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|
module WarfaceMapper
(
input m2,
input romsel,
input cpu_rw,
input ppu_rd,
input ppu_wr,
input cpu_a0,
input cpu_a13,
input cpu_a14,
input [7:0] cpu_data,
output [16:13] prg_addr,
output prg_ce,
output prg_oe,
output prg_prog,
input [13:10] ppu_addr_in,
output [16:10] ppu_addr_out,
output chr_ce,
output chr_oe,
output chr_prog,
output ciram_a10,
output irq
);
// scanline counter, counts dummy PPU reads, detects v-blank automatically
reg [3:0] ppu_rd_hi_time = 0; // counts how long there is no reads from PPU to detect v-blank
reg [1:0] ppu_nt_read_count; // nametable read counter
reg [7:0] scanline = 0; // current scanline
reg new_screen = 0; // stores 1 when v-blank detected ("in frame" flag)
reg new_screen_clear = 0; // flag to clear new_screen flag
assign prg_ce = romsel;
assign prg_oe = romsel | !cpu_rw;
assign prg_prog = romsel | cpu_rw;
reg [2:0] prg_bank;
assign prg_addr = cpu_a14
? {3'b111, cpu_a13} // fixed last bank @ $C000-$FFFF
: {prg_bank, cpu_a13}; // selectable @ $8000-$BFFF
assign chr_ce = ppu_addr_in[13];
assign chr_oe = ppu_addr_in[13] | ppu_rd;
assign chr_prog = ppu_addr_in[13] | ppu_wr;
reg prg_auto_switch = 0;
reg [4:0] chr_bank;
reg [1:0] chr_latch;
assign ppu_addr_out[16:10] = !ppu_addr_in[12]
? (
prg_auto_switch
? {chr_bank[4:2], chr_latch[1:0], ppu_addr_in[11:10]} // $0000-$0FFF is autoswitchable
: {chr_bank[4:0], ppu_addr_in[11:10]} // $0000-$0FFF is switchable manually
)
: {5'b11111, ppu_addr_in[11:10]}; // $1000-$1FFF is fixed to last
assign ciram_a10 = ppu_addr_in[11]; // horizontal mirroring
reg [13:0] timer = 0;
reg timer_elapsed = 0;
assign irq = timer_elapsed ? 1'b0 : 1'bZ;
always @ (negedge m2)
begin
if (timer != 0)
begin
timer = timer - 1'b1;
if (timer == 0) timer_elapsed = 1;
end
if (romsel && !cpu_rw && cpu_a13 && cpu_a14) // write to $6000-$7FFF
begin
if (!cpu_a0)
begin // even
prg_bank[2:0] = cpu_data[2:0];
if (cpu_data[7]) timer = 4095;
timer_elapsed = 0;
end else begin // odd
prg_auto_switch = cpu_data[7];
chr_bank[4:0] = cpu_data[4:0];
end
end
end
// V-blank detector
always @ (negedge m2, negedge ppu_rd)
begin
if (~ppu_rd)
begin
ppu_rd_hi_time = 0;
if (new_screen_clear) new_screen = 0;
end else if (ppu_rd_hi_time < 4'b1111)
begin
// Counting how long there is no PPU reads
ppu_rd_hi_time = ppu_rd_hi_time + 1'b1;
end else begin
// Too long, v-blank detected
new_screen = 1;
end
end
// Scanline counter
always @ (negedge ppu_rd)
begin
if (!new_screen && new_screen_clear) new_screen_clear = 0;
if (new_screen & !new_screen_clear)
begin
scanline = 0;
new_screen_clear = 1;
chr_latch = 0;
end else
if (ppu_addr_in[13:12] == 2'b10)
begin
if (ppu_nt_read_count < 3)
begin
ppu_nt_read_count = ppu_nt_read_count + 1'b1;
end else begin
if (scanline == 64) chr_latch = 1;
if (scanline == 128) chr_latch = 2;
if (scanline == 192) chr_latch = 3;
scanline = scanline + 1'b1;
end
end else begin
ppu_nt_read_count = 0;
end
end
endmodule
|
