COMP541
Video Monitors
Montek Singh
Oct 1, 2014
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Outline
 Last Friday’s lab
 Tips/discussion
 How to generate video signal
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How about making a BCD stop watch?
 Each digit counts 0 to 9, and then wraps around
 i.e., display is decimal number, not hex
 Do the following:
 Separate the 16-bit number into four 4-bit numbers
 reg [3:0] A3, A2, A1, A0;
 For A0: on each clock tick…
– if this digit is 9, change it to 0, else add 1 to it
 For A1, A2, A3: on each clock tick…
– if all lower A’s are at 9, then
» if this digit is 9, change it to 0, else add 1 to it
– else this digit does not change
 Slow it down to tick once per 1/100 second
 have a separate counter to count 220 clock ticks (~1/100th sec)
 update the 4-digit number only whenever this counter fills up!
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Reminder: Good Verilog Practices
 Best to use single clock for all FFs
 Make all signals synchronous to one clock
 No: @(posedge button) etc.
 Yes: @(posedge clock)
 Avoids “weird” and frustrating problems
 Multiple modules
 Tested individually
 Top level has input and outputs
 One module per file
 Just to make it easier to follow and test
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VGA Monitors
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How Do Monitors Work?
 Origin is TV, so let’s look at that
 LCDs work on different principle, but all signaling still derived
from TV of 1940s
 Relies on your brain to do two things
 Integrate over space
 Integrate over time
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Many Still Images
 Video (and movies) are a series of stills
 If stills go fast enough your brain interprets as moving
imagery
 50-60 Hz or more to not see flicker
– “1 Hz” means once per second
 In fact, even if the scene does not change…
 … a single “still” image is displayed repeatedly over time
 Why? Phosphor persistence varies
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Cathode Ray Tube (CRT)
From wikipedia: http://en.wikipedia.org/wiki/Cathode_ray_tube
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Deflection Coils
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Simple Scanning TV
 Electron beam scans across
 Turned off when
 Scanning back to the left (horizontal retrace ----)
 Scanning to the top (vertical retrace ____)
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Scanning: Interlaced vs. Progressive
 (Some) TVs use interlacing
 Every other scan line is swept per field
 Two fields per frame (30Hz)
 Way to make movement less disturbing
 Computers use progressive scan
 Whole frame refreshed at once
 60Hz or more, 72Hz looks better
 Similar notation used for HD
 i = interlaced (1080i)
 p = progressive (1080p)
 which better?
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Color
 Three colors of phosphor
 three beams, one each for the three phosphors
 Black: all beams off
 White: all beams on
Picture is a bit misleading.
Mask (or aperture grill)
ensures beams hit only
correct color phosphor.
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What about LCD?
 How do LCD monitors work?
 internals are very different
 no beams, tubes
 made up of tiny LCD cells
 However, external signaling is the same!
 for compatibility
 Same goes for micro-mirror projectors
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VGA Signaling
ual
 Timing signals
 horizontal sync (hsync) & vertical sync (vsync)
tus byte
YS XY YY P
X direction byte
1
0
Y direction byte
X0 X1 X2 X3 X4 X5 X6 X7 P
1
0
Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P
1
 Color values: R, G, B
Mouse Data Format
 total 8 bits for Nexys 3 (rrrgggbb), 12 bits for Nexys 4 (rrrrggggbbbb)
Stop bit
Start bit
s 10 FPGA signals to
8-bit color and the two
(HS – Horizontal Sync,
c). The color signals use
that work in conjunction
nation resistance of the
eight signal levels on the
gnals, and four on blue
sensitive to blue levels).
gure 13, produces video
eed in equal increments
and 0.7V (fully on). Using
nt colors can be
h unique 8-bit pattern. A
must be created in the
c and color signals with
der to produce a working
e specified, published,
y the VESA organization
ollowing VGA system
ovided as an example of
ght be driven in 640 by
recise information, or for
Stop bit
Start bit
5
1
10
Stop bit
Idle state
Pin 1: Red
Pin 2: Grn
Pin 3: Blue
Pin 13: HS
Pin 14: VS
6
15
11
RED0
2K
RED1
1K
RED2
510
P8
T6
V6
GRN0
2K
GRN1
1K
GRN2
510
R7
T7
BLU1
1K
BLU2
510
U7
V7
N7
N6
P7
Pin 5: GND
Pin 6: Red GND
Pin 7: Grn GND
Pin 8: Blu GND
Pin 10: Sync GND
digital to analog
converter
RED
GRN
HSYNC 100
VSYNC
100
Spartan 6
Nexys 3
BLU
HS
VS
HD-DB15
Nexys 4
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VGA Timing
 You supply two pulses
 hsync and vsync
 allow the monitor to lock onto
timing
 One vsync per frame
 One hsync per scan line
 hsync does not stop during vsync
pulse
Image from dell.com
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Horizontal Timing Terms
 Horizontal timing:
 hsync pulse
 Back porch (left side of display)
 Active Video
 Video should be blanked (not sent) at other times
 Front porch (right side)
Picture not accurate
for our case; just for
illustration.
Video and HSYNC not
on same wire
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Horizontal Timing
640 Horizontal Dots
Horiz. Sync Polarity NEG
Scanline time (A) 31.77 us
Sync pulse length (B) 3.77 us
Back porch (C) 1.89 us
Active video (D) 25.17 us
Front porch (E) 0.94 us
This diagram shows
video as a digital
signal. It’s not – video
is an analog level.
us = microsecond
Image from http://www.epanorama.net/documents/pc/vga_timing.html
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Vertical Timing (note ms, not us)
Vert. Sync Polarity NEG
Vertical Frequency 60Hz
Total frame time (O) 16.68 ms
Sync length (P) 0.06 ms
Back porch (Q) 1.02 ms
Active video (R) 15.25 ms
Front porch (S) 0.35 ms
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Timing as Pixels
 Easiest to derive all timing from single-pixel timing
 How “long” is a pixel?
 Active video / number of pixels
 25.17 us / 640 = 39.32ns
 Conveniently close to 25 MHz – just use that
 Actual VESA spec is 25.175 MHz
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Standards
 640 x 480 (sometimes x 60Hz) is “VGA”
 I will give you spec sheets in lab
 You can try for 800x600 at 60 Hz (40 MHz exactly)
 or 800x600 at 72 Hz (50 MHz exactly)
 Note that some standards have vsync and hsync
positive true, some negative true
 choose correct polarity
 determine by experimentation!
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Color Depth
Nexys3 Reference Manual
 Voltage of each of RGB determines color
 Nexys 3:
1 0 L R 0 1 XS YS XY YY
Mouse status byte
X direction byte
P
1
0
Y direction byte
X0 X1 X2 X3 X4 X5 X6 X7 P
1
0
Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P
bit
Start blue
bit
 3-bit for
red Start
and
2-bit
for
Stop bit
bit green, Stop
Start bit
Idle state
 Nexys 4:
1
Stop bit
Idle state
Mouse Data Format
 4-bit for red, green and blue
VGA Port
 All on for white
The Nexys3 board uses 10 FPGA signals to
create a VGA port with 8-bit color and the two
standard sync signals (HS – Horizontal Sync,
and VS – Vertical Sync). The color signals use
resistor-divider circuits that work in conjunction
with the 75-ohm termination resistance of the
VGA display to create eight signal levels on the
red and green VGA signals, and four on blue
(the human eye is less sensitive to blue levels).
This circuit, shown in figure 13, produces video
color signals that proceed in equal increments
between 0V (fully off) and 0.7V (fully on). Using
this circuit, 256 different colors can be
displayed, one for each unique 8-bit pattern. A
video controller circuit must be created in the
FPGA to drive the sync and color signals with
the correct timing in order to produce a working
display system.
VGA System Timing
5
Pin 1: Red
Pin 2: Grn
Pin 3: Blue
Pin 13: HS
Pin 14: VS
1
10
6
15
11
RED0
2K
RED1
1K
RED2
510
P8
T6
V6
GRN0
2K
GRN1
1K
GRN2
510
R7
T7
BLU1
1K
BLU2
510
U7
V7
N7
Pin 5: GND
Pin 6: Red GND
Pin 7: Grn GND
Pin 8: Blu GND
Pin 10: Sync GND
RED
GRN
Nexys 4
BLU
HSYNC 100
HS
N6
VGA signal timings are specified, published,
100
VSYNC
copyrighted and sold by the VESA organization
VS
P7
(www.vesa.org). The following VGA system
HD-DB15
Spartan 6
timing information is provided as an example of
how a VGA monitor might be driven in 640 by
480 mode. For more precise information, or for
information on other VGA frequencies, refer to documentation available at the VESA website.
Nexys 3
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What To Do Friday
1. Show previous lab’s demo
2. Make Verilog module to generate
 hsync, vsync, horizontal count, vertical count, and signal to
indicate active video
3. Use higher-level module to drive RGB using counts
gated by active
 Just do something simple (stripes, checkerboard, diagonals)
4. Later we will use memory addressed by counts to
make terminal
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What do you Need for VGA?
 Think first
 Need counter(s)?
 Will you need a state machine?
 Sketch out a design
 Block diagram
 Test individually in lab
 Keep in mind
 Verilog has all these operators (and more; see Verilog ref.)
==, <, >, <=, >=
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VGA Links
 VGA Timing
 Recommended: http://tinyvga.com/vga-timing
 http://www.epanorama.net/documents/pc/vga_timing.html
 Interesting
 http://www.howstuffworks.com/tv.htm
 http://computer.howstuffworks.com/monitor.htm
 http://www.howstuffworks.com/lcd.htm
 http://plc.cwru.edu/
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