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PsNee, an open source stealth modchip for the Sony Playstation 1

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This version is from Is developed by the psxdev team

This PsNee version is meant for Arduino boards.

  • Arduino Pro Mini @8Mhz and @16Mhz (supported, tested)

  • Arduino Uno @8Mhz and @16Mhz (supported, tested)

  • Pro Micro @8Mhz and @16Mhz (supported, tested)

  • Arduino Leonardo @8Mhz and @16Mhz: Should work the same as Pro Micro, but two pins are on the ICSP header (supported, untested)

  • ATtiny85: Should work the same as ATtiny45 (supported, untested)

  • ATtiny45: LFUSE 0xE2 HFUSE 0xDF > internal oscillator, full 8Mhz speed (supported, tested)

  • ATtiny25: Should work the same as ATtiny45 but doesn't have enough Flash nor RAM for PSNEEDEBUG (supported, untested)

Be sure to use the 3.5V / 3.3V PSX power supply, * AND NOT THE 5V! * for your PsNee. (The ps chipset works in 3.3v, and their injected 5v is not good.) The installation pictures include an example.

Some extra libraries might be required, depending on the board / chip used. This code defaults to multi-region, meaning it will unlock PAL, NTSC-U and NTSC-J machines. You can optimize boot times for your console further. See "// inject symbols now" in the main loop.

BIOS patch for PM-41

For now it only supports Arduino boards (ATmega chips). Also, the Arduino must either be powered on first or have no bootloader present (flashed using SPI) since I expect a signal ~1 second after power on. 8Mhz boards are also supported.

Choose your hardware!
You must uncomment the line that corresponds to your chips.

2 main branches are available:

  • ATmega based: easy to use, fast and nice features for development
  • ATtiny based: fewer features, internal clock has 10% variation

This code is multi-region, meaning it will unlock PAL, NTSC-U and NTSC-J machines.

                  Pin assignments
                  PSNee psxdev                                PlayStation
  Arduino   Atinny   ProMicro   Leonardo    name           ps pin         Name

  pin-5v   = VCC    = VCC      = pin-5v    = 3.5v         3.5v           = supply
             3      =          =           = debugtx
  pin-9    = 4      = pin-15   = icsp-sck  = gate_wfck    IC732.Pin-5    = WFCK           
  pin-8    = 2      = pin-14   = icsp-miso = data         IC732.Pin-42   = CEO
  pin-7    = 1      = pin-3    = pin-3     = subq         IC304.Pin-24   = SUBQ
  pin-6    = 0      = pin-2    = pin-2     = sqck         IC304.Pin-26   = SQCK
  pin 5             = pin-9    = pin-9     = BIOS D2      IC102.Pin-15   = D2
  pin 4             = pin-8    = pin-8     = BIOS A18     Ic102.Pin-31   = A18
  pin-gnd  = GND    = GND      = pin-gnd   = gnd          GND            = gnd

General Info


Sony didn't really go through great lenghts to protect its precious Playstation from running unauthorised software: the main security is based on a simple ASCII string of text that is read from a part of an original Playstation disc that cannot be reproduced by an ordinary PC CD burner. As most of you will know, a CD is basically a very long rolled up (carrier) string in which very little pits and ehm... little not-pits are embedded that represent the data stored on the disc. The nifty Sony engineers did not use the pits and stuff to store the security checks for Playstation discs, but went crazy with the rolled up carrier string. In an ordinary CD, the string is rolled up so that the spacing between the tracks is as equal as possible. If that is not the case, the laser itself needs to move a bit to keep track of the track and reliably read the data off the disc. If you wonder how the laser knows when it follows the track optimally: four photodiodes, light intensity measurement, difference measurements, servo. There. To the point: the Sony engineers decidedly "fumbled up" the track of sector 4 on a Playstation disc (the track was modulated, in nerd-speak) so that the error correction circuit outputs a recognisable signal, as the laser needs to be corrected to follow the track optimally. This output signal actually is a 250bps serial bitstream (with 1 start bit and 2 stop bits) which in plain ASCII says SCEA (Sony Computer Entertainment of America), SCEE (Sony Computer Entertainment of Europe) or SCEI (Sony Computer Entertainment of Japan), depending on the region of the disc inserted. The security thus functions not only as copy protection, but also as region protection. The text string from the disc is compared with the text string that is embedded in the Playstation hardware. When these text strings are the same, the disc is interpreted to be authentic and from the correct region. Bingo!

The master branch is completely redesigned!

The original code doesn't have a mechanism to turn the injections off. It bases everything on a timer. After power on, it will start sending injections for some time, then turns off. It also doesn't know when it's required to turn on again (except for after a reset), so it gets detected by anti-mod games.

The mechanism to know when to inject and when to turn it off.

This is the 2 wires for SUBQ / SQCK. The PSX transmits the current subchannel Q data on this bus. It tells the console where on the disc the read head is. We know that the protection symbols only exist on the earliest sectors, and that anti-mod games exploit this by looking for the symbols elsewhere on the disk. If they get those symbols, a modchip must be generating them!

So with that information, my code knows when the PSX wants to see the unlock symbols, and when it's "fake" / anti-mod. The chip is continously looking at that subcode bus, so you don't need the reset wire or any other timing hints that other modchips use. That makes it compatible and fully functional with all revisions of the PSX, not just the later ones. Also with this method, the chip knows more about the current CD. This allows it to not send unlock symbols for a music CD, which means the BIOS starts right into the CD player, instead of after a long delay with other modchips.

This has some drawbacks, though:

  • It's more logic / code. More things to go wrong. The testing done so far suggests it's working fine though.
  • It's not a good example anymore to demonstrate PSX security, and how modchips work in general.


PsNee, an open source stealth modchip for the Sony Playstation 1



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