Nintendo GAME BOY Programming Manual page 83

Example: When NR12 = 94h, the Amp Gain is as follows.
Amp.
Note: By Setting the envelope register only nothing will be reflected in the output. Always set
the initial flag.
Name Address Bit 7
FF13
NR13
Name Address Bit 7
NR14 FF14
Counter/Continuous Selection
0: Outputs continuous sound regardless of length data in register NR11.
1: Outputs sound for the duration specified by the length data in register NR11.
When sound output is finished, bit 0 of register NR52, the Sound 1 ON flag,
is reset.
Initialize
Setting this bit to 1 restarts Sound 1.
With the 11-bit frequency data specified in NR13 and NR14 represented by x, the
frequency, f, is determined by the following formula.
f = 4194304 / (4 x 2 x (2048 - X)) Hz
Thus, the minimum frequency is 64 Hz and the maximum is 131.1 KHz.
Gain
4/64 sec.
6 5 4 3 2 1
6 5 4 3 2 1
0
R/W(Low-order Frequency Data
0
R/W(Only the shaded portion
( ) can be read.)
High-order Frequency Data(3 bits)
Counter Continuous Selection
Initialize
83
Chapter 3: Sound Functions