Question about MMC3's scanline counter

This is an archive of a topic from NESdev BBS, taken in mid-October 2019 before a server upgrade.
View original topic
Question about MMC3's scanline counter
by on (#158498)
I'm currently implementing MMC3's scanline counter and I just have a question to clarify.

If I have both sprite and BG patterns set to $0000 (meaning no transition of A12 from low to high), does the scanline counter get clocked every 2 scanlines due to increment of PPU's Fine Y on PPU clock 256? (I say every 2 scanlines because it flips between odd and even fine Y values)
Re: Question about MMC3's scanline counter
by on (#158499)
breathermachine wrote:
I'm currently implementing MMC3's scanline counter and I just have a question to clarify.

If I have both sprite and BG patterns set to $0000 (meaning no transition of A12 from low to high), does the scanline counter get clocked every 2 scanlines due to increment of PPU's Fine Y on PPU clock 256? (I say every 2 scanlines because it flips between odd and even fine Y values)


No, because that fine Y bit does not appear on the address bus at A12 - it only affects A0, and even then only during pattern table fetches.