Amstrad Cartridge Identification Device
The ACID (Amstrad Cartridge Identification Device) protection chip is used in the cartridges of the Plus series and the GX4000 game console.
Contents
Technical
The 16pin ACID chip (in the cartridge) is sending a serial data stream to the 160pin ASIC chip (in the computer). The data stream is generated by some shift/xor logic, and is also affected by the EPROM address/enable signals. If the data stream isn't correct, then the ASIC does scatter access to RAM, making the cartridge unusable.
;ACID reverse-engineered 13-16 February 2010 by nocash (Martin Korth) ;below is repeated on every CLK cycle... CmpVal=13596h, XorVal=0c820h if PinA0=1 then CmpVal=CmpVal XOR 0000ch, XorVal=XorVal XOR 00004h if PinA1=1 then CmpVal=CmpVal XOR 06000h, XorVal=XorVal XOR 06000h if PinA2=1 then CmpVal=CmpVal XOR 000c0h, XorVal=XorVal XOR 00080h if PinA3=1 then CmpVal=CmpVal XOR 00030h, XorVal=XorVal XOR 00020h if PinA4=1 then CmpVal=CmpVal XOR 18000h, XorVal=XorVal XOR 08000h if PinA5=1 then CmpVal=CmpVal XOR 00003h, XorVal=XorVal XOR 00000h if PinA6=1 then CmpVal=CmpVal XOR 00600h, XorVal=XorVal XOR 00000h if PinA7=1 then CmpVal=CmpVal XOR 01800h, XorVal=XorVal XOR 00800h if PinCE=0 AND (ShiftReg OR 100h)=CmpVal then ShiftReg=ShiftReg XOR XorVal NewBit=ShiftRegBit0 XOR ShiftRegBit9 XOR ShiftRegBit12 XOR ShiftRegBit16 ShiftReg=(ShiftReg SHR 1) + (NewBit SHL 16) Wait for falling edge on PinCLK if PinCCLR=0 then ShiftReg=1FFFFh ;\done at falling CLK edge PinSIN=ShiftRegBit0 ;/
Replacing or overwriting the EPROM in existing cartridges is easy, so the ACID doesn't prevent piracy or homebrew programming. However, it prevents commercial developers from producing unlicensed cartridges.
Pinout
+--()--+
GND |1 16| +5V AMSTRAD 40908
A7 |2 15| A6 1L03P1003
CLK4|3 14| A5 JAPAN
NC |4 13| A4
CCLR|5 12| A3
SIN |6 11| A2
/CE |7 10| A1
GND |8 9| A0
+------+
| Pin number | Name | Direction | Description |
| 1 | GND | - | Ground |
| 2 | A7 | Input | EPROM Address A7 |
| 3 | CLK4 | Input | Clock (4 MHz) |
| 4 | NC | - | Not connected |
| 5 | CCLR | Input | Reset ACID |
| 6 | SIN | Output | Serial Data |
| 7 | /CE | Input | EPROM Chip enable |
| 8 | GND | - | Ground |
| 9 | A0 | Input | EPROM Address A0 |
| 10 | A1 | Input | EPROM Address A1 |
| 11 | A2 | Input | EPROM Address A2 |
| 12 | A3 | Input | EPROM Address A3 |
| 13 | A4 | Input | EPROM Address A4 |
| 14 | A5 | Input | EPROM Address A5 |
| 15 | A6 | Input | EPROM Address A6 |
| 16 | +5V | - | Vcc |
Note: The two GND pins are interconnected with each other (inside of the chip). The NC pin seems to be always high.
