Nintendo DMG-01 - Game Boy Console Manual page 45

TM
Game Boy CPU Manual
18. FF1A (NR 30)
Name
Contents - Sound Mode 3 register, Sound on/off (R/W)
19. FF1B (NR 31)
Name
Contents - Sound Mode 3 register, sound length (R/W)
by DP
restarts)
Bit 6 - Counter/consecutive selection
Bit 2-0 - Frequency's higher 3 bits (x)
Frequency = 4194304/(32*(2048-x)) Hz
Counter/consecutive Selection
0 = Regardless of the length data in
NR21 sound can be produced
consecutively.
1 = Sound is generated during the time
period set by the length data in
NR21. After this period the sound 2
ON flag (bit 1 of NR52) is reset.
- NR 30
Only bit 7 can be read
Bit 7 - Sound OFF
0: Sound 3 output stop
1: Sound 3 output OK
- NR 31
Bit 7-0 - Sound length (t1: 0 - 255)
Sound Length = (256-t1)*(1/2) seconds
= 131072/(2048-x) Hz
2.13.1. I/O Registers
Page 45