Nintendo DMG-01 - Game Boy Console Manual page 43

TM
Game Boy CPU Manual
14. FF16 (NR 21)
Name
Contents - Sound Mode 2 register, Sound Length; Wave
Wave Duty: (default: 10)
by DP
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
NR11 sound can be produced
consecutively.
1 = Sound is generated during the time
period set by the length data in
NR11. After this period the sound 1
ON flag (bit 0 of NR52) is reset.
- NR 21
Pattern Duty (R/W)
Only bits 7-6 can be read.
Bit 7-6 - Wave pattern duty
Bit 5-0 - Sound length data (t1: 0-63)
00: 12.5% ( _--------_--------_-------- )
01: 25%
10: 50%
11: 75%
Sound Length = (64-t1)*(1/256) seconds
= 131072/(2048-x) Hz
( __-------__-------__------- )
( ____-----____-----____----- )
( ______---______---______--- )
2.13.1. I/O Registers
Page 43