Reference Notes; Communication; Sound - Nintendo GAME BOY Programming Manual

Game Boy Programming Manual

Reference Notes:

1. Accessing VRAM Outside of a V-blanking period
In early DMGs, accessing VRAM outside of a V-blanking period would corrupt the screen.
2. Length of H-blanking
The length of the H-blanking period changes depending on the conditions of OBJ use, so
caution is recommended when using H-blanking.

2.2 Communication

2.2.1 Communication Rate (Required)
Covers: DMG, SGB2, and CGB
Data may be corrupted if the data transfer rate is too high.
The maximum external clock setting should be 512KHz between CGBs.
It should be 500KHz for others including DMG and SGB2.
Also, it should be 256KHz for communication between CGB and DMG.
2.2.2 Communication Errors (Recommended)
Covers: DMG, SGB2, and CGB
When using the communication function (infrared), the communicating data may be corrupted by
noise and such. Therefore, the program should not go out of control by such data corruption on
both the sending and receiving side.
When using the communication function (serial), depending on how the program is made, it is
confirmed that communication errors happen rarely.
SIO interrupt processing may be delayed by factors such as the processing of other interrupts.
This type of error should be avoided by establishing a proper communication interval that allows a
problem-free exchange of data.
2.2.3 Effects of Other Infrared Devices (Recommended)
Covers: CGB
Adequate care should be taken to ensure against faulty operation and loss of program control
even when infrared communication signal input is received from other game software and devices.
Note that such problems may particularly occur in communication between multiple games that
use the same subroutines. (Before performing data communication, use means such exchanging
a unique key code to check whether the same game is on the other hardware.)

2.3 Sound

2.3.1 Using Sounds 1, 2 , and 3 (Required)
Covers: CGB
With continuous operation mode selected (bit 6 of NR*4 set to 0) for sounds 1, 2, and 3, if the
higher-order frequency data (lower-order 3 bits of NR*4) are changed, the sound length (bits 0-5 of
NR*1) must to set to 0 after the frequency data is set. If the sound length is not set to 0, the
sound may stop during playback.
2.3.2 Using Sound 3 (Required)
Covers: DMG, SGB, and CGB
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