Serial Data Transfer (Link Cable) - Revision history

Nitro2k01 at 02:14, 5 April 2020

2020-04-05T02:14:59Z

PinoBatch: /* INT 58 - Serial Interrupt */ No wake-on-LAN, per gekkio

2019-06-02T20:40:42Z

‎INT 58 - Serial Interrupt: No wake-on-LAN, per gekkio

Raldi: Added notes from https://www.reddit.com/r/emulation/comments/64a7kl/cant_make_heads_or_tails_of_gameboy_video_link/

2017-04-09T22:57:36Z

Added notes from https://www.reddit.com/r/emulation/comments/64a7kl/cant_make_heads_or_tails_of_gameboy_video_link/

Xzakox at 23:03, 9 December 2012

2012-12-09T23:03:34Z

Xzakox at 12:50, 28 June 2009

2009-06-28T12:50:14Z

Xzakox: New page: ===F01 - 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...

2009-06-28T10:37:30Z

New page: ===F01 - 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...

New page

===F01 - 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.

===Internal 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

===External Clock===
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.

===Timeouts===
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.

'''XXXXXX...'''

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

← Older revision		Revision as of 02:14, 5 April 2020
Line 1:		Line 1:
−	{{Pandocs\|~~serialdatatransferlinkcable~~}}	+	{{Pandocs\|serial-data-transfer}}

	Communication between two Gameboys happens one byte at a time. One Gameboy acts as the master, uses its internal clock, and thus controls when the exchange happens. The other one uses an external clock (i.e., the one inside the other Gameboy) and has no control over when the transfer happens. If it hasn't gotten around to loading up the next data byte at the time the transfer begins, the last one will go out again. Alternately, if it's ready to send the next byte but the last one hasn't gone out yet, it has no choice but to wait.		Communication between two Gameboys happens one byte at a time. One Gameboy acts as the master, uses its internal clock, and thus controls when the exchange happens. The other one uses an external clock (i.e., the one inside the other Gameboy) and has no control over when the transfer happens. If it hasn't gotten around to loading up the next data byte at the time the transfer begins, the last one will go out again. Alternately, if it's ready to send the next byte but the last one hasn't gone out yet, it has no choice but to wait.

← Older revision		Revision as of 20:40, 2 June 2019
Line 76:		Line 76:
	ld a,$81		ld a,$81
	ld ($FF02),a		ld ($FF02),a
		+
		+	The Game Boy does not support wake-on-LAN. Completion of an externally clocked serial transfer does not exit STOP mode.

@@ Line 1: / Line 1: @@
 {{Pandocs|serialdatatransferlinkcable}}
-===F01 - SB - Serial transfer data (R/W)===
+===FF01 - SB - Serial transfer data (R/W)===
 Bits of data to be read/written

← Older revision		Revision as of 12:50, 28 June 2009
Line 1:		Line 1:
		+	{{Pandocs\|serialdatatransferlinkcable}}
	===F01 - SB - Serial transfer data (R/W)===		===F01 - SB - Serial transfer data (R/W)===
	8 Bits of data to be read/written		8 Bits of data to be read/written

@@ Line 1: / Line 1: @@
 {{Pandocs|serialdatatransferlinkcable}}
 ===FF01 - SB - Serial transfer data (R/W)===
-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 7 - Transfer Start Flag (0=No transfer is in progress or requested, 1=Transfer in progress, or requested)
    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.
+The gameboy acting as master will load up a data byte in SB and then set SC to 0x81 (Transfer requested, use internal clock). It will be notified that the transfer is complete in two ways: SC's Bit 7 will be cleared (i.e., SC will be set up 0x01), and also the Serial Interrupt handler will be called (i.e., the CPU will jump to 0x0058).
 ===Internal Clock===
@@ Line 21: / Line 42: @@
 ===External Clock===
-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.
+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 limitation 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.
 ===Timeouts===
@@ Line 29: / Line 50: @@
 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.
+Transfer is initiated by setting the master Gameboy setting its Transfer Start Flag, regardless of the value of this flag on the other device. This bit is automatically set to 0 (on both) at the end of Transfer. Reading this bit can be used to determine if the transfer is still active.
 ===INT 58 - Serial Interrupt===
@@ Line 49: / Line 70: @@
 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:
+The following code initiates the process of shifting $75 out the serial port and a byte to be shifted into $FF01:
      ld   a,$75