Serial Data Transfer (Link Cable)
FF01 - SB - Serial transfer data (R/W)
8 Bits of data to be read/written
FF02 - SC - Serial Transfer Control (R/W)
Bit 7 - Transfer Start Flag (0=No Transfer, 1=Start) Bit 1 - Clock Speed (0=Normal, 1=Fast) ** CGB Mode Only ** Bit 0 - Shift Clock (0=External Clock, 1=Internal Clock)
The clock signal specifies the rate at which the eight data bits in SB (FF01) are transferred. When the gameboy is communicating with another gameboy (or other computer) then either one must supply internal clock, and the other one must use external clock.
In Non-CGB Mode the gameboy supplies an internal clock of 8192Hz only (allowing to transfer about 1 KByte per second). In CGB Mode four internal clock rates are available, depending on Bit 1 of the SC register, and on whether the CGB Double Speed Mode is used:
8192Hz - 1KB/s - Bit 1 cleared, Normal 16384Hz - 2KB/s - Bit 1 cleared, Double Speed Mode 262144Hz - 32KB/s - Bit 1 set, Normal 524288Hz - 64KB/s - Bit 1 set, Double Speed Mode
The external clock is typically supplied by another gameboy, but might be supplied by another computer (for example if connected to a PCs parallel port), in that case the external clock may have any speed. Even the old/monochrome gameboy is reported to recognizes external clocks of up to 500KHz. And there is no limitiation into the other direction - even when suppling an external clock speed of "1 bit per month", then the gameboy will still eagerly wait for the next bit(s) to be transferred. It isn't required that the clock pulses are sent at an regular interval either.
When using external clock then the transfer will not complete until the last bit is received. In case that the second gameboy isn't supplying a clock signal, if it gets turned off, or if there is no second gameboy connected at all) then transfer will never complete. For this reason the transfer procedure should use a timeout counter, and abort the communication if no response has been received during the timeout interval.
Delays and Synchronization
The gameboy that is using internal clock should always execute a small delay between each transfer, in order to ensure that the opponent gameboy has enough time to prepare itself for the next transfer, ie. the gameboy with external clock must have set its transfer start bit before the gameboy with internal clock starts the transfer. Alternately, the two gameboys could switch between internal and external clock for each transferred byte to ensure synchronization.
Transfer is initiated by setting the Transfer Start Flag. This bit is automatically set to 0 at the end of Transfer. Reading this bit can be used to determine if the transfer is still active.
INT 58 - Serial Interrupt
When the transfer has completed (ie. after sending/receiving 8 bits, if any) then an interrupt is requested by setting Bit 3 of the IF Register (FF0F). When that interrupt is enabled, then the Serial Interrupt vector at 0058 is called.
Transmitting and receiving serial data is done simultaneously. The received data is automatically stored in SB.
The serial I/O port on the Gameboy is a very simple setup and is crude compared to standard RS-232 (IBM-PC) or RS-485 (Macintosh) serial ports. There are no start or stop bits.
During a transfer, a byte is shifted in at the same time that a byte is shifted out. The rate of the shift is determined by whether the clock source is internal or external. The most significant bit is shifted in and out first.
When the internal clock is selected, it drives the clock pin on the game link port and it stays high when not used. During a transfer it will go low eight times to clock in/out each bit.
The state of the last bit shifted out determines the state of the output line until another transfer takes place.
If a serial transfer with internal clock is performed and no external GameBoy is present, a value of $FF will be received in the transfer.
The following code causes $75 to be shifted out the serial port and a byte to be shifted into $FF01:
ld a,$75 ld ($FF01),a ld a,$81 ld ($FF02),a