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 (when the length counter halt flag is clear). Once it reaches zero, the corresponding channel is silenced.

Address	Bitfield	Description
$4015	---d.nt21	DMC control and length counter enabled flags (write)
$4000	ssHc.vvvv	Pulse channel 1 duty cycle, length counter halt, constant volume flag, and volume/envelope (write)
$4004	ssHc.vvvv	Pulse channel 2 duty cycle, length counter halt, constant volume flag, and volume/envelope (write)
$400C	--Hc.vvvv	Noise channel length counter halt, constant volume flag, and volume/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 linear counter reload 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

Table structure

The structure of the length table becomes clearer when rearranged like in the following index to length map (which corresponds to the order in which the values appear in the internal APU lookup table):

Legend:
<bit pattern> (<value of bit pattern>) => <note length>

Linear length values:
1 1111 (1F) => 30
1 1101 (1D) => 28
1 1011 (1B) => 26
1 1001 (19) => 24
1 0111 (17) => 22
1 0101 (15) => 20
1 0011 (13) => 18
1 0001 (11) => 16
0 1111 (0F) => 14
0 1101 (0D) => 12
0 1011 (0B) => 10
0 1001 (09) => 8
0 0111 (07) => 6
0 0101 (05) => 4
0 0011 (03) => 2
0 0001 (01) => 254

Notes with base length 12 (4/4 at 75 bpm):
1 1110 (1E) => 32  (96 times 1/3, quarter note triplet)
1 1100 (1C) => 16  (48 times 1/3, eighth note triplet)
1 1010 (1A) => 72  (48 times 1 1/2, dotted quarter)
1 1000 (18) => 192 (Whole note)
1 0110 (16) => 96  (Half note)
1 0100 (14) => 48  (Quarter note)
1 0010 (12) => 24  (Eighth note)
1 0000 (10) => 12  (Sixteenth)

Notes with base length 10 (4/4 at 90 bpm, with relative durations being the same as above):
0 1110 (0E) => 26  (Approx. 80 times 1/3)
0 1100 (0C) => 14  (Approx. 40 times 1/3)
0 1010 (0A) => 60
0 1000 (08) => 160 
0 0110 (06) => 80
0 0100 (04) => 40
0 0010 (02) => 20
0 0000 (00) => 10

With the least significant bit set, the remaining bits select a linear length (with the exception of the 0 entry). Otherwise, we get note lengths based on a base length of 10 (MSB clear) or 12 (MSB set).

Length counter internals

In the actual APU, the length counter silences the channel when clocked while already zero (provided the length counter halt flag isn't set). The values in the above table are the actual values the length counter gets loaded with plus one, to allow us to use a model where the channel is silenced when the length counter becomes zero.

The triangle's linear counter works differently, and does silence the channel when it reaches zero.