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.

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

$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 sequencer is clocked by a timer whose period is the 12-bit value (%HHHL.LLLLLLL0, incorporating a left shift) formed by timer high and timer low, plus two. It further divides the timer's output by 8 to produce the audio frequency. So given the following:

• f_CPU = the clock rate of the CPU
• tval = the 11-bit value that the program writes to the timer high and low registers
• f = the frequency of the wave generated by this channel

The following relationships hold:

• f = f_CPU/(16*(tval + 1))
• tval = f_CPU/(16*f) - 1

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.