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)
|}
|}
: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.