1

COMP541

Video Monitors

Montek Singh

Oct 1, 2014

Outline Last Friday’s lab

Tips/discussion How to generate video signal

2

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 itFor 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 secondhave a separate counter to count 220 clock ticks (~1/100th

sec)update the 4-digit number only whenever this counter fills

up!

3

Reminder: Good Verilog Practices Best to use single clock for all FFs

Make all signals synchronous to one clockNo: @(posedge button) etc.Yes: @(posedge clock)

Avoids “weird” and frustrating problems Multiple modules

Tested individuallyTop level has input and outputs

One module per file Just to make it easier to follow and test

4

VGA Monitors

5

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

Many Still Images Video (and movies) are a series of stills

If stills go fast enough your brain interprets as moving imagery50-60 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 timeWhy? Phosphor persistence varies

7

Cathode Ray Tube (CRT)

8

From wikipedia: http://en.wikipedia.org/wiki/Cathode_ray_tube

Deflection Coils

9

Simple Scanning TV Electron beam scans across Turned off when

Scanning back to the left (horizontal retrace ----)Scanning to the top (vertical retrace ____)

10

Scanning: Interlaced vs. Progressive (Some) TVs use interlacing

Every other scan line is swept per fieldTwo fields per frame (30Hz)Way to make movement less disturbing

Computers use progressive scanWhole frame refreshed at once60Hz or more, 72Hz looks better

Similar notation used for HD i = interlaced (1080i)p = progressive (1080p)which better?

11

Color Three colors of phosphor

three beams, one each for the three phosphorsBlack: all beams offWhite: all beams on

12

Picture is a bit misleading. Mask (or aperture grill) ensures beams hit only correct color phosphor.

What about LCD? How do LCD monitors work?

internals are very differentno beams, tubesmade up of tiny LCD cells

However, external signaling is the same!for compatibility

Same goes for micro-mirror projectors

13

14

VGA Signaling Timing signals

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 3 Nexys 4

digital to analog converter

VGA Timing You supply two pulses

hsync and vsyncallow the monitor to lock onto

timing

One vsync per frameOne hsync per scan line

hsync does not stop during vsync pulse

15

Image from dell.com

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)

16

Picture not accurate for our case; just for illustration.

Video and HSYNC not on same wire

Horizontal Timing

17

640 Horizontal Dots Horiz. Sync Polarity NEG Scanline time (A) 31.77 usSync pulse length (B) 3.77 usBack porch (C) 1.89 usActive video (D) 25.17 us Front porch (E) 0.94 us

Image from http://www.epanorama.net/documents/pc/vga_timing.html

This diagram shows video as a digital signal. It’s not – video is an analog level.

us = microsecond

Vertical Timing (note ms, not us)

18

Vert. Sync Polarity NEGVertical Frequency 60HzTotal frame time (O) 16.68 ms Sync length (P) 0.06 msBack porch (Q) 1.02 msActive video (R) 15.25 msFront porch (S) 0.35 ms

Timing as Pixels Easiest to derive all timing from single-pixel

timing

How “long” is a pixel?Active video / number of pixels25.17 us / 640 = 39.32nsConveniently close to 25 MHz – just use thatActual VESA spec is 25.175 MHz

19

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 truechoose correct polaritydetermine by experimentation!

20

Color Depth Voltage of each of RGB determines color

Nexys 3: 3-bit for red and green, 2-bit for blue

Nexys 4: 4-bit for red, green and blue

All on for white

21Nexys 3

Nexys 4

What To Do Friday1. Show previous lab’s demo2. 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

22

23

What do you Need for VGA? Think first

Need counter(s)?Will you need a state machine?

Sketch out a designBlock diagram

Test individually in lab Keep in mind

Verilog has all these operators (and more; see Verilog ref.)

==, <, >, <=, >=

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/

24