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 Post subject: PPU bus activity traces
PostPosted: Thu Jun 30, 2016 1:05 am 

Joined: Sun Apr 13, 2008 11:12 am
Posts: 6283
Location: Seattle
Tried to get all of AWJ's requests.

Rather than a separate trace for each PPU mode, I used HDMA to switch to the next mode every 32 scanlines. Layer 1 is using 16x16 tiles, layers 2-4 are using 8x8 tiles. Sprites #0-95 are 16x16, sprites #96-127 are 8x8.

Every sixteenth word of the nametable for layer 1 has the "flipped horizontally" bit set.

Sample rate is now 12MHz, so the aliasing will be a bit more annoying (alternating between 167ns and 250ns instead of the true value of 186ns)

I couldn't get all the logic analyzer clips on one RAM, so this is a random mixture of lines on U5 and U4.

Pertinent details: BGnSC of $70, $74, $78, $7C; BGnNBA of 0, 3, 5, 6; OBSEL=$0A. HDMA changes BGMODE from $10...$16, each value lasting for 32 scanlines. HDMA changes BG3VOFS to get offset-per-tile to be visible.

File comment: 9MB uncompressed, 1million lines; first column is CSYNC, other column is PPU bus address in hexadecimal.
ppubusactivity_rev2_csv.7z [272.23 KiB]
Downloaded 67 times
File comment: just for reference
ppubusactivity_rev2_sfc.7z [12.36 KiB]
Downloaded 73 times
File comment: rename without .zip; open with sigrok pulseview [506.43 KiB]
Downloaded 69 times
PostPosted: Fri Jul 01, 2016 7:38 pm 

Joined: Sun Apr 13, 2008 11:12 am
Posts: 6283
Location: Seattle
Well, let's try to recapitulate AWJ's previous analysis, with the extra data:

Mode 0: no new insights.
Modes 1 & 3: Lower-addressed bitplane is fetched first
Mode 2: Also lower-addressed OPT row is fetched first
Mode 5: Fetch cadence appears to be completely identical to mode 3. Horizontal flip flag does reverse left-right fetch order.
Modes 4,6: No new insights beyond above.

Sprites: Lower-addressed bitplane is fetched first. Horizontally flipped sprites reverse sliver fetch order.

Each scanline fetches 33 slivers for tiles (taking 8 cycles per tile, for 264 total cycles), followed by 8 idle cycles (bus unchanging), followed by 34 slivers for sprites (taking 2 cycles per sliver, 68 total cycles).
That's only 340 cycles, but there's 341 usually; two extra half-cycles appear to be be inserted during hsync and immediately after it ends.

Hsync timing is not obviously aligned to sprite fetch cycles. It's hard to tell anything more precise with the comparatively low sample rate.

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