A simple horizontal frequency doubler PLL for TV line double

[Jan Panteltje]

Now I am very happy I got this working, PLL always did cost me
many hours of thinking and tinkering...
This takes a 50 MHz clock, and a horizontal trigger of about 1 to 5 clocks
high (purified H sync in).
It generates a 32KHz sync pulse for the monitor.
The stability seems very good.
So anyways I got a picture from the linedoubler, only BW for now.
Also some test on timebase correction worked..
Thought I share this with you, as it is the first thing needed for
a line doubler, and could be used for other things too I am sure.

Input is htrigger64, output is test_h, clock is real_clock.
Never mind the reset, it will run continuously.
It will go wild without input, and kill your monitor perhaps.. so have some
fail safe circuit.

// start frequency multiplier
parameter MULTIP = 2;

// 2h oscillator
reg [10:0] reload;
reg test_h;
reg [15:0] th_count;
reg th;
always @(posedge real_clock)
begin
case(th_count)
0:
begin
test_h = 0;
th = 1;
th_count = 1;
end
10:
begin
th = 0;
th_count = 11;
end
200:
begin
test_h = 1;
th_count = 201;
end
reload:
begin
th_count = 0;
end
default:
th_count = th_count + 1;
endcase
end

// incoming line length detection
reg [15:0] llen;
reg [15:0] cllen;
reg hflag;
always @(posedge real_clock)
begin
if(htrigger64 == 1)
begin
if(! hflag)
begin
cllen = llen;
llen = 0;
hflag = 1;
end
end
else
begin
llen = llen + 1;
hflag = 0;
end
end

// frequency discriminator
reg [10:0] sh;
reg [7:0] lcnt;
reg [10:0] ccnt;
always @(posedge real_clock)
begin
if(htrigger64 == 1)
begin
if(test_h2 == 0)
begin
if(sh > 0) sh = sh - 1;
end
else
begin
if(sh < 400) sh = sh + 1;
end

// hehe, the trick
reload = (cllen / MULTIP) + sh;
end
end
// end frequency multiplier