode to General Purpose. Required to initialize the library!
setMode(pin, direction)	-	Configures a pin to use a direction (input or output).
getMode(pin)	LinkGPIO::Direction	Returns the direction set at pin.
readPin(pin)	bool	Returns whether a pin is HIGH or not (when set as an input).
writePin(pin, isHigh)	-	Sets a pin to be high or not (when set as an output).
setSIInterrupts(isEnabled)	-	If it isEnabled, a IRQ will be generated when SI changes from HIGH to LOW.

⚠️ always set the SI terminal to an input!

🔗 LinkSPI

(aka Normal Mode)

This is GBA's implementation of SPI. In this library, packets are set to 32-bit, as there's no benefit to using the 8-bit version. You can use this to interact with other GBAs or computers that know SPI.

Methods

Name	Return type	Description
isActive()	bool	Returns whether the library is active or not.
activate(mode)	-	Activates the library in a specific mode (one of LinkSPI::Mode::SLAVE, LinkSPI::Mode::MASTER_256KBPS, or LinkSPI::Mode::MASTER_2MBPS).
deactivate()	-	Deactivates the library.
transfer(data)	u32	Exchanges data with the other end. Returns the received data.
transfer(data, cancel)	u32	Like transfer(data) but accepts a cancel() function. The library will continuously invoke it, and abort the transfer if it returns true.
transferAsync(data, [cancel])	-	Schedules a data transfer and returns. After this, call getAsyncState() and getAsyncData(). Note that until you retrieve the async data, normal transfer(...)s won't do anything!
getAsyncState()	LinkSPI::AsyncState	Returns the state of the last async transfer (one of LinkSPI::AsyncState::IDLE, LinkSPI::AsyncState::WAITING, or LinkSPI::AsyncState::READY).
getAsyncData()	u32	If the async state is READY, returns the remote data and switches the state back to IDLE.
getMode()	LinkSPI::Mode	Returns the current mode.
setWaitModeActive(isActive)	-	Enables or disables waitMode (*).
isWaitModeActive()	bool	Returns whether waitMode (*) is active or not.

(*) waitMode: The GBA adds an extra feature over SPI. When working as master, it can check whether the other terminal is ready to receive, and wait if it's not. That makes the connection more reliable, but it's not always supported on other hardware units (e.g. the Wireless Adapter), so it must be disabled in those cases.

waitMode is disabled by default.

⚠️ when using Normal Mode between two GBAs, use a GBC Link Cable!

⚠️ only use the 2Mbps mode with custom hardware (very short wires)!

⚠️ don't send 0xFFFFFFFF, it's reserved for errors!

📻 LinkWireless

(aka GBA Wireless Adapter)

This is a driver for an accessory that enables wireless games up to 5 players. The inner workings of the adapter are highly unknown, but this article is very helpful. I've updated the blog post to add more details about the things I learnt by the means of reverse engineering brute force and trial&error.

The library, by default, implements a lightweight protocol (on top of the adapter's message system) that sends packet IDs and checksums. This allows detecting disconnections, forwarding messages to all nodes, and retransmitting to prevent packet loss.

Constructor

new LinkWireless(...) accepts these optional parameters:

Name	Type	Default	Description
forwarding	bool	true	If true, the server forwards all messages to the clients. Otherwise, clients only see messages sent from the server (ignoring other peers).
retransmission	bool	true	If true, the library handles retransmission for you, so there should be no packet loss.
maxPlayers	u8 (2~5)	5	Maximum number of allowed players. The adapter will accept connections after reaching the limit, but the library will ignore them. If your game only supports -for example- two players, set this to 2 as it will make transfers faster.
timeout	u32	8	Number of frames without receiving any data to reset the connection.
remoteTimeout	u32	10	Number of successful transfers without a message from a client to mark the player as disconnected.
interval	u16	50	Number of 1024cycles (61.04μs) ticks between transfers (50 = 3.052ms). It's the interval of Timer #sendTimerId.
sendTimerId	u8 (0~3)	3	GBA Timer to use for sending.

You can also change these compile-time constants:

• LINK_WIRELESS_QUEUE_SIZE: to set a custom buffer size (how many incoming and outcoming messages the queues can store at max). The default value is 30, which seems fine for most games.
• LINK_WIRELESS_MAX_COMMAND_RESPONSE_LENGTH: to set the biggest allowed response from the adapter. The default value is 50, which allows reading all user messages (max receive length is 21) and -in theory- up to 7 broadcasting servers (7 values per broadcast * 7 = 49 responses). This library was only tested with 4 adapters, so the real maximum is unknown.

Methods

• Most of these methods return a boolean, indicating if the action was successful. If not, you can call getLastError() to know the reason. Usually, unless it's a trivial error (like buffers being full), the connection with the adapter is reset and the game needs to start again.
• You can check the connection state at any time with getState().
• Until a session starts, all actions are synchronic.
• During sessions (when the state is SERVING or CONNECTED), the message transfers are IRQ-driven, so send(...) and receive(...) won't waste extra cycles.

Name	Return type	Description
isActive()	bool	Returns whether the library is active or not.
activate()	bool	Activates the library. When an adapter is connected, it changes the state to AUTHENTICATED. It can also be used to disconnect or reset the adapter.
deactivate()	-	Deactivates the library.
serve([gameName], [userName])	bool	Starts broadcasting a server and changes the state to SERVING. You can, optionally, provide a gameName (max 14 characters) and userName (max 8 characters) that games will be able to read.
getServers(servers, [onWait])	bool	Fills the servers vector with all the currently broadcasting servers. This action takes 1 second to complete, but you can optionally provide an onWait() function which will be invoked each time VBlank starts.
getServersAsyncStart()	bool	Starts looking for broadcasting servers and changes the state to SEARCHING. After this, call getServersAsyncEnd(...) 1 second later.
getServersAsyncEnd(servers)	bool	Fills the servers vector with all the currently broadcasting servers. Changes the state to AUTHENTICATED again.
connect(serverId)	bool	Starts a connection with serverId and changes the state to CONNECTING.
keepConnecting()	bool	When connecting, this needs to be called until the state is CONNECTED. It assigns a player id. Keep in mind that isConnected() and playerCount() won't be updated until the first message from server arrives.
send(data)	bool	Enqueues data to be sent to other nodes.
receive(messages)	bool	Fills the messages vector with incoming messages, forwarding if needed.
getState()	LinkWireless::State	Returns the current state (one of LinkWireless::State::NEEDS_RESET, LinkWireless::State::AUTHENTICATED, LinkWireless::State::SEARCHING, LinkWireless::State::SERVING, LinkWireless::State::CONNECTING, or LinkWireless::State::CONNECTED).
isConnected()	bool	Returns true if the player count is higher than 1.
isSessionActive()	bool	Returns true if the state is SERVING or CONNECTED.
playerCount()	u8 (1~5)	Returns the number of connected players.
currentPlayerId()	u8 (0~4)	Returns the current player id.
getLastError([clear])	LinkWireless::Error	If one of the other methods returns false, you can inspect this to know the cause. After this call, the last error is cleared if clear is true (default behavior).

⚠️ 0xFFFF is a reserved value, so don't send it!

🌎 LinkUniversal

A multiuse library that doesn't care whether you plug a Link Cable or a Wireless Adapter. It continuously switches between both and try to connect to other peers, supporting hot swapping cables with adapters and all the features from 👾 LinkCable and 📻 LinkWireless.

https://user-images.githubusercontent.com/1631752/218244610-99618911-0be9-4861-a10f-8b4bdf7259dd.mp4

Constructor

new LinkUniversal(...) accepts these optional parameters:

Name	Type	Default	Description
protocol	LinkUniversal::Protocol	AUTODETECT	Specifies what protocol should be used (one of LinkUniversal::Protocol::AUTODETECT, LinkUniversal::Protocol::CABLE, LinkUniversal::Protocol::WIRELESS_AUTO, or LinkUniversal::Protocol::WIRELESS_CLIENT).
gameName	std::string	""	The game name that will be broadcasted in wireless sessions. The library uses this to only connect to servers from the same game.
cableOptions	LinkUniversal::CableOptions	same as LinkCable	All the 👾 LinkCable constructor parameters in one struct.
wirelessOptions	LinkUniversal::WirelessOptions	same as LinkWireless	All the 📻 LinkWireless constructor parameters in one struct.

Methods

The interface is the same as 👾 LinkCable, with one exception: instead of calling consume() at the end of your game loop, you call sync() at the start.

Aditionally, it supports these methods:

Name	Return type	Description
getState()	LinkUniversal::State	Returns the current state (one of LinkUniversal::State::INITIALIZING, LinkUniversal::State::WAITING, or LinkUniversal::State::CONNECTED).
getMode()	LinkUniversal::Mode	Returns the active mode (one of LinkUniversal::Mode::LINK_CABLE, or LinkUniversal::Mode::LINK_WIRELESS).
getWirelessState()	LinkWireless::State	Returns the wireless state (same as 📻 LinkWireless's getState()).