Each of the two NES APU pulse (square) wave channels generate a pulse wave with variable duty.
Sweep -----> Timer | | | | | v | Sequencer Length Counter | | | | | | v v v Envelope -------> Gate -----> Gate -------> Gate --->(to mixer)
|$4000||DDlc.vvvv||Pulse 1 Duty cycle, length counter halt, constant volume/envelope flag, and volume/envelope divider period|
|$4004||DDlc.vvvv||Pulse 2 Duty cycle, length counter halt, constant volume/envelope flag, and volume/envelope divider period|
|Side effects||The duty cycle is changed (see table below), but the sequencer's current position isn't affected.|
|$4001||EPPP.NSSS||See APU Sweep|
|$4005||EPPP.NSSS||See APU Sweep|
|$4002||LLLL.LLLL||Pulse 1 timer Low 8 bits|
|$4006||LLLL.LLLL||Pulse 2 timer Low 8 bits|
|$4003||llll.lHHH||Pulse 1 length counter load and timer High 3 bits|
|$4007||llll.lHHH||Pulse 2 length counter load and timer High 3 bits|
|Side effects||The sequencer is immediately restarted at the first value of the current sequence. The envelope is also restarted. The period divider is not reset.|
Duty Cycle Sequences
|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 goes from 0 to t. 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.
- fpulse = fCPU/(16*(t+1)) (where fCPU is 1.789773 MHz for NTSC, 1.662607 MHz for PAL, and 1.773448 MHz for Dendy)
- t = fCPU/(16*fpulse) - 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 sequencer output is zero, or
- overflow from the sweep unit's adder is silencing the channel, or
- the length counter is zero, or
- the timer has a value less than eight.
The behavior of the two pulse channels differs only in the effect of the negate mode of their sweep units.
Notice that a few Famiclone units have swapped APU duty cycles, as 12.5, 50, 25 and 25 negated instead.