diff options
Diffstat (limited to 'docs/research.md')
| -rw-r--r-- | docs/research.md | 32 |
1 files changed, 23 insertions, 9 deletions
diff --git a/docs/research.md b/docs/research.md index 9fe4ab5..9b87324 100644 --- a/docs/research.md +++ b/docs/research.md @@ -100,7 +100,7 @@ int main() { Here's a list of features our PPU should have: <!-- TODO: expand list with PPU spreadsheet --> -- 256x224 @ 60Hz VGA output +- 320x240 @ 60Hz VGA output - single tilemap with room for 1024 tiles of 16x16 pixels - 8 colors per palette, with 4096 possible colors (12-bit color depth) - 512x448 background canvas with scrolling @@ -116,6 +116,11 @@ Notable differences: - NES nametable equivalent is called BAM (background attribute register) - NES OAM equivalent is called FAM (foreground attribute register) +- 320x240 @ 60Hz output + + Since we're using VGA, we can't use custom resolutions without an + upscaler/downscaler. This resolution was chosen because it's exactly half of + the lowest standard VGA resolution 640x480. - No scanline sprite limit Unless not imposing any sprite limit makes the hardware implementation @@ -176,13 +181,22 @@ Notable differences: Important notes: -- The STM32 receives 'feedback' lines from the PPU. These are mirrors of the - VSYNC and HSYNC lines from the VGA signal generator. These lines can be used - to trigger interrupts for counting frames, and to make sure no read/write - conflicts occur for protected memory regions in the PPU. -- The STM32 can enable and reset the PPU. These lines will also be connected to - physical buttons on the FPGA. +- The STM32 can reset the PPU. This line will also be connected to a physical + button on the FPGA. - The STM32 uses direct memory access to control the PPU. +- The PPU's native resolution is 320x240. It works in this resolution as if it + is a valid VGA signal. The STM32 is also only aware of this resolution. This + resolution is referred to as "tiny" resolution. Because VGA-compatible LCD's + likely don't support this resolution due to low clock speed, a built-in + pixel-perfect 2X upscaler is chained after the PPU's "tiny" output. This + means that the display sees the resolution as 640x480, but the PPU and STM32 + only work in 320x240. +- The STM32 receives the TVSYNC and THSYNC lines from the PPU. These are the + VSYNC and HSYNC lines from the tiny VGA signal generator. These lines can be + used to trigger interrupts for counting frames, and to make sure no + read/write conflicts occur for protected memory regions in the PPU. +- NVSYNC, NHSYNC and the RGB signals refer to the output of the native VGA + signal generator. #### Level 2 @@ -194,8 +208,8 @@ Important notes: 1. - (Foreground sprite info) calculate if foreground sprite exists at current pixel using FAM register - (Background sprite info) get background sprite info from BAM register - 3. - (Sprite render) calculate pixel to read from TMM based on sprite info - 5. - (Compositor) get pixel with 'highest' priority (pick first foreground + 2. - (Sprite render) calculate pixel to read from TMM based on sprite info + 3. - (Compositor) get pixel with 'highest' priority (pick first foreground sprite with non-transparent color at current pixel in order, fallback to background) - (Palette lookup) lookup palette color using palette register |