chore: start work on APU cycles

src/apu/README.md

@@ -1,18 +1,17 @@
 # APU - Audio Processing Unit
 
-
-
-
 Rust libs:
+
 - synthesis, has osc for common wavs.. will it work well to generate one for each register?
 https://docs.rs/twang/latest/twang/
 - SDL2 can play sounds
   - ex of squarewave: https://github.com/Rust-SDL2/rust-sdl2/blob/master/examples/audio-queue-squarewave.rs
   - can play multiple sounds with mixer:
     - https://rust-sdl2.github.io/rust-sdl2/sdl2/mixer/index.html
     - usage ex: https://github.com/Rust-SDL2/rust-sdl2/blob/master/examples/mixer-demo.rs
+  - https://nicktasios.nl/posts/making-sounds-using-sdl-and-visualizing-them-on-a-simulated-oscilloscope.html
 
-Combing wave forms:
+Combining wave forms:
 https://0xc45.com/blog/digital-audio-synthesizer-in-rust/
 
 Other synthesizer examples:

src/apu/mod.rs

@@ -14,18 +14,35 @@ pub struct Apu {
     dmc: [u8; 4],
     status: u8,
     frame_counter: u8,
+
+    cpu_cycles: usize,
+    is_between_apu_cycle: bool,
 }
 
 impl Apu {
     pub fn new() -> Self {
         Self {
+            // TODO: Pulse2 differs from Pulse1 slightly.
+            // Known diff is how in APU sweep "Calculating the target period"
             pulse1: PulseRegister::new(),
             pulse2: PulseRegister::new(),
             triangle: [0; 4],
             noise: [0; 4],
             dmc: [0; 4],
             status: 0,
             frame_counter: 0,
+
+            cpu_cycles: 0,
+            is_between_apu_cycle: false,
+        }
+    }
+
+    /// tick tracks CPU cycles. APU cycles occur every 2 CPU cycles
+    pub fn tick_cpu_cycles(&mut self, cycles: usize) {
+        for _ in 0..cycles {
+            self.cpu_cycles += 1;
+            // or just cpu_cycles %2
+            self.is_between_apu_cycle = !self.is_between_apu_cycle
         }
     }
 }

src/bus.rs

@@ -52,6 +52,8 @@ impl<'a> Bus<'a> {
         if should_rerender {
             (self.gameloop_callback)(&self.ppu, &mut self.gamepad1, &mut self.gamepad2)
         }
+
+        self.apu.tick_cpu_cycles(cycles);
     }
 
     pub fn poll_nmi_interrupt(&mut self) -> bool {