An NES APU sweep unit can be made to periodically adjust a pulse channel's period up or down.
Each sweep unit contains the following:
- reload flag
|$4001||EPPP.NSSS||Pulse channel 1 sweep setup (write)|
|$4005||EPPP.NSSS||Pulse channel 2 sweep setup (write)|
|bit 7||E--- ----||Enabled flag|
|bits 6-4||-PPP ----||The divider's period is P + 1 half-frames|
|bit 3||---- N---||Negate flag|
0: add to period, sweeping toward lower frequencies
1: subtract from period, sweeping toward higher frequencies
|bits 2-0||---- -SSS||Shift count (number of bits)|
|Side effects||Sets the reload flag|
Calculating the target period
The sweep unit continuously calculates each pulse channel's target period in this way:
- A barrel shifter shifts the pulse channel's 11-bit raw timer period right by the shift count, producing the change amount.
- If the negate flag is true, the change amount is made negative.
- The target period is the sum of the current period and the change amount.
For example, if the negate flag is false and the shift amount is zero, the change amount equals the current period, making the target period equal to twice the current period.
The two pulse channels have their adders' carry inputs wired differently, which produces different results when each channel's change amount is made negative:
- Pulse 1 adds the ones' complement (−c − 1). Making 20 negative produces a change amount of −21.
- Pulse 2 adds the two's complement (−c). Making 20 negative produces a change amount of −20.
Whenever the current period changes for any reason, whether by $400x writes or by sweep, the target period also changes.
Muting means that the pulse channel sends 0 to the mixer instead of the current volume. Muting happens regardless of whether the sweep unit is disabled (because either the Enabled flag or the Shift count are zero) and regardless of whether the sweep divider is outputting a clock signal.
Two conditions cause the sweep unit to mute the channel:
- If the current period is less than 8, the sweep unit mutes the channel.
- If at any time the target period is greater than $7FF, the sweep unit mutes the channel.
In particular, if the negate flag is false, the shift count is zero, and the current period is at least $400, the target period will be large enough to mute the channel. (This is why several publishers' NES games never seem to use the bottom octave of the pulse waves.)
Because the target period is computed continuously, a target period overflow from the sweep unit's adder can silence a channel even when the sweep unit is disabled and even when the sweep divider is not outputting a clock signal. Thus to fully disable the sweep unit, a program must additionally turn on the Negate flag, such as by writing $08. This ensures that the target period is not greater than the current period and therefore not greater than $7FF.
Updating the period
When the frame counter sends a half-frame clock (at 120 or 96 Hz), two things happen:
- If the divider's counter is zero, the sweep is enabled, and the sweep unit is not muting the channel: The pulse's period is set to the target period.
- If the divider's counter is zero or the reload flag is true: The divider counter is set to P and the reload flag is cleared. Otherwise, the divider counter is decremented.
When the sweep unit is muting a pulse channel, the channel's current period remains unchanged, but the sweep unit's divider continues to count down and reload the divider's period as normal. Otherwise, if the enable flag is set and the shift count is non-zero, when the divider outputs a clock, the pulse channel's period is updated to the target period.
If the shift count is zero, the pulse channel's period is never updated, but muting logic still applies.