Nintendo 1504166 - Game Boy Advance SP Edition Console Programming Manual page 131

AGB Programming Manual
SIOCNT [d03] Transfer Enable Flag Send
A setting of 0 enables transfer; 1 disables it.
This flag is output from the SO terminal until the start of a transfer. When
the transfer starts, serial data are output from the SO terminal.
SIOCNT [d02] Transfer Enable Flag Receive
It is possible to read the status of SI terminal (transfer-enable flag
transmitting of the other party's hardware) before communication starts.
It becomes invalid after communication has started.(receive data bit during
communication is reflected.)
SIOCNT [d01] Internal Shift Clock Selection
If 0, 256KHz is selected for the shift clock.
If 1, 2MHz is selected for the shift clock.
SIOCNT [d00] Shift Clock Selection
If 0, an external clock is used as a shift clock. (slave)
The external clock is input by the SC terminal from another hardware unit.
SD terminal will go to LO output.
If 1, an internal clock is used as a shift clock. (master)
The internal clock is output from the SC terminal, and SD terminal will be in
the pull-up input status.
[Cautions for Normal Serial Communications]
The shift clock should be selected before the start bit of the SIOCNT register is set.
Extra shift operations may result if the serial transfer is started before or at the same
time as the shift clock is selected.
Do not use a value of the transfer enable flag receiving bit for SIOCNT register while
the start bit of SIOCNT register is being set. (Because it transforms to a receiving data
bit that is being communicated.)
The 8 bit transfer mode is compatible in terms of modes with DMG/CGB, but the
voltage with the communication terminal varies. Therefore, communication between
AGB and DMG/CGB is not possible.
Using a Game Link cable for DMG/MGB is prohibited in normal serial communication
mode. It is possible to communicate at 256KHz and 2MHz with peripheral equipment
that does not use a cable.
©1999 - 2001 Nintendo of America Inc.
131
Communication Functions
D.C.N. AGB-06-0001-002B4