Nintendo DMG-01 - Game Boy Console Manual page 42

2.13.1. I/O Registers
11. FF12 (NR12)
Name
Contents - Sound Mode 1 register, Envelope (R/W)
12. FF13 (NR 13)
Name
Contents - Sound Mode 1 register, Frequency lo (W)
13. FF14 (NR 14)
Name
Contents - Sound Mode 1 register, Frequency hi (R/W)
Page 42
- NR 12
Bit 7-4 - Initial volume of envelope
Bit 3 -
Bit 2-0 - Number of envelope sweep
Initial volume of envelope is from 0 to
$F. Zero being no sound.
Length of 1 step = n*(1/64) seconds
- NR 13
Lower 8 bits of 11 bit frequency (x).
Next 3 bit are in NR 14 ($FF14)
- NR 14
Only Bit 6 can be read.
Bit 7 - Initial (when set, sound
restarts)
Bit 6 - Counter/consecutive selection
Envelope UP/DOWN
0: Attenuate
1: Amplify
(n: 0-7) (If zero, stop
envelope operation.)
TM
Game Boy CPU Manual
V 1.01