diff --git a/snes-core/src/ppu/interface.rs b/snes-core/src/ppu/interface.rs
index 4b7656b..74232b7 100644
--- a/snes-core/src/ppu/interface.rs
+++ b/snes-core/src/ppu/interface.rs
@@ -76,7 +76,7 @@ impl PPU {
         // (0xFF, 0x00, 0xFF)
     }
 
-    // TODO: wrte tests for this function
+    // TODO: wirte tests for this function
     fn compute_background_pixel(&self, background: Background) -> (u8, u8, u8) {
         // 0. detect video mode?
         // 1. get base tileset vram address
@@ -127,7 +127,6 @@ impl PPU {
         }
     }
 
-    // TODO: write tests
     fn mix_pixel_bitplanes(lsb_bitplane: u8, msb_bitplane: u8) -> [u8; 8] {
         [
             (
@@ -244,4 +243,28 @@ mod ppu_general_test {
         assert_eq!(ppu.framebuffer[2049], 22);
         assert_eq!(ppu.framebuffer[2050], 33);
     }
+
+    #[test]
+    fn test_mix_pixel_bitplanes() {
+        assert_eq!(
+            PPU::mix_pixel_bitplanes(0b00000000, 0b00000000),
+            [0b00, 0b00, 0b00, 0b00, 0b00, 0b00, 0b00, 0b00],
+        );
+        assert_eq!(
+            PPU::mix_pixel_bitplanes(0b11111111, 0b11111111),
+            [0b11, 0b11, 0b11, 0b11, 0b11, 0b11, 0b11, 0b11],
+        );
+        assert_eq!(
+            PPU::mix_pixel_bitplanes(0b11111111, 0b00000000),
+            [0b01, 0b01, 0b01, 0b01, 0b01, 0b01, 0b01, 0b01],
+        );
+        assert_eq!(
+            PPU::mix_pixel_bitplanes(0b00000000, 0b11111111),
+            [0b10, 0b10, 0b10, 0b10, 0b10, 0b10, 0b10, 0b10],
+        );
+        assert_eq!(
+            PPU::mix_pixel_bitplanes(0b11110000, 0b00001111),
+            [0b01, 0b01, 0b01, 0b01, 0b10, 0b10, 0b10, 0b10],
+        );
+    }
 }
diff --git a/snes-core/src/ppu/registers.rs b/snes-core/src/ppu/registers.rs
index 52aab98..73be983 100644
--- a/snes-core/src/ppu/registers.rs
+++ b/snes-core/src/ppu/registers.rs
@@ -45,7 +45,7 @@ pub const M7Y: u16          = 0x2120;  // Rotation/Scaling Center Coordinate Y (
 
 // PPU CGRAM
 pub const CGADD: u16        = 0x2121;  // Palette CGRAM Address
-pub const CGDATA: u16       = 0x2122;  // Palette CGRAM Address
+pub const CGDATA: u16       = 0x2122;  // Palette CGRAM data write
 
 // PPU Window
 pub const W12SEL: u16       = 0x2123;  // Window BG1/BG2 Mask Settings (W)