about timing.

[Amit]

Hello group,

I'm looking at a source code which generates a vsync and hsync signal
for VGA (640x480 pixel by pixel) and in there I see there is a
checking if v_count and h_count condition is checked as follow:



For horizontal and then vertical:

--Generate horizontal and vertical timing signals for video signla
--H_count counts pixels (640 + extra time for sync signals)


if (h_count = 799 ) Then
h_count <= "0000000000";
else
h_count <= h_count + 1;
end if;


if (v_count <= 755) AND (h_count =>659 ) Then
horiz_sync <= '0';
else
horiz_sync <= '1';
end if;



if (v_count >= 524) AND (h_count =>699 ) Then
v_count <= "0000000000";
elsif
v_count <= v_count + 1;
end if;


Questions:

Why does it check for 799 when the resolution is 640?
if it is extra time how is it calculated then?!
same for the vertical? how should I know what number should I use in
condition part?

Thanks,
Amit

 

[kennheinrich]

Hello group,

I'm looking at a source code which generates a vsync and hsync signal
for VGA (640x480 pixel by pixel) and in there I see there is a
checking if v_count and h_count condition is checked as follow:

For horizontal and then vertical:

--Generate horizontal and vertical timing signals for video signla
--H_count counts pixels (640 + extra time for sync signals)

if (h_count = 799 ) Then
h_count <= "0000000000";
else
h_count <= h_count + 1;
end if;

if (v_count <= 755) AND (h_count =>659 ) Then
horiz_sync <= '0';
else
horiz_sync <= '1';
end if;

if (v_count >= 524) AND (h_count =>699 ) Then
v_count <= "0000000000";
elsif
v_count <= v_count + 1;
end if;

Questions:

Why does it check for 799 when the resolution is 640?
if it is extra time how is it calculated then?!
same for the vertical? how should I know what number should I use in
condition part?

Thanks,
Amit

There's always some dead time between when the active pixels on one
line end and those in the the next line start. This called the HBI
(horizontal blanking interval). In the olden days, this time was
needed for the magnetic field of the monitor's deflection coils to
collapse and put the beam back at the left edge of the screen. Same
idea for the dead time between bottom of active video and top of
active picture. I'd expect you could google for a chart, or look for
some Xilinx/Altera app notes, although I think there's more
flexibility (read: slop) in the VGA/computer world than there is in
real (read: television broadcast) video.

- Kenn

 

[Amit]

There's always some dead time between when the active pixels on one
line end and those in the the next line start. This called the HBI
(horizontal blanking interval). In the olden days, this time was
needed for the magnetic field of the monitor's deflection coils to
collapse and put the beam back at the left edge of the screen. Same
idea for the dead time between bottom of active video and top of
active picture. I'd expect you could google for a chart, or look for
some Xilinx/Altera app notes, although I think there's more
flexibility (read: slop) in the VGA/computer world than there is in
real (read: television broadcast) video.

- Kenn

Hello Keen,

Thanks for the explanation. Now, how should we calculate it? how is it
calculated?

Regards,
Amit

 

[David Spencer]

On Mar 1, 3:36 pm, (e-mail address removed) wrote:

Hello Keen,

Thanks for the explanation. Now, how should we calculate it? how is it
calculated?

Regards,
Amit

Have a look at http://www.epanorama.net/documents/pc/vga_timing.html.

 

[Amit]

Have a look athttp://www.epanorama.net/documents/pc/vga_timing.html.

Hi David,

Thank you so much. That was a big help.

Regards,
Amit

 

[Tricky]

Hi David,

Thank you so much. That was a big help.

Regards,
Amit

Video signals are made up of all sorts of timings. If you look at a
video signal on a scope you will see that the actual video data only
makes up about 2/3 of the overall signal. You have to worry about your
syncs, plus flyback as modern monitors are disigned to work with
standardised signals, that were made standard with analogue screens,
which are still in major use.

The counters may also be set to odd numbers depending of the clock
speed of the system.

 

[JK]

Hi David,

Thank you so much. That was a big help.

Regards,
Amit

Take a look at
http://www.tkk.fi/Misc/Electronics/faq/vga2rgb/calc.html

Regards,
JK