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`timescale 1ns / 1ps
//////////////////////////////////////////////////////////////////////////////////
// Company:        Dept. Architecture and Computing Technology. University of Seville
// Engineer:       Miguel Angel Rodriguez Jodar. rodriguj@atc.us.es
// 
// Create Date:    19:13:39 4-Apr-2012 
// Design Name:    ZX Spectrum
// Module Name:    ula 
// Project Name: 
// Target Devices: 
// Tool versions: 
// Description: 
//
// Dependencies: 
//
// Revision: 
// Revision 1.00 - File Created
// Additional Comments: GPL License policies apply to the contents of this file.
//
//////////////////////////////////////////////////////////////////////////////////
 
`define cyclestart(a,b) ((a)==(b))
`define cycleend(a,b) ((a)==(b+1))
 
module ula(
    input clk14,                        // 14MHz master clock
         input reset,                   // to reset the ULA to normal color mode.
         // CPU interfacing
    input [15:0] a,              // Address bus from CPU (not all lines are used)
    input [7:0] din,             // Input data bus from CPU
         output [7:0] dout,      // Output data bus to CPU
    input mreq_n,                       // MREQ from CPU
    input iorq_n,                       // IORQ from CPU
    input rd_n,                 // RD from CPU
    input wr_n,                 // WR from CPU
         input rfsh_n,                  // RFSH from CPU
         output clkcpu,         // CLK to CPU
         output msk_int_n,      // Vertical retrace interrupt, to CPU
         // VRAM interfacing
    output [13:0] va,     // Address bus to VRAM (16K)
         input [7:0] vramdout,// Data from VRAM to ULA/CPU
         output [7:0] vramdin,// Data from CPU to VRAM
    output vramoe,               // 
    output vramcs,               // Control signals for VRAM
    output vramwe,               //
         // ULA I/O
    input ear,                             //
    output mic,                    // I/O ports
    output spk,            //
         output [7:0] kbrows,   // Keyboard rows
    input [4:0] kbcolumns,       //  Keyboard columns
         // Video output
    output r,                           //
    output g,                           // RGB TTL signal
    output b,                           // with separate bright
    output i,                           // and composite sync
    output csync                        //               
    );
 
        reg [2:0] BorderColor = 3'b100;
        reg TimexHiColorMode = 0;
 
        reg ULAPlusConfig = 0;
        reg [7:0] ULAPlusAddrReg = 0;
        wire addrportsel = !iorq_n && a[0] && !a[2] && (a[7:6]==2'b00) && (a[15:14]==2'b10); // port BF3Bh
        wire dataportsel = !iorq_n && a[0] && !a[2] && (a[7:6]==2'b00) && (a[15:14]==2'b11); // port FF3Bh
        wire cpu_writes_palette = dataportsel && !wr_n && (ULAPlusAddrReg[7:6]==2'b00);
        reg [5:0] paletteaddr;
        wire [7:0] palettedout;
        reg palettewe;
 
        ram64bytes palette (
                .clk(clk14),
                .a(paletteaddr),
                .din(din),
                .dout(palettedout),
                .we(palettewe)
                );
 
        // Pixel clock
        reg clk7 = 0;
        always @(posedge clk14)
                clk7 <= !clk7;
 
        // Horizontal counter
        reg [8:0] hc = 0;
        always @(posedge clk7) begin
                if (hc==447)
                        hc <= 0;
                else
                        hc <= hc + 1;
        end
 
        // Vertical counter
        reg [8:0] vc = 0;
        always @(posedge clk7) begin
                if (hc==447) begin
                        if (vc == 311)
                                vc <= 0;
                        else
                                vc <= vc + 1;
                end
        end
 
        // HBlank generation
        reg HBlank_n = 1;
        always @(negedge clk7) begin
                if (`cyclestart(hc,320))
                        HBlank_n <= 0;
                else if (`cycleend(hc,415))
                        HBlank_n <= 1;
        end
 
        // HSync generation (6C ULA version)
        reg HSync_n = 1;
        always @(negedge clk7) begin
                if (`cyclestart(hc,344))
                        HSync_n <= 0;
                else if (`cycleend(hc,375))
                        HSync_n <= 1;
        end
 
        // VBlank generation
        reg VBlank_n = 1;
        always @(negedge clk7) begin
                if (`cyclestart(vc,248))
                        VBlank_n <= 0;
                else if (`cycleend(vc,255))
                        VBlank_n <= 1;
        end
 
        // VSync generation (PAL)
        reg VSync_n = 1;
        always @(negedge clk7) begin
                if (`cyclestart(vc,248))
                        VSync_n <= 0;
                else if (`cycleend(vc,251))
                        VSync_n <= 1;
        end
 
        // INT generation
        reg INT_n = 1;
        assign msk_int_n = INT_n;
        always @(negedge clk7) begin
                if (`cyclestart(vc,248) && `cyclestart(hc,0))
                        INT_n <= 0;
                else if (`cyclestart(vc,248) && `cycleend(hc,31))
                        INT_n <= 1;
        end
 
        // Border control signal (=0 when we're not displaying paper/ink pixels)
        reg Border_n = 1;
        always @(negedge clk7) begin
                if ( (vc[7] & vc[6]) | vc[8] | hc[8])
                        Border_n <= 0;
                else
                        Border_n <= 1;
        end
 
        // VidEN generation (delaying Border 8 clocks)
        reg VidEN_n = 1;
        always @(negedge clk7) begin
                if (hc[3])
                        VidEN_n <= !Border_n;
        end
 
        // DataLatch generation (posedge to capture data from memory)
        reg DataLatch_n = 1;
        always @(negedge clk7) begin
                if (hc[0] & hc[1] & Border_n & hc[3])
                        DataLatch_n <= 0;
                else
                        DataLatch_n <= 1;
        end
 
        // AttrLatch generation (posedge to capture data from memory)
        reg AttrLatch_n = 1;
        always @(negedge clk7) begin
                if (hc[0] & !hc[1] & Border_n & hc[3])
                        AttrLatch_n <= 0;
                else
                        AttrLatch_n <= 1;
        end
 
        // SLoad generation (negedge to load shift register)
        reg SLoad = 0;
        always @(negedge clk7) begin
                if (!hc[0] & !hc[1] & hc[2] & !VidEN_n)
                        SLoad <= 1;
                else
                        SLoad <= 0;
        end
 
        // AOLatch generation (negedge to update attr output latch)
        reg AOLatch_n = 1;
        always @(negedge clk7) begin
                if (hc[0] & !hc[1] & hc[2])
                        AOLatch_n <= 0;
                else
                        AOLatch_n <= 1;
        end
 
        // First buffer for bitmap
        reg [7:0] BitmapReg = 0;
        always @(negedge DataLatch_n) begin
                BitmapReg <= vramdout;
        end
 
        // Shift register (second bitmap register)
        reg [7:0] SRegister = 0;
        always @(negedge clk7) begin
                if (SLoad)
                        SRegister <= BitmapReg;
                else
                        SRegister <= {SRegister[6:0],1'b0};
        end
 
        // First buffer for attribute
        reg [7:0] AttrReg = 0;
        always @(negedge AttrLatch_n) begin
                AttrReg <= vramdout;
        end
 
        // Second buffer for attribute
        reg [7:0] AttrOut = 0;
        always @(negedge AOLatch_n) begin
                if (!VidEN_n)
                        AttrOut <= AttrReg;
                else
                        AttrOut <= {2'b00,BorderColor,BorderColor};
        end
 
        // Flash counter and pixel generation
        reg [4:0] FlashCnt = 0;
        always @(negedge VSync_n) begin
                FlashCnt <= FlashCnt + 1;
        end
        wire Pixel = SRegister[7] ^ (AttrOut[7] & FlashCnt[4]);
 
        // RGB generation
        reg rI,rG,rR,rB;
        assign r = rR;
        assign g = rG;
        assign b = rB;
        assign i = rI;
        always @(*) begin
                if (HBlank_n && VBlank_n)
                        {rI,rG,rR,rB} = (Pixel)? {AttrOut[6],AttrOut[2:0]} : {AttrOut[6],AttrOut[5:3]};
                else
                        {rI,rG,rR,rB} = 4'b0000;
        end
 
        //CSync generation
        assign csync = HSync_n & VSync_n;
 
        // VRAM address and control line generation
        reg [13:0] rVA = 0;
        reg rVCS = 0;
        reg rVOE = 0;
        reg rVWE = 0;
        assign va = rVA;
        assign vramcs = rVCS;
        assign vramoe = rVOE;
        assign vramwe = rVWE;
        // Latches to hold delayed versions of V and H counters
        reg [8:0] v = 0;
        reg [8:0] c = 0;
        // Address and control line multiplexor ULA/CPU
        always @(negedge clk7) begin
                if (Border_n && (hc[3:0]==4'b0111 || hc[3:0]==4'b1011)) begin     // cycles 7 and 11: load V and C from VC and HC
                        c <= hc;
                        v <= vc;
                end
        end
        // Address and control line multiplexor ULA/CPU
        always @(*) begin
                if (Border_n && (hc[3:0]==4'b1000 || hc[3:0]==4'b1001 || hc[3:0]==4'b1100 || hc[3:0]==4'b1101)) begin       // cycles 8 and 12: present attribute address to VRAM
                        rVA = (TimexHiColorMode)?       {1'b1,v[7:6],v[2:0],v[5:3],c[7:3]} :                                                                                                             // (cycles 9 and 13 load attr byte). 
                                                                                                {4'b0110,v[7:3],c[7:3]};                                                                                                                                                // Attribute address depends upon the mode selected
                        rVCS = 1;
                        rVOE = !hc[0];
                        rVWE = 0;
                end
                else if (Border_n && (hc[3:0]==4'b1010 || hc[3:0]==4'b1011 || hc[3:0]==4'b1110 || hc[3:0]==4'b1111)) begin  // cycles 10 and 14: present display address to VRAM 
                        rVA = {1'b0,v[7:6],v[2:0],v[5:3],c[7:3]};                                                // (cycles 11 and 15 load display byte)
                        rVCS = 1;
                        rVOE = !hc[0];
                        rVWE = 0;
                end
                else if (Border_n && hc[3:0]==4'b0000) begin
                        rVA = a[13:0];
                        rVCS = 0;
                        rVOE = 0;
                        rVWE = 0;
                end
                else begin      // when VRAM is not in use by ULA, give it to CPU
                        rVA = a[13:0];
                        rVCS = !a[15] & a[14] & !mreq_n;
                        rVOE = !rd_n;
                        rVWE = !wr_n;
                end
        end
 
        // Palette addr and control bus multiplexing
        always @(*) begin
                if (Border_n && (hc[3:0]==10 || hc[3:0]==14)) begin         // present address of paper to palette RAM
                        palettewe = 0;
                        paletteaddr = { AttrReg[7:6],1'b1,AttrReg[5:3] };
                end
                else if (Border_n && (hc[3:0]==11 || hc[3:0]==15)) begin    // present address of ink to palette RAM
                        palettewe = 0;
                        paletteaddr = { AttrReg[7:6],1'b0,AttrReg[2:0] };
                end
                else if (dataportsel) begin                                                                             // if CPU requests access, give it palette control
                        paletteaddr = ULAPlusAddrReg[5:0];
                        palettewe = cpu_writes_palette;
                end
                else begin
                        palettewe = 0;           // blocking assignment, so we will first deassert WE at palette RAM...
                        paletteaddr = {3'b001, BorderColor};  // ... then, we can change the palette RAM address
                end
        end
 
        // CPU contention
        reg CPUClk = 0;
        assign clkcpu = CPUClk;
        reg ioreqtw3 = 0;
        reg mreqt23 = 0;
        wire ioreq_n = (a[0] | iorq_n) & ~dataportsel & ~addrportsel;
        wire Nor1 = (~(a[14] | ~ioreq_n)) |
                    (~(~a[15] | ~ioreq_n)) |
                                        (~(hc[2] | hc[3])) |
                                        (~Border_n | ~ioreqtw3 | ~CPUClk | ~mreqt23);
        wire Nor2 = (~(hc[2] | hc[3])) |
                    ~Border_n |
                                        ~CPUClk |
                                        ioreq_n |
                                        ~ioreqtw3;
        wire CLKContention = ~Nor1 | ~Nor2;
 
        always @(posedge clk7) begin    // change clk7 by clk14 for 7MHz CPU clock operation
                if (CPUClk && !CLKContention)   // if there's no contention, the clock can go low
                        CPUClk <= 0;
                else
                        CPUClk <= 1;
        end
        always @(posedge CPUClk) begin
                ioreqtw3 <= ioreq_n;
                mreqt23 <= mreq_n;
        end
 
        // ULA+ palette management
        always @(posedge clk7 or posedge reset) begin
                if (reset)
                        ULAPlusConfig <= 0;
                else begin
                        if (addrportsel && !wr_n)
                                ULAPlusAddrReg <= din;
                        else if (dataportsel && !wr_n && ULAPlusAddrReg[7:6]==2'b01)
                                ULAPlusConfig <= din[0];
                end
        end
 
        // ULA-CPU interface
        assign dout = (!a[15] && a[14] && !mreq_n)? vramdout : // CPU reads VRAM through ULA as in the +3, not directly
                      (!iorq_n && !a[0])?          {1'b1,ear,1'b1,kbcolumns} :   // CPU reads keyboard and EAR state
                                          (!iorq_n && a[7:0]==8'hFF && !rd_n)? {6'b000000,TimexHiColorMode,1'b0} : // Timex hicolor config port. Only bit 1 is reported.
                                          (addrportsel && !rd_n)? ULAPlusAddrReg :  // ULA+ addr register
                                          (dataportsel && !rd_n && ULAPlusAddrReg[7:6]==2'b01)? {7'b0000000, ULAPlusConfig} :
                                          (dataportsel && !rd_n && ULAPlusAddrReg[7:6]==2'b00)? palettedout :
                                          (Border_n)?                  AttrReg :  // to emulate
                                                                      8'hFF;     // port FF (well, cannot be actually FF anymore)
        assign vramdin = din;           // The CPU doesn't need to share the memory input data bus with the ULA
        assign kbrows = {a[11]? 1'bz : 1'b0,    // high impedance or 0, as if diodes were been placed in between
                                                  a[10]? 1'bz : 1'b0,   // if the keyboard matrix is to be implemented within the FPGA, then
                                                  a[9]?  1'bz : 1'b0,   // there's no need to do this.
                                                  a[12]? 1'bz : 1'b0,
                                                  a[13]? 1'bz : 1'b0,
                                                  a[8]?  1'bz : 1'b0,
                                                  a[14]? 1'bz : 1'b0,
                                                  a[15]? 1'bz : 1'b0 };
        reg rMic = 0;
        reg rSpk = 0;
        assign mic = rMic;
        assign spk = rSpk;
        always @(negedge clk7 or posedge reset) begin
                if (reset)
                        TimexHiColorMode <= 0;
                else if (!iorq_n && a[7:0]==8'hFF && !wr_n)
                        TimexHiColorMode <= din[1];
                else if (!iorq_n & !a[0] & !wr_n)
                        {rSpk,rMic,BorderColor} <= din[5:0];
        end
endmodule

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Subversion Repositories zx_ula

[/] [zx_ula/] [branches/] [xilinx/] [spectrum_48k_spartan3_starter_kit_timex_hicolor_ulaplus/] [ula.v] - Blame information for rev 15

Line No.	Rev	Author	Line
1	15	mcleod_ide	`timescale 1ns / 1ps
2			`//////////////////////////////////////////////////////////////////////////////////`
3			`// Company: Dept. Architecture and Computing Technology. University of Seville`
4			`// Engineer: Miguel Angel Rodriguez Jodar. rodriguj@atc.us.es`
5			`//`
6			`// Create Date: 19:13:39 4-Apr-2012`
7			`// Design Name: ZX Spectrum`
8			`// Module Name: ula`
9			`// Project Name:`
10			`// Target Devices:`
11			`// Tool versions:`
12			`// Description:`
13			`//`
14			`// Dependencies:`
15			`//`
16			`// Revision:`
17			`// Revision 1.00 - File Created`
18			`// Additional Comments: GPL License policies apply to the contents of this file.`
19			`//`
20			`//////////////////////////////////////////////////////////////////////////////////`
21
22			`define cyclestart(a,b) ((a)==(b))
23			`define cycleend(a,b) ((a)==(b+1))
24
25			`module ula(`
26			`input clk14, // 14MHz master clock`
27			`input reset, // to reset the ULA to normal color mode.`
28			`// CPU interfacing`
29			`input [15:0] a, // Address bus from CPU (not all lines are used)`
30			`input [7:0] din, // Input data bus from CPU`
31			`output [7:0] dout, // Output data bus to CPU`
32			`input mreq_n, // MREQ from CPU`
33			`input iorq_n, // IORQ from CPU`
34			`input rd_n, // RD from CPU`
35			`input wr_n, // WR from CPU`
36			`input rfsh_n, // RFSH from CPU`
37			`output clkcpu, // CLK to CPU`
38			`output msk_int_n, // Vertical retrace interrupt, to CPU`
39			`// VRAM interfacing`
40			`output [13:0] va, // Address bus to VRAM (16K)`
41			`input [7:0] vramdout,// Data from VRAM to ULA/CPU`
42			`output [7:0] vramdin,// Data from CPU to VRAM`
43			`output vramoe, //`
44			`output vramcs, // Control signals for VRAM`
45			`output vramwe, //`
46			`// ULA I/O`
47			`input ear, //`
48			`output mic, // I/O ports`
49			`output spk, //`
50			`output [7:0] kbrows, // Keyboard rows`
51			`input [4:0] kbcolumns, // Keyboard columns`
52			`// Video output`
53			`output r, //`
54			`output g, // RGB TTL signal`
55			`output b, // with separate bright`
56			`output i, // and composite sync`
57			`output csync //`
58			`);`
59
60			`reg [2:0] BorderColor = 3'b100;`
61			`reg TimexHiColorMode = 0;`
62
63			`reg ULAPlusConfig = 0;`
64			`reg [7:0] ULAPlusAddrReg = 0;`
65			`wire addrportsel = !iorq_n && a[0] && !a[2] && (a[7:6]==2'b00) && (a[15:14]==2'b10); // port BF3Bh`
66			`wire dataportsel = !iorq_n && a[0] && !a[2] && (a[7:6]==2'b00) && (a[15:14]==2'b11); // port FF3Bh`
67			`wire cpu_writes_palette = dataportsel && !wr_n && (ULAPlusAddrReg[7:6]==2'b00);`
68			`reg [5:0] paletteaddr;`
69			`wire [7:0] palettedout;`
70			`reg palettewe;`
71
72			`ram64bytes palette (`
73			`.clk(clk14),`
74			`.a(paletteaddr),`
75			`.din(din),`
76			`.dout(palettedout),`
77			`.we(palettewe)`
78			`);`
79
80			`// Pixel clock`
81			`reg clk7 = 0;`
82			`always @(posedge clk14)`
83			`clk7 <= !clk7;`
84
85			`// Horizontal counter`
86			`reg [8:0] hc = 0;`
87			`always @(posedge clk7) begin`
88			`if (hc==447)`
89			`hc <= 0;`
90			`else`
91			`hc <= hc + 1;`
92			`end`
93
94			`// Vertical counter`
95			`reg [8:0] vc = 0;`
96			`always @(posedge clk7) begin`
97			`if (hc==447) begin`
98			`if (vc == 311)`
99			`vc <= 0;`
100			`else`
101			`vc <= vc + 1;`
102			`end`
103			`end`
104
105			`// HBlank generation`
106			`reg HBlank_n = 1;`
107			`always @(negedge clk7) begin`
108			if (`cyclestart(hc,320))
109			`HBlank_n <= 0;`
110			else if (`cycleend(hc,415))
111			`HBlank_n <= 1;`
112			`end`
113
114			`// HSync generation (6C ULA version)`
115			`reg HSync_n = 1;`
116			`always @(negedge clk7) begin`
117			if (`cyclestart(hc,344))
118			`HSync_n <= 0;`
119			else if (`cycleend(hc,375))
120			`HSync_n <= 1;`
121			`end`
122
123			`// VBlank generation`
124			`reg VBlank_n = 1;`
125			`always @(negedge clk7) begin`
126			if (`cyclestart(vc,248))
127			`VBlank_n <= 0;`
128			else if (`cycleend(vc,255))
129			`VBlank_n <= 1;`
130			`end`
131
132			`// VSync generation (PAL)`
133			`reg VSync_n = 1;`
134			`always @(negedge clk7) begin`
135			if (`cyclestart(vc,248))
136			`VSync_n <= 0;`
137			else if (`cycleend(vc,251))
138			`VSync_n <= 1;`
139			`end`
140
141			`// INT generation`
142			`reg INT_n = 1;`
143			`assign msk_int_n = INT_n;`
144			`always @(negedge clk7) begin`
145			if (`cyclestart(vc,248) && `cyclestart(hc,0))
146			`INT_n <= 0;`
147			else if (`cyclestart(vc,248) && `cycleend(hc,31))
148			`INT_n <= 1;`
149			`end`
150
151			`// Border control signal (=0 when we're not displaying paper/ink pixels)`
152			`reg Border_n = 1;`
153			`always @(negedge clk7) begin`
154			`if ( (vc[7] & vc[6]) \| vc[8] \| hc[8])`
155			`Border_n <= 0;`
156			`else`
157			`Border_n <= 1;`
158			`end`
159
160			`// VidEN generation (delaying Border 8 clocks)`
161			`reg VidEN_n = 1;`
162			`always @(negedge clk7) begin`
163			`if (hc[3])`
164			`VidEN_n <= !Border_n;`
165			`end`
166
167			`// DataLatch generation (posedge to capture data from memory)`
168			`reg DataLatch_n = 1;`
169			`always @(negedge clk7) begin`
170			`if (hc[0] & hc[1] & Border_n & hc[3])`
171			`DataLatch_n <= 0;`
172			`else`
173			`DataLatch_n <= 1;`
174			`end`
175
176			`// AttrLatch generation (posedge to capture data from memory)`
177			`reg AttrLatch_n = 1;`
178			`always @(negedge clk7) begin`
179			`if (hc[0] & !hc[1] & Border_n & hc[3])`
180			`AttrLatch_n <= 0;`
181			`else`
182			`AttrLatch_n <= 1;`
183			`end`
184
185			`// SLoad generation (negedge to load shift register)`
186			`reg SLoad = 0;`
187			`always @(negedge clk7) begin`
188			`if (!hc[0] & !hc[1] & hc[2] & !VidEN_n)`
189			`SLoad <= 1;`
190			`else`
191			`SLoad <= 0;`
192			`end`
193
194			`// AOLatch generation (negedge to update attr output latch)`
195			`reg AOLatch_n = 1;`
196			`always @(negedge clk7) begin`
197			`if (hc[0] & !hc[1] & hc[2])`
198			`AOLatch_n <= 0;`
199			`else`
200			`AOLatch_n <= 1;`
201			`end`
202
203			`// First buffer for bitmap`
204			`reg [7:0] BitmapReg = 0;`
205			`always @(negedge DataLatch_n) begin`
206			`BitmapReg <= vramdout;`
207			`end`
208
209			`// Shift register (second bitmap register)`
210			`reg [7:0] SRegister = 0;`
211			`always @(negedge clk7) begin`
212			`if (SLoad)`
213			`SRegister <= BitmapReg;`
214			`else`
215			`SRegister <= {SRegister[6:0],1'b0};`
216			`end`
217
218			`// First buffer for attribute`
219			`reg [7:0] AttrReg = 0;`
220			`always @(negedge AttrLatch_n) begin`
221			`AttrReg <= vramdout;`
222			`end`
223
224			`// Second buffer for attribute`
225			`reg [7:0] AttrOut = 0;`
226			`always @(negedge AOLatch_n) begin`
227			`if (!VidEN_n)`
228			`AttrOut <= AttrReg;`
229			`else`
230			`AttrOut <= {2'b00,BorderColor,BorderColor};`
231			`end`
232
233			`// Flash counter and pixel generation`
234			`reg [4:0] FlashCnt = 0;`
235			`always @(negedge VSync_n) begin`
236			`FlashCnt <= FlashCnt + 1;`
237			`end`
238			`wire Pixel = SRegister[7] ^ (AttrOut[7] & FlashCnt[4]);`
239
240			`// RGB generation`
241			`reg rI,rG,rR,rB;`
242			`assign r = rR;`
243			`assign g = rG;`
244			`assign b = rB;`
245			`assign i = rI;`
246			`always @(*) begin`
247			`if (HBlank_n && VBlank_n)`
248			`{rI,rG,rR,rB} = (Pixel)? {AttrOut[6],AttrOut[2:0]} : {AttrOut[6],AttrOut[5:3]};`
249			`else`
250			`{rI,rG,rR,rB} = 4'b0000;`
251			`end`
252
253			`//CSync generation`
254			`assign csync = HSync_n & VSync_n;`
255
256			`// VRAM address and control line generation`
257			`reg [13:0] rVA = 0;`
258			`reg rVCS = 0;`
259			`reg rVOE = 0;`
260			`reg rVWE = 0;`
261			`assign va = rVA;`
262			`assign vramcs = rVCS;`
263			`assign vramoe = rVOE;`
264			`assign vramwe = rVWE;`
265			`// Latches to hold delayed versions of V and H counters`
266			`reg [8:0] v = 0;`
267			`reg [8:0] c = 0;`
268			`// Address and control line multiplexor ULA/CPU`
269			`always @(negedge clk7) begin`
270			`if (Border_n && (hc[3:0]==4'b0111 \|\| hc[3:0]==4'b1011)) begin // cycles 7 and 11: load V and C from VC and HC`
271			`c <= hc;`
272			`v <= vc;`
273			`end`
274			`end`
275			`// Address and control line multiplexor ULA/CPU`
276			`always @(*) begin`
277			`if (Border_n && (hc[3:0]==4'b1000 \|\| hc[3:0]==4'b1001 \|\| hc[3:0]==4'b1100 \|\| hc[3:0]==4'b1101)) begin // cycles 8 and 12: present attribute address to VRAM`
278			`rVA = (TimexHiColorMode)? {1'b1,v[7:6],v[2:0],v[5:3],c[7:3]} : // (cycles 9 and 13 load attr byte).`
279			`{4'b0110,v[7:3],c[7:3]}; // Attribute address depends upon the mode selected`
280			`rVCS = 1;`
281			`rVOE = !hc[0];`
282			`rVWE = 0;`
283			`end`
284			`else if (Border_n && (hc[3:0]==4'b1010 \|\| hc[3:0]==4'b1011 \|\| hc[3:0]==4'b1110 \|\| hc[3:0]==4'b1111)) begin // cycles 10 and 14: present display address to VRAM`
285			`rVA = {1'b0,v[7:6],v[2:0],v[5:3],c[7:3]}; // (cycles 11 and 15 load display byte)`
286			`rVCS = 1;`
287			`rVOE = !hc[0];`
288			`rVWE = 0;`
289			`end`
290			`else if (Border_n && hc[3:0]==4'b0000) begin`
291			`rVA = a[13:0];`
292			`rVCS = 0;`
293			`rVOE = 0;`
294			`rVWE = 0;`
295			`end`
296			`else begin // when VRAM is not in use by ULA, give it to CPU`
297			`rVA = a[13:0];`
298			`rVCS = !a[15] & a[14] & !mreq_n;`
299			`rVOE = !rd_n;`
300			`rVWE = !wr_n;`
301			`end`
302			`end`
303
304			`// Palette addr and control bus multiplexing`
305			`always @(*) begin`
306			`if (Border_n && (hc[3:0]==10 \|\| hc[3:0]==14)) begin // present address of paper to palette RAM`
307			`palettewe = 0;`
308			`paletteaddr = { AttrReg[7:6],1'b1,AttrReg[5:3] };`
309			`end`
310			`else if (Border_n && (hc[3:0]==11 \|\| hc[3:0]==15)) begin // present address of ink to palette RAM`
311			`palettewe = 0;`
312			`paletteaddr = { AttrReg[7:6],1'b0,AttrReg[2:0] };`
313			`end`
314			`else if (dataportsel) begin // if CPU requests access, give it palette control`
315			`paletteaddr = ULAPlusAddrReg[5:0];`
316			`palettewe = cpu_writes_palette;`
317			`end`
318			`else begin`
319			`palettewe = 0; // blocking assignment, so we will first deassert WE at palette RAM...`
320			`paletteaddr = {3'b001, BorderColor}; // ... then, we can change the palette RAM address`
321			`end`
322			`end`
323
324			`// CPU contention`
325			`reg CPUClk = 0;`
326			`assign clkcpu = CPUClk;`
327			`reg ioreqtw3 = 0;`
328			`reg mreqt23 = 0;`
329			`wire ioreq_n = (a[0] \| iorq_n) & ~dataportsel & ~addrportsel;`
330			`wire Nor1 = (~(a[14] \| ~ioreq_n)) \|`
331			`(~(~a[15] \| ~ioreq_n)) \|`
332			`(~(hc[2] \| hc[3])) \|`
333			`(~Border_n \| ~ioreqtw3 \| ~CPUClk \| ~mreqt23);`
334			`wire Nor2 = (~(hc[2] \| hc[3])) \|`
335			`~Border_n \|`
336			`~CPUClk \|`
337			`ioreq_n \|`
338			`~ioreqtw3;`
339			`wire CLKContention = ~Nor1 \| ~Nor2;`
340
341			`always @(posedge clk7) begin // change clk7 by clk14 for 7MHz CPU clock operation`
342			`if (CPUClk && !CLKContention) // if there's no contention, the clock can go low`
343			`CPUClk <= 0;`
344			`else`
345			`CPUClk <= 1;`
346			`end`
347			`always @(posedge CPUClk) begin`
348			`ioreqtw3 <= ioreq_n;`
349			`mreqt23 <= mreq_n;`
350			`end`
351
352			`// ULA+ palette management`
353			`always @(posedge clk7 or posedge reset) begin`
354			`if (reset)`
355			`ULAPlusConfig <= 0;`
356			`else begin`
357			`if (addrportsel && !wr_n)`
358			`ULAPlusAddrReg <= din;`
359			`else if (dataportsel && !wr_n && ULAPlusAddrReg[7:6]==2'b01)`
360			`ULAPlusConfig <= din[0];`
361			`end`
362			`end`
363
364			`// ULA-CPU interface`
365			`assign dout = (!a[15] && a[14] && !mreq_n)? vramdout : // CPU reads VRAM through ULA as in the +3, not directly`
366			`(!iorq_n && !a[0])? {1'b1,ear,1'b1,kbcolumns} : // CPU reads keyboard and EAR state`
367			`(!iorq_n && a[7:0]==8'hFF && !rd_n)? {6'b000000,TimexHiColorMode,1'b0} : // Timex hicolor config port. Only bit 1 is reported.`
368			`(addrportsel && !rd_n)? ULAPlusAddrReg : // ULA+ addr register`
369			`(dataportsel && !rd_n && ULAPlusAddrReg[7:6]==2'b01)? {7'b0000000, ULAPlusConfig} :`
370			`(dataportsel && !rd_n && ULAPlusAddrReg[7:6]==2'b00)? palettedout :`
371			`(Border_n)? AttrReg : // to emulate`
372			`8'hFF; // port FF (well, cannot be actually FF anymore)`
373			`assign vramdin = din; // The CPU doesn't need to share the memory input data bus with the ULA`
374			`assign kbrows = {a[11]? 1'bz : 1'b0, // high impedance or 0, as if diodes were been placed in between`
375			`a[10]? 1'bz : 1'b0, // if the keyboard matrix is to be implemented within the FPGA, then`
376			`a[9]? 1'bz : 1'b0, // there's no need to do this.`
377			`a[12]? 1'bz : 1'b0,`
378			`a[13]? 1'bz : 1'b0,`
379			`a[8]? 1'bz : 1'b0,`
380			`a[14]? 1'bz : 1'b0,`
381			`a[15]? 1'bz : 1'b0 };`
382			`reg rMic = 0;`
383			`reg rSpk = 0;`
384			`assign mic = rMic;`
385			`assign spk = rSpk;`
386			`always @(negedge clk7 or posedge reset) begin`
387			`if (reset)`
388			`TimexHiColorMode <= 0;`
389			`else if (!iorq_n && a[7:0]==8'hFF && !wr_n)`
390			`TimexHiColorMode <= din[1];`
391			`else if (!iorq_n & !a[0] & !wr_n)`
392			`{rSpk,rMic,BorderColor} <= din[5:0];`
393			`end`
394			`endmodule`