Nintendo DMG-01 - Game Boy Console Manual page 44

2.13.1. I/O Registers
15. FF17 (NR 22)
Name
Contents - Sound Mode 2 register, envelope (R/W)
16. FF18 (NR 23)
Name
Contents - Sound Mode 2 register, frequency
17. FF19 (NR 24)
Name
Contents - Sound Mode 2 register, frequency
Page 44
- NR 22
Bit 7-4 - Initial volume of envelope
Bit 3 -
Bit 2-0 - Number of envelope sweep
Initial volume of envelope is from 0 to
$F. Zero being no sound.
Length of 1 step = n*(1/64) seconds
- NR 23
lo data (W)
Frequency's lower 8 bits of 11 bit data
(x). Next 3 bits are in NR 14 ($FF19).
- NR 24
hi data (R/W)
Only bit 6 can be read.
Bit 7 - Initial (when set, sound
Envelope UP/DOWN
0: Attenuate
1: Amplify
(n: 0-7) (If zero, stop
envelope operation.)
TM
Game Boy CPU Manual
V 1.01