Games with Issues on 1CHIP Consoles

This article covers games that have graphical or audio problems on 1CHIP (S-CPUN) SNES and Super Famicom consoles, including the SNES Jr. and Super Famicom Jr. These problems are not found on older multi-chip ("3-Chip") console revisions unless noted otherwise. They are caused by differences between the S-CPUN's combined PPU design and the separate PPU chips used in earlier boards.

Technical Background

The 1CHIP SNES combines the two PPU chips (PPU1 and PPU2) and the CPU into a single chip called the S-CPUN. This reduced manufacturing cost and board complexity, but it also changed how the analog video output behaves.

The main issue involves the brightness register ($2100). The S-CPUN's internal DAC does not respond well to fast changes in the brightness register value. Many games change this register in the middle of a frame, for example by turning forced blanking on or off with an instant brightness change. The 1CHIP's DAC cannot settle fast enough, which causes visible problems like darkened or broken scanlines near the top of the screen, or incorrect shadow and fade effects.

The common C11 capacitor replacement mod makes this worse. This mod replaces the stock 47nF capacitor with a larger value (typically 220nF to 470nF) to reduce ghosting. The larger capacitor slows down the DAC's settling time, which makes the brightness transition problems more noticeable. Even with the stock 47nF capacitor, the issue still exists to a smaller degree on all 1CHIP consoles.

The root cause was found and documented by members of the Shmups Forum hardware community,^[1] including Voultar, FBX, borti4938, and ikari_01, among others.

SD2SNES / FXPak Brightness Patching

Starting with firmware v1.8.0 (released June 29, 2018), the SD2SNES (now FXPak / FXPak Pro) includes an experimental FPGA-based brightness register patch for 1CHIP consoles. From the v1.8.0 release notes:^[2]

1CHIP transient fix – tries to alleviate some graphical issues with 1CHIP consoles where it reacts unfavorably to sudden changes to the brightness register. This fixes the shadow in Air Strike Patrol, warped scanlines in Rudra's Treasure, and faded scanlines at the top of the screen on certain Capcom games when the console is equipped with a de-ghosting fix involving the replacement of the C11 capacitor.

The patch intercepts CPU writes to the brightness register ($2100) using the FPGA. When a game switches to forced blanking, the patch keeps the previous brightness value in the DAC instead of letting the game reset it to zero. This avoids the voltage change that causes the visual problems. When a game tries to briefly darken the picture, a brightness value of zero is used instead. This was first committed by ikari (mrehkopf) on April 20, 2018.^[3]

Note: This fix has limitations. Games that use DMA to write to the brightness register (such as Star Fox and some MSU1 video players) may have problems when the patch is active. Users should turn off the feature generally and only enable it for known affected games. The patch can be toggled on or off in the SD2SNES/FXPak menu.

Known affected Games

List originally compiled on Reddit by /u/morli.^[4]

Removed Entries

The following games were previously listed but have been removed after further review:

• Super Ghouls 'n Ghosts (JP version, Chō Makaimura): The reported sprite glitches and heavy slowdown came from a single user who was playing the game through a Super UFO Pro 8 copier. Testing has confirmed that these problems are caused by the copier and do not occur when the game is played from an original cartridge on a 1CHIP console.

• Super Street Fighter II: The reported sprite glitches and heavy slowdown came from the same user as the Super Ghouls 'n Ghosts report. The issue has not been reproduced on 1CHIP hardware by other users.

• Earthbound: The reported flickering pixels during intro cinematic transitions are not specific to 1CHIP consoles. This behavior has been seen on multiple SNES board revisions, including clone hardware, and appears to happen randomly across all models. It is not a 1CHIP-specific problem.

• Pocky and Rocky: The original Reddit thread listed this game without describing a specific issue. Multiple users have been unable to find any problem on 1CHIP consoles.

• Soul Blazer: Bad lag in the opening music as the title text scrolls across the screen during the intro is not specific to 1CHIP consoles. Since the issue is present across multiple board revisions, it is not a 1CHIP-specific problem. ^[10]

References