gba-link-connection/README.md

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!

(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	LinkCable::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 use 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. 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
protocol	LinkWireless::Protocol	LinkWireless::Protocol::RETRANSMIT	LinkWireless::Protocol::BASIC: Clients only see messages sent from the server, ignoring other peers. Packet loss can occur, so games need to always send the full game state or implement retransmission on top of this. LinkWireless::Protocol::FORWARD: The server forwards all messages to the clients. LinkWireless::Protocol::RETRANSMIT: Same as FORWARD, but the library also handles retransmission for you, so there should be no packet loss.
maxPlayers	u8	5	Maximum number of allowed players.
msgTimeout	u32	5	Number of receive(...) calls without a message from other connected player to disconnect.

Methods

✔️ Most of the methods return a boolean, indicating if the action was successful. If not, the connection with the adapter is reset and the game needs to start again. All actions are synchronic.

Name	Return type	Description
isActive()	bool	Returns whether the library is active or not.
activate()	bool	Activates the library.
deactivate()	-	Deactivates the library.
serve()	bool	Starts broadcasting a server and changes the state to SERVING.
acceptConnections()	bool	Accepts new clients and updates the player count.
connect(serverId)	bool	Starts a connection with serverId and changes the state to CONNECTING.
keepConnecting()	bool	When connecting, needs to be called until the state is CONNECTED. It assigns a player id.
getServerIds(serverIds)	bool	Fills the serverIds vector with all the currently broadcasting servers.
send(data)	bool	Enqueues data to be sent to other nodes. Note that this data will be sent in the next receive(...) call.
receive(messages)	bool	Sends the pending data and fills the messages vector with incoming messages, checking for timeouts and forwarding if needed. This call doesn't block the hardware waiting for messages, it returns if there are no incoming messages.
disconnect()	bool	Disconnects and resets the adapter.
getState()	LinkWireless::State	Returns the current state (one of LinkWireless::State::NEEDS_RESET, LinkWireless::State::AUTHENTICATED, LinkWireless::State::SERVING, LinkWireless::State::CONNECTING, or LinkWireless::State::CONNECTED).
getLastError()	LinkWireless::Error	If one of the other methods return false, you can inspect this to know the cause. After this call, the last error is cleared.
getPlayerId()	u8 (0~4)	Returns the current player id.
getPlayerCount()	u8 (1~5)	Returns the connected players.

⚠️ the library can transfer a maximum of 14 words of 32 bits at a time, and messages are often concatenated together, so keep things way below this limit (specially when the protocol is FORWARD or RETRANSMIT)!