/* vim: set ai et ts=4 sw=4: */

`default_nettype none

module top (
input logic clkin ,
output logic D5 ,

output logic vga_r ,
output logic vga_g ,
output logic vga_b ,
output logic vga_hs ,
output logic vga_vs ,

input logic wclk ,
input logic dc ,
input logic din ,
input logic cs
) ;

logic clk ;

// 640×480 @ 60Hz
// 25.125 Mhz, see `icepll -i 12 -o 25`
SB_PLL40_CORE # (
.FEEDBACK_PATH ( «SIMPLE» ) ,
.PLLOUT_SELECT ( «GENCLK» ) ,
.DIVR ( 4’b0000 ) ,
.DIVF ( 7’b1000010 ) ,
.DIVQ ( 3’b101 ) ,
.FILTER_RANGE ( 3’b001 ) ,
) uut (
.REFERENCECLK ( clkin ) ,
.PLLOUTCORE ( clk ) ,
.LOCK ( D5 ) , // keep this!
.RESETB ( 1’b1 ) ,
.BYPASS ( 1’b0 )
) ;

parameter addr_width = 13 ;
logic mem [ ( 1 << addr_width ) — 1 : 0 ] ;

logic [ addr_width — 1 : 0 ] raddr = 0 ;
logic [ addr_width — 1 : 0 ] waddr = 0 ;

always_ff @ ( posedge wclk ) // write memory
begin
if ( cs == 0 ) // chip select
begin
if ( dc ) // dc = high, accept data
begin
mem [ waddr ] <= din ;
waddr <= waddr + 1 ;
end // dc = low, ignore command
end
else
waddr <= 0 ;
end

parameter h_pulse = 96 ; // h-sunc pulse width
parameter h_bp = 48 ; // back porch pulse width
parameter h_pixels = 640 ; // number of pixels horizontally
parameter h_fp = 16 ; // front porch pulse width
parameter h_frame = h_pulse + h_bp + h_pixels + h_fp ;

parameter v_pulse = 2 ; // v-sync pulse width
parameter v_bp = 31 ; // back porch pulse width
parameter v_pixels = 480 ; // number of pixels vertically
parameter v_fp = 11 ; // front porch pulse width
parameter v_frame = v_pulse + v_bp + v_pixels + v_fp ;

parameter border = 10 ;
parameter h_offset = ( h_pixels — ( 128 * 4 ) ) / 2 ;
parameter v_offset = ( v_pixels — ( 64 * 4 ) ) / 2 ;

logic [ addr_width — 1 : 0 ] h_pos = 0 ;
logic [ addr_width — 1 : 0 ] v_pos = 0 ;

assign vga_hs = ( h_pos < h_pixels + h_fp ) ? 0 : 1 ;
assign vga_vs = ( v_pos < v_pixels + v_fp ) ? 0 : 1 ;

logic color = 0 ;
assign vga_r = color ;
assign vga_g = color ;
assign vga_b = color ;

always_ff @ ( posedge clk ) begin
// update current position
if ( h_pos < h_frame — 1 )
h_pos <= h_pos + 1 ;
else
begin
h_pos <= 0 ;
if ( v_pos < v_frame — 1 )
v_pos <= v_pos + 1 ;
else
v_pos <= 0 ;
end

// are we inside centered 512×256 area plus border?
if ( ( h_pos >= h_offset — border ) &&
( h_pos < ( h_pixels — h_offset + border ) ) &&
( v_pos >= v_offset — border ) &&
( v_pos < ( v_pixels — v_offset + border ) ) )
begin
if ( ( h_pos >= h_offset ) && ( h_pos < h_pixels — h_offset ) &&
( v_pos >= v_offset ) && ( v_pos < v_pixels — v_offset ) )
begin // inside centered area
color <= mem [ raddr ] ;

// addr = (X + (Y / 8) * 128)*8 + (7 — Y % 8)
// X = (h_pos — h_offset) >> 2
// Y = (v_pos — v_offset) >> 2
raddr <= (
(
// X
( ( h_pos — h_offset ) >> 2 ) |
(
//  Y  div 8
( ( ( ( v_pos — v_offset ) >> 2 ) >> 3 ) & 3’b111 )
// mul 128
<< 7
)
// mul 8 (size of byte in bits)
) << 3
// + (7 — (Y % 8))
) | ( 7 — ( ( ( v_pos — v_offset ) >> 2 ) & 3’b111 ) ) ;
end
else // outside centered area, draw the border
color <= 1 ;
end
else // everything else is black
color <= 0 ;
end

endmodule

parameter addr_width = 13 ;
logic mem [ ( 1 << addr_width ) — 1 : 0 ] ;

always_ff @ ( posedge wclk ) // write memory
begin
if ( cs == 0 ) // chip select
begin
if ( dc ) // dc = high, accept data
begin
mem [ waddr ] <= din ;
waddr <= waddr + 1 ;
end // dc = low, ignore command
end
else
waddr <= 0 ;
end