APU Pulse

Each of the two NES APU pulse (square) wave channels generate a pulse wave with variable duty.

Each pulse channel contains the following: envelope generator, sweep unit, timer, 8-step sequencer, length counter.

                     Sweep -----> Timer
                       |            |
                       |            |
                       |            v 
                       |        Sequencer   Length Counter
                       |            |             |
                       |            |             |
                       v            v             v
    Envelope -------> Gate -----> Gate -------> Gate --->(to mixer)

Address	Bitfield	Description
$4000	DDle.eeee	Pulse 1 duty, length counter halt, and envelope (write)
$4004	DDle.eeee	Pulse 2 duty, length counter halt, and envelope (write)
bits 7-6	DD-- ----	The duty cycle is set (see table below). The sequencer's current position is not changed.
$4002	LLLL.LLLL	Pulse 1 timer low (write)
$4006	LLLL.LLLL	Pulse 2 timer low (write)
bits 7-0	LLLL LLLL	Timer low 8 bits
$4003	llll.lHHH	Pulse 1 length counter load and timer high (write)
$4007	llll.lHHH	Pulse 2 length counter load and timer high (write)
bits 2-0	---- -HHH	Timer high 3 bits
Side effects		The sequencer is restarted at the first value of the current sequence. The envelope is also restarted.

Duty Cycle Sequences

Duty	Waveform sequence
0	0 1 0 0 0 0 0 0 (12.5%)
1	0 1 1 0 0 0 0 0 (25%)
2	0 1 1 1 1 0 0 0 (50%)
3	1 0 0 1 1 1 1 1 (25% negated)

The reason for these odd sequences is that the sequence counter is initialized to zero but counts downward rather than upward

The sequencer is clocked by an 11-bit timer. Given the timer value t = HHHLLLLLLLL formed by timer high and timer low, this timer is updated every APU cycle (i.e., every second CPU cycle), and counts t, t-1, ..., 0, t, t-1, ..., clocking the waveform generator when it reaches 0. Since the period of the timer is t+1 APU cycles and the sequencer has 8 steps, the period of the waveform is 8*(t+1) APU cycles, or equivalently 16*(t+1) CPU cycles.

Hence

• f_pulse = f_CPU/(16*(t+1)) (where f_CPU is 1.78977267 MHz for NTSC, 1.662607 MHz for PAL, and 1.773448 MHz for Dendy)
• t = f_CPU/(16*f_pulse) - 1

A period of t < 8, either set explicitly or via a sweep period update, silences the corresponding pulse channel. The highest frequency a pulse channel can output is hence about 12.4 KHz for NTSC. (TODO: PAL behavior?)

The mixer receives the current envelope volume except when

• The sequencer output is zero, or
• The sweep unit is silencing the channel, or
• The length counter is zero

The behavior of the two pulse channels differs only in the effect of the negate mode of their sweep units.