diff --git a/TODO.md b/TODO.md
index a277ee9..e8f5983 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,10 +1,17 @@
+- [ ] Why are some sprites flipped around?
 - [x] Figure out why sprites aren't drawing
   - wasn't writing to OAM DMA correctly
-- [ ] try out code coverage
+- [x] try out code coverage
   https://github.com/xd009642/tarpaulin
   https://medium.com/@gnanaganesh/robust-rust-how-code-coverage-powers-rust-software-quality-417ef3ac2360
   https://blog.balthazar-rouberol.com/measuring-the-coverage-of-a-rust-program-in-github-actions
-- [ ] Why are some sprites flipped around?
+- [ ] integrate code coverage into CI
+- [ ] migrate to bitfields (from bitflags)
+  - https://docs.rs/bilge/latest/bilge/
+  - https://gitlab.com/SmartAcoustics/sparrow/-/blob/master/sparrow-bitpacker/src/bitfields.rs
+  - https://github.com/wrenger/bitfield-struct-rs
+  - https://github.com/gregorygaines/bitfields-rs
+
 - [ ] Try an external debugger and setting breakpts
 - [ ] Debugger view
   - show both pattern tables beside the UI
diff --git a/src/apu/README.md b/src/apu/README.md
index 067e506..c2df875 100644
--- a/src/apu/README.md
+++ b/src/apu/README.md
@@ -1,5 +1,20 @@
 # APU - Audio Processing Unit
 
+Another great APU ref:
+https://www.slack.net/~ant/nes-emu/apu_ref.txt
+
+Read about relevant synthesis:
+https://www.slack.net/~ant/bl-synth/
+
+Deep divin
+https://forums.nesdev.org/viewtopic.php?t=8602
+
+Other people asking about APU:
+- https://github.com/amhndu/SimpleNES/issues/24
+- https://www.reddit.com/r/rust/comments/16kx8i2/how_to_implement_apu_for_my_nes_emulator/
+- https://www.reddit.com/r/EmuDev/comments/16kx80n/how_to_implement_apu/
+- https://github.com/OneLoneCoder/olcNES/blob/master/Part%20%237%20-%20Mappers%20%26%20Basic%20Sounds/olcNes_Sounds1.cpp
+
 Rust libs:
 
 - synthesis, has osc for common wavs.. will it work well to generate one for each register?
diff --git a/src/apu/mod.rs b/src/apu/mod.rs
index 96a0315..a01d12c 100644
--- a/src/apu/mod.rs
+++ b/src/apu/mod.rs
@@ -41,7 +41,7 @@ impl Apu {
     pub fn tick_cpu_cycles(&mut self, cycles: usize) {
         for _ in 0..cycles {
             self.cpu_cycles += 1;
-            // or just cpu_cycles %2
+            // or just `cpu_cycles %2`
             self.is_between_apu_cycle = !self.is_between_apu_cycle
         }
     }