fb.modes - frame buffer modes file
DESCRIPTION
/etc/fb.modes contains an unlimited number of video mode
descriptions. The general format of a video mode is:
mode
geometry <xres> <yres> <vxres> <vyres> <depth>
timings <pixclock> <left> <right> <upper> <lower>
<hslen> <vslen>
options <value>
endmode
OPTIONS
geometry options:
xres visible horizontal resolution (in pixels)
yres visible vertical resolution (in pixels)
vxres virtual horizontal resolution (in pixels)
vyres virtual vertical resolution (in pixels)
depth display depth (in bits per pixel)
timing options:
pixclock
length of one pixel (in picoseconds)
left left margin (in pixels)
right right margin (in pixels)
upper upper margin (in pixel lines)
lower lower margin (in pixel lines)
hslen horizontal sync length (in pixels)
vslen vertical sync length (in pixel lines)
other options:
the first value of this options is the default
hsync {low|high}
the horizontal sync polarity
vsync {low|high}
the vertical sync polarity
the composite sync polarity
extsync {false|true}
enable or disable external resync. If
enabled the sync timings are not generated
by the frame buffer device and must be pro
vided externally instead. Note that this
option may not be supported by every frame
buffer device
laced {false|true}
enable or disable interlace. If enabled the
display will be split in two frames, each
frame contains only even and odd lines
respectively. These two frames will be dis
played alternating, this way twice the lines
can be displayed and the vertical frequency
for monitor stays the same, but the visible
vertical frequency gets halved
double {false|true}
enable or disable doublescan. If enabled
every line will be displayed twice and this
way the horizontal frequency can easily be
doubled, so that the same resolution can be
displayed on different monitors, even if the
horizontal frequency specification differs.
Note that this option may not be supported
by every frame buffer device
INTERNALS
Generally a frame buffer display is organized as follows:
+---+---------------+---+---+
| | ^ | | |
| | | 5 | | |
| | v | | |
+---#################---+---+
| # ^ # | |
| # | # | |
| # | # | |
| 1 # | 2 # 3 | 4 |
|<->#<------+------>#<->|<->|
| # | # | |
| # | 6 # | |
| # | # | |
| # v # | |
+---#################---+---+
| | ^ | | |
| | | 7 | | |
| | v | | |
+---+---------------+---+---+
| | ^ | | |
| | v | | |
+---+---------------+---+---+
1 left margin
2 xres
3 right margin
4 horizontal sync len
5 upper margin
6 yres
7 lower margin
8 vertical sync len
The area bordered with `#' is the visible display area.
Horizontal and vertical frequencies can now easily be cal
culated, for this the sum of horizontal or vertical values
are important
htotal = left + xres + right + hslen
vtotal = upper + yres + lower + vslen
The length of one line can now be calculated with pixclock
line = pixclock * htotal
and we have the horizontal frequency
hfreq = 1E12 / line = 1E12 / (pixclock * htotal)
To get the vertical frequency vtotal must eventually
adjusted. If the display is laced, vtotal must be halved
or if the display is a doublescan one, vtotal must be dou
bled. Now we can calculate the length of one frame
if (lace) vtotal /= 2
if (double) vtotal *= 2
frame = vtotal * line
and we get also the vertical frequency
vfreq = 1E12 / frame = hfreq / vtotal
SEE ALSO
fbset(8), fbdev(4)
Man(1) output converted with
man2html