APU Length Counter

The length counter provides automatic duration control for the NES APU waveform channels. Once loaded with a value, it can optionally count down and silence the channel when it reaches zero.

Address	Bitfield	Description
$4015	---d.nt21	DMC control and length counter enabled flags (write)
$4000	ssHe.eeee	Pulse channel 1 sequence, length counter halt, and envelope (write)
$4004	ssHe.eeee	Pulse channel 2 sequence, length counter halt, and envelope (write)
$400C	--He.eeee	Noise channel length counter halt and envelope (write)
bit 5	--H- ----	Halt length counter (this bit is also the envelope's loop flag)
$4008	Hlll.llll	Triangle channel length counter halt and linear counter load (write)
bit 7	H--- ----	Halt length counter (this bit is also the linear counter's control flag)
$4003	LLLL.Lttt	Pulse channel 1 length counter load and timer (write)
$4007	LLLL.Lttt	Pulse channel 2 length counter load and timer (write)
$400B	LLLL.Lttt	Triangle channel length counter load and timer (write)
$400F	LLLL.L---	Noise channel length counter load (write)
bits 7-3	LLLL L---	If the enabled flag is set, the length counter is loaded with entry L of the length table: | 0 1 2 3 4 5 6 7 8 9 A B C D E F -----+---------------------------------------------------------------- 00-0F 10,254, 20, 2, 40, 4, 80, 6, 160, 8, 60, 10, 14, 12, 26, 14, 10-1F 12, 16, 24, 18, 48, 20, 96, 22, 192, 24, 72, 26, 16, 28, 32, 30
Side effects		The envelope is restarted, for pulse channels phase is reset, for triangle the halt flag is set.

Clocking

When the enabled bit is cleared (via $4015), the length counter is forced to 0 and cannot be changed until enabled is set again (the length counter's previous value is lost). There is no immediate effect when enabled is set.

When clocked by the frame counter, the length counter is decremented except when:

• The length counter is 0, or
• The halt flag is set