1. COMP541 Video Monitors
Montek Singh
Oct 2, 2015

2. Outline
Last Friday’s lab
Tips/discussion
How to generate video signal

3. Lab 6: 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
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!

4. 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

5. VGA Monitors

6. 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

7. 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

8. Cathode Ray Tube (CRT)
From wikipedia:

9. Deflection Coils

10. Simple Scanning TV Electron beam scans across Turned off when
Scanning back to the left (horizontal retrace ----)
Scanning to the top (vertical retrace ____)

11. 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?

12. 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.

13. 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
tiny mirrors (one/pixel) that allow light through or send it out of the way
possible to change pixels selectively

14. VGA Signaling Timing signals Color values: R, G, B
horizontal sync (hsync) & vertical sync (vsync)
Color values: R, G, B
total 8 bits for Nexys 3 (rrrgggbb), 12 bits for Nexys 4 (rrrrggggbbbb)
Nexys 4
digital to analog converter

15. 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

16. Horizontal Timing Terms
hsync pulse
Back porch (left side of display)
Active Video
Video should be blanked (not sent) at other times
Front porch (right side)

17. Horizontal Timing 640 Horizontal Dots Horiz. Sync Polarity NEG
This diagram shows video as a digital signal. It’s not – video is an analog level.
640 Horizontal Dots
Horiz. Sync Polarity NEG
Scanline time (A) us
Sync pulse length (B) 3.77 us
Back porch (C) 1.89 us
Active video (D) us
Front porch (E) 0.94 us
us = microsecond
Image from

18. Vertical Timing (note ms, not us)
Vert. Sync Polarity NEG
Vertical Frequency 60Hz
Total frame time (O) ms
Sync length (P) 0.06 ms
Back porch (Q) 1.02 ms
Active video (R) ms
Front porch (S) 0.35 ms

19. 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 MHz

20. The overall picture
(Picture from

21. 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!

22. Color Depth Voltage of each of RGB determines color Nexys 3: Nexys 4:
3-bit for red and green, 2-bit for blue
Nexys 4:
4-bit for red, green and blue
All on for white
Nexys 4

23. Lab 7 Notes: Make Verilog module to generate
hsync, vsync, horizontal count, vertical count, and signal to indicate active video
Use higher-level module to drive RGB using counts gated by active
Just do something simple (stripes, checkerboard, diagonals)
Later we will use memory addressed by counts to make a terminal
Notes:
Be careful about the “Sync Polarity”
A “1” means a downward going pulse
sync signal is normally high, but goes low during the pulse
A “0” means an upward going pulse
Use my self-checking text bench!
simulates my VGA driver …
… and compares your outputs with mine
flags any mismatches

24. What do you need for VGA? Think first Sketch out a design
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.)
==, <, >, <=, >=

25. VGA Links VGA Timing Interesting
Recommended:
Interesting