From 8f3ab6e751fcc6f380e46e625ce6351dce8da071 Mon Sep 17 00:00:00 2001
From: Theron Spiegl <tspiegl@gmail.com>
Date: Tue, 31 Dec 2019 17:22:44 -0600
Subject: [PATCH] troubleshooting, experimenting

---
 src/apu/mod.rs    |  2 --
 src/apu/square.rs | 12 ++++++++++--
 src/main.rs       |  8 +++++++-
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/src/apu/mod.rs b/src/apu/mod.rs
index c2ab8e2..c87eab9 100644
--- a/src/apu/mod.rs
+++ b/src/apu/mod.rs
@@ -60,8 +60,6 @@ impl Apu {
     pub fn new() -> Self {
         let square_table = (0..31).map(|x| 95.52/((8128.0 / x as f32) + 100.0)).collect();
         let tnd_table = (0..203).map(|x| 163.67/((24329.0 / x as f32) + 100.0)).collect();
-        println!("square_table: {:?}", square_table);
-        println!("tnd_table: {:?}", tnd_table);
         Apu {
             square1:    Square::new(false),
             square2:    Square::new(true),
diff --git a/src/apu/square.rs b/src/apu/square.rs
index 0b359cd..21569fe 100644
--- a/src/apu/square.rs
+++ b/src/apu/square.rs
@@ -80,7 +80,7 @@ impl Square {
         // Update volume for this channel
         // The mixer receives the current envelope volume except when
         self.sample = if self.duty_cycle[self.duty_counter] == 0 // the sequencer output is zero, or
-            || self.target_period > 0x7FF // overflow from the sweep unit's adder is silencing the channel,
+            || self.timer_period > 0x7FF // overflow from the sweep unit's adder is silencing the channel,
             || self.length_counter == 0 // the length counter is zero, or
             || self.timer_period < 8 { // the timer has a value less than eight.
                 0
@@ -126,9 +126,10 @@ impl Square {
     }
 
     pub fn clock_sweep(&mut self) {
+        self.calculate_target_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 adjusted.
-        if self.sweep_counter == 0 && self.sweep_enabled && !(self.timer_period < 8 || self.target_period > 0x7FF) {
+        if self.sweep_counter == 0 && self.sweep_enabled && !(self.timer_period < 8 || self.timer_period > 0x7FF) {
             self.timer_period = self.target_period;
             println!("timer period adjusted to {}", self.timer_period);
         }
@@ -139,7 +140,10 @@ impl Square {
         } else {
             self.sweep_counter -= 1;
         }
+    }
 
+    // Whenever the current period changes for any reason, whether by $400x writes or by sweep, the target period also changes. 
+    pub fn calculate_target_period(&mut self) {
         // The sweep unit continuously calculates each channel's target period in this way:
         // A barrel shifter shifts the channel's 11-bit raw timer period right by the shift count, producing the change amount.
         let change = self.timer_period >> self.shift_count;
@@ -185,17 +189,21 @@ impl Square {
     pub fn write_timer_low(&mut self, value: u8) {
         self.timer_period &= 0b00000111_00000000; // mask off everything but high 3 bits of 11-bit timer
         self.timer_period |= value as u16; // apply low 8 bits
+        self.calculate_target_period();
     }
 
     // $4003/$4007
     pub fn write_timer_high(&mut self, value: u8) {
         // LLLL.Lttt 	Pulse channel 1 length counter load and timer (write)
+        // TODO: thought the below meant that the length counter was only set if the channel was enabled, but apparently not as not having it fixes start game noise in DK.
+        // When the enabled bit is cleared (via $4015), the length counter is forced to 0 and cannot be changed until enabled is set again (the length counter's previous value is lost).
         if self.enabled {
             self.length_counter = super::LENGTH_COUNTER_TABLE[value as usize >> 3];
         }
         let timer_high = value as u16 & 0b0000_0111;
         self.timer_period &= 0b11111000_11111111; // mask off high 3 bits of 11-bit timer
         self.timer_period |= timer_high << 8; // apply high timer bits in their place
+        self.calculate_target_period();
         // The sequencer is immediately restarted at the first value of the current sequence. The envelope is also restarted. The period divider is not reset.
         self.duty_counter = 0;
         self.start = true;
diff --git a/src/main.rs b/src/main.rs
index 342f6a5..fcaa57d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -69,7 +69,7 @@ fn main() -> Result<(), String> {
             match cpu.apu.clock() {
                 Some(sample) => {
                     sps += 1;
-                    audio_device.queue(&vec![sample]);
+                    if sps < 44100 {audio_device.queue(&vec![sample]);}
                 },
                 None => (),
             };
@@ -126,8 +126,14 @@ TODO:
 - remaining APU channels
 - common mappers
 - battery-backed RAM solution
+- fix mysterious Mario pipe non-locations
 - GUI? drag and drop ROMs?
 - reset function
 - save/load/pause functionality
 
+
+Timing notes:
+The PPU is throttled to 60Hz by sleeping in the main loop. This locks the CPU to roughly its intended speed, 1.789773MHz NTSC. The APU runs at half that.
+The SDL audio device samples/outputs at 44,100Hz, so as long as the APU queues up 44,100 samples per second, it works.
+
 */