Sound 3 Mode Registers - Nintendo GAME BOY Programming Manual

Name Address Bit 7
NR23 FF18
Name Address Bit 7
NR24 FF19
Counter/Continous Selection
0: Outputs continuous sound regardless of length data in register NR21.
1: Outputs sound for the duration specified by the length data in register
NR21. When sound output is finished, bit 1 of register NR52, the Sound 2
ON flag, is reset.
Initialize
Setting this bit to 1 restarts Sound 2.
♦ ♦ ♦ ♦ Sound 2 Usage Notes
When a value is written in the envelope register, the sound output becomes unstable until the initial flag
is set. Hence, set the initial flag immediately after writing a value in the envelope register.

2.3 Sound 3 Mode Registers

Sound 3 is a circuit that generates user-defined waveforms. It automatically reads a waveform pattern (1
cycle)written to waveform RAM at FF30h-FF3Fh, and it can output a sound while changing its length,
frequency, and level by registers NR30, NR31, NR32, NR33, and NR34.
The settings of the sound length and frequency functions and data are the same as for the Sound 1 circuit.
Name Address Bit 7
NR30 FF1A
Name Address Bit 7
NR31 FF1B
Sound Length = (256-t1) x (1/256) sec
6 5 4 3 2 1
6 5 4 3 2 1
6 5 4 3 2 1
6 5 4 3 2 1
0
W Lower Frequency Data
0
R/W(Only the shaded portion
( ) can be read.)
High-order Frequency Data(3 bits)
Counter/Continuous Selection
Initialize
0
R/W(Only the shaded portion
( ) can be read.)
Sound Off
0:Stop Sound 3 Output
1:Enable Sound 3 Output
0
R/W Selects the Sound Length
Sound Length Data, t1(0 to 255)
85
Chapter 3: Sound Functions