APU Length Counter
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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. See channel's page for any further effects. | |
Clocking
When enabled is clear, the length counter is forced to 0 and cannot be changed until enabled is set again (the length counter's previous value is lost). Enabled has no effect when 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
