From: jenny@zelator.in-berlin.de (Sebastian Hetze)
Subject: How to measure the VGA clocks
Organization: Puplic-Access-Xenix-System
Date: Thu, 20 Aug 92 23:25:54 GMT
Summary: a method to measure VGA clocks
Keywords: X11 clocks VGA


Hi *!
Since there seem to be a lot of people having trouble to find out the
clocks of their VGA-card, here's a suggestion how to measure them:

First of all you need an oscilloscope or an frequency-counter with an
range up to 100kHz (no megaHerz). Then you take something like the
following as Xconfig:


-- start -- CUT HERE --

FontPath	"/usr/lib/X11/fonts/misc/,/usr/lib/X11/fonts/75dpi/"
#  DontZap

#Microsoft	"/dev/ttys0"
MouseSystems	"/dev/ttys0"
#MMSeries	"/dev/ttys2"
#Logitech	"/dev/ttys2"
#MouseMan	"/dev/ttys2"
#Busmouse	"/dev/ttys2"
  BaudRate	1200
#  SampleRate	150
#  Emulate3Buttons

#
# The graphics drivers
#
vga256
  #Virtual	800x600
  ViewPort      0 0
  Modes	      "1" "2" "3" "4" "5" "6" "7" "8" "9" "10" "11" "12" "13"
Clocks 25 28 50 51 52 53 54 55 56 57 58 59 60 61 62
  #Staticgray
  #Grayscale
  #Staticcolor
  #Pseudocolor
  #Truecolor
  #Directcolor
  Chipset	"et4000"
  #Chipset	"et3000"
  #Chipset	"gvga"
  #Chipset	"pvga1"
  #Displaysize	1024 768
  #Screenno	0
  #Vendor	"legend"
  #Vendor "hiclock"
  #Videoram	1024

ModeDB
# clock  horzontal timing      vertical timing
  "640x480"   28     640  672  768  800    480  490  492  525

  "1"   50     920  944 1032 1168    690  714  718  742
  "2"   51     920  944 1032 1168    690  714  718  742
  "3"   52     920  944 1032 1168    690  714  718  742
  "4"   53     920  944 1032 1168    690  714  718  742
  "5"   54     920  944 1032 1168    690  714  718  742
  "6"   55     920  944 1032 1168    690  714  718  742
  "7"   56     920  944 1032 1168    690  714  718  742
  "8"   57     920  944 1032 1168    690  714  718  742
  "9"   58     920  944 1032 1168    690  714  718  742
  "10"   59     920  944 1032 1168    690  714  718  742
  "11"   60     920  944 1032 1168    690  714  718  742
  "12"   61     920  944 1032 1168    690  714  718  742
  "13"   62     920  944 1032 1168    690  714  718  742

#
# 1     2     3     4     5     6     7     8     9     10    11    12    13
# 
# 50    51    52    53    54    55    56    57    58    59    60    61    62
# oscilloscope (usec)
# 23,3  22,9  22,5  22,0  21,6  21,2  20,9  20,5  20,1  19,8  19,5  19,1  18,8
# frequency-counter (kHz)
# 42,8  43,7  44,5  45,4  46,2  47,1  47,9  48,8  49,7  50,5  51,4  52,2  53,1
#
# 

-- end -- CUT HERE --

Then you disconnect your Monitor from the videocard and do an blind
"startx". After a few seconds you can connect the oscilloscope or the
counter to pin 13 of the videobuchse(howeveryoucallitinenglish).

            5       1
         ---------------
         \  o o o o o  /
      10  \  o o o o o/ 6
-->        \o o o o o/ 11
|           ---------
|
|               ^
|               |
--------------------- This is the one
(I hope the "graphic" is ok. Pin 13 is the midst in the
short-side-row)

What you can see (or count) now, are the horizontal sync-pulses of the
video signal.

To find out the clocks you continue
1. Using an oscilloscope:
To get the true frequency, you have to get the length of one sync-period
in usec (micro seconds). If you are in mode 1 (50MHz)and get for example 29
usec, you take the first number of the usec line in the table (23.3)
and calculate:
		23.3/29*50 = 40.17
that means you have a 40MHz Clock at the third position.

2. Using an frequency-counter
You measure the h-sync frequency (for example 34 kHz) and calculate
		34/42.8*50 = 39.71
which is again a 40MHz clock

The clocks on the first two positions are allways 25 and 28 for
compatibility with VGA standard.
To find ot the next clock, switch into the next mode.
To swich in mode 2, you have to do a Alt-Ctrl-Key+ .
After measuring the time (frequency) again, you do the same
calculation, but you use 22.9 instead of 23.3 (43.7 instead of 42.8) 
and so on.

Finaly, you stop the X-server by typing Alt-Ctrl-Backsp and that's it.

If you send me your results (including the exact type of your VGA-card)
i'll post an update of the vga.dbase.

Sebastian


-- 
--
                                                   she@soho.in-berlin.de
Sebastian Hetze
                                              jenny@zelator.in-berlin.de
Newsgroups: comp.os.linux