Re: Blurry picture

From: Segher Boessenkool <>
Date: Wed, 16 Nov 2011 06:46:50 +0100
Message-Id: <>
>> The MAX machine used dual crystal oscillators (in discrete parts).
> The first C64 board had a layout that suggested that they had  
> planned the same there:

[That link doesn't work, but I think I found the picture]

What makes you say that?  I see two missing ICs there (U33 and U34),
14-pin and 16-pin, but no other missing components.  Looks to me
like they changed the divider design; cannot really tell without
seeing the bottom side of the board though.

> I read somewhere that 2 crystals resulted in poor picture quality  
> compared to using a PLL and only one crystal. Never understood why.

I'm not sure if it is true, we only got hearsay.  If it is true, I can
think of two things: a) the two clocks gave beating from interference;
b) the 8MHz clock derived from a crystal is less stable than one from
a PLL from a 14MHz/18MHz crystal, and it wasn't stable enough.

The colour clock is used completely independently from the pixel clock,
there is no reason they need to be perfectly synchronised, as far as I
can see.

>> I don't think it would have been possible to make an 8701 in the
>> older 65xx NMOS process, 36MHz is too much.
> I'm surprised they were able to get 36MHz working with HMOS-II.

The 8701 uses really big transistors all over the place, super drivers
in many many places, and some bootstrapped drivers.  It looks like 36MHz
was quite close to the limits of the process (if you want a decent  

>> Are there die photos of the VIC-IIe somewhere? It would be  
>> interesting
>> to see what changed on there (what exactly changed functionally,  
>> anyway?)
> The VIC-IIe still uses the 8701, so the clock part didn't change.
> Otherwise it got a few I/O-Bits and the ability to switch the CPU  
> clock between 1 and 2 MHz. More details can be found here:
> Looks like there were some extra 'undocumented' features added as well

So there was some change in the clocking, for the 2MHz mode, and by the
sound of it is was rather hackish, not resulting in a good picture;  
is the test bit, with which you can upset the state machines some more;
and there is the added GPIO port?

Still want pics :-)


       Message was sent through the cbm-hackers mailing list
Received on 2011-11-16 06:00:12

Archive generated by hypermail 2.2.0.