Nilquader and I decided to release our Verilog code (based on Nocash’s decryption of the algorithm) to “emulate” the ACID protection chip of the Amstrad Plus. So, you can now find it here (for more information have a look at the [[Amstrad Cartridge Identification Device|ACID article]] of the CPCWiki):

`timescale 1ns / 1ps
//////////////////////////////////////////////////////////////////////////////////
// Company:
// Engineer: Octoate, Nilquader
// ACID Reverse engineering by nocash
//
// Create Date   : 00:45:53 09/03/2010
// Design Name   : amsacid
// Module Name   : amsacid
// Project Name  :
// Target Devices: Xilinx XC9572
// Tool versions :
// Description   : Reverse engineered Amstrad 40908 "ACID" Chip
//
// Revision:
// Revision 0.05
// Additional Comments:
//
//////////////////////////////////////////////////////////////////////////////////
module amsacid(PinCLK, PinA, PinOE, PinCCLR, PinSIN);

input PinCLK;
input [7:0]PinA;
input PinOE;
input PinCCLR;

output [7:0]PinSIN;

wire PinCLK;

reg [16:0]ShiftReg = 17'h1FFFF;
wire [16:0]CmpVal;
wire [16:0]XorVal;

assign CmpVal = 17'h13596	^ (PinA[0] ? 17'h0000c : 0)
				^ (PinA[1] ? 17'h06000 : 0)
				^ (PinA[2] ? 17'h000c0 : 0)
				^ (PinA[3] ? 17'h00030 : 0)
				^ (PinA[4] ? 17'h18000 : 0)
				^ (PinA[5] ? 17'h00003 : 0)
				^ (PinA[6] ? 17'h00600 : 0)
				^ (PinA[7] ? 17'h01800 : 0);
assign XorVal = 17'h0C820	^ (PinA[0] ? 17'h00004 : 0)
				^ (PinA[1] ? 17'h06000 : 0)
				^ (PinA[2] ? 17'h00080 : 0)
				^ (PinA[3] ? 17'h00020 : 0)
				^ (PinA[4] ? 17'h08000 : 0)
				^ (PinA[5] ? 17'h00000 : 0)
				^ (PinA[6] ? 17'h00000 : 0)
				^ (PinA[7] ? 17'h00800 : 0);

always@(negedge PinCLK)
	begin

	if (PinCCLR) // not in reset state
		begin
		if (!PinOE && ((ShiftReg | 17'h00100) == CmpVal))
			begin
			ShiftReg <= (ShiftReg ^ XorVal) >> 1;
			ShiftReg[16] <= ShiftReg[0] ^ ShiftReg[9] ^ ShiftReg[12] ^ ShiftReg[16] ^ XorVal[0];  // hier xorval mit berüchsichtigen
			end
		else
			begin
			ShiftReg <= ShiftReg >> 1;
			ShiftReg[16] <= ShiftReg[0] ^ ShiftReg[9] ^ ShiftReg[12] ^ ShiftReg[16];
			end
		end
	else
		begin
			ShiftReg <= 17'h1FFFF;
		end
	end

//assign PinSIN = ShiftReg[7:0] ^ 8'hff;
assign PinSIN = ShiftReg[7:0];
//assign PinSIN[0] = PinCLK;

endmodule

But this isn’t everything about the ACID. We also found a timing problem during the investigation with a logic analyzer, which you can see here:

ACID initialisation (by Grim)

You will find glitches on the /CCLR line which show that the SIN contact should be changed… All in all it is possible to use a fast flip-flop to change the SIN signal when such a glitch occurs (see picture below).

So, have fun with it :-).