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<A HREF="S3.html"><EM> Information for S3 Chipset Users</EM></A>
 <b>:</b> <EM>Reference clock value for IBM RGB 5xx  RAMDACs</EM><BR>
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<HR NOSHADE>
  <H2><A NAME="6"></A>6. Reference clock value for IBM RGB 5xx  RAMDACs</H2>

<p>Cards with IBM RGB5xx RAMDACs use several different input frequencies 
for the clock synthesizer which can't be probed without some knowledge 
of the text mode clocks (which may be a wrong assumption if you're using
non-standard text modes).  Here is the procedure you should use to
find out the input frequency:</p>
<p>First run </p>
<p>
<blockquote><code>
<pre>
    X -probeonly &gt;&amp; outfile 
</pre>
</code></blockquote>
</p>
<p>and check the output for the probed clock chip which might look like this:</p>
<p>
<blockquote><code>
<pre>
(--) S3: Using IBM RGB52x programmable clock (MCLK 66.000 MHz)
(--) S3: with refclock 16.000 MHz (probed 15.952 &amp; 16.041)
                       ^^^^^^      ^^^^^^^^^^^^^^^^^^^^^
</pre>
</code></blockquote>
</p>
<p>there will be a "good guessed" value which will be used 
and two probed values in brackets based on the 25MHz and 28MHz
text clocks. This probing can only work if you run a normal 80x25
or 80x28 text mode!</p>
<p>The refclock values known so far are:</p>
<p>
<pre>
	STB Velocity 64		24 Mhz
	Genoa VideoBlitz II AV	50 MHz
	Hercules S3 964 	50 MHz
	Hercules S3 968 	16 MHz
	#9 Motion 771		16 MHz
</pre>
</p>
<p>depending on the quartz on your card and maybe other features like
an additional clock chip on the Genoa card (which as a 14.3MHz quartz).</p>
<p>If you claim that your card has a 16MHz clock, but it really uses 50MHz,
all pixel clocks will be tripled and a 640x480 mode with 25MHz will
use a 75MHz pixel clock, so be very careful.</p>
<p>If you found the right refclock, you should set it in the config file
(device section) e.g. with</p>
<p>
<blockquote><code>
<pre>
	s3RefClk  16
      
</pre>
</code></blockquote>

or
<blockquote><code>
<pre>
	s3RefClk  50
      
</pre>
</code></blockquote>
</p>
<p>so that that this value will be used even if you use another text
mode and probing fails!</p>


 <HR NOSHADE>
<A HREF="S3.html"><EM> Information for S3 Chipset Users</EM></A>
 <b>:</b> <EM>Reference clock value for IBM RGB 5xx  RAMDACs</EM><BR>
<b>Previous:</b> <A HREF="S35.html"><EM>Additional Notes</EM></A><BR>
<b>Next:</b> <A HREF="S37.html"><EM>Hints for LCD configuration (S3 Aurora64V+)</EM></A>



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