Sound 2 Mode Registers - Nintendo GAME BOY Programming Manual

Game Boy Programming Manual
♦ ♦ ♦ ♦ Sound 1 Usage Notes
-When no sweep function is used with Sound 1, the sweep time should be set to 0
(sweep OFF). In addition, either the sweep increase/decrease flag should be set to 1
or the sweep shift number set to 0 (set to 08h-0Fh or 00h in NR10).
-Sound may not be produced if the sweep increase/decrease flag of NR10 is set to 0
(addition mode), the sweep shift number set to a value other than 0, and the mode set
to sweep OFF (e.g. NR10 = 01h)
-When a value is written in the envelope register, the sound output becomes unstable
till the initial flag is set. Therefore, set the initial flag immediately after writing a value in
the envelope register.

2.2 Sound 2 Mode Registers

Sound 2 is a circuit that generates a rectangle waveform with an envelope function. It is set by registers
NR21, NR22, NR23, and NR24.
Name Address Bit 7
NR21 FF16
Name Address Bit 7
NR22 FF17
Note: By Setting the envelope register only nothing will be reflected in the output. Always set
the initial flag.
6 5 4 3 2 1
6 5 4 3 2 1
0
R/W(Only the shaded portion
( ) can be read.)
Sound Length Data, t1(0 to 63)
Waveform Duty(00-11 Binary)
0
R/W
Length of Envelope Steps n(n=0 to 7)
Envelope Up/Down
0: Decrease
1: Increase
Default Envelope Value
84