APU Pulse: Difference between revisions
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(pretty table) |
(The duty cycle counter actually counts downward (since it's triggered by frequency counter underflow), but it's initialized to 000 rather than 111, which explains the waveform sequences) |
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| 3 || <tt>1 0 0 1 1 1 1 1</tt> (25% negated) | | 3 || <tt>1 0 0 1 1 1 1 1</tt> (25% negated) | ||
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: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 a [[APU Misc|timer]] whose period is the 12-bit value (<tt>%HHHL.LLLLLLL0</tt>, incorporating a left shift) formed by timer high and timer low, ''plus two''. | The sequencer is clocked by a [[APU Misc|timer]] whose period is the 12-bit value (<tt>%HHHL.LLLLLLL0</tt>, incorporating a left shift) formed by timer high and timer low, ''plus two''. | ||
Revision as of 16:37, 14 May 2011
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)
| 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 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:
- fCPU = 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 = fCPU/(16*(tval + 1))
- tval = fCPU/(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.
