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 volume/envelope divider period (write) |
$4004 | DDle.eeee | Pulse 2 duty, length counter halt, and volume/envelope divider period (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
- fpulse = fCPU/(16*(t+1)) (where fCPU is 1.78977267 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 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.