Re: clock unit

From: Gerrit Heitsch <>
Date: Tue, 14 May 2013 13:36:20 +0200
Message-ID: <>
On 05/14/2013 01:21 PM, wrote:
> On 2013-05-14, at 09:18, Gerrit Heitsch wrote:
>>>>> I had a moment of inspiration today (after Michau asked about the SX ROMs) to check how it is done in the SX (I thought it was like in the older 64s) and.. it occured that there are actually /two/ crystals piggybacked one on another and just three 74LS chips next to the passive stuff on the daughterboard called "clock unit".
>>>> Would it be possible to post the frequencies of the 2 crystals and what 74LSxxx are on the clock board?
>>> One is phicolor (17.734 on PAL) the other one is dotclock (7.8819 on PAL )frequency.
>> Ouch... It's already a bit of a pain to get your hands on a 17.73447 MHz crystal, but where do you get a 7.8819 MHz one?
> Well, unlike ICs, in the worst case crystals can still be grown for a desired frequency but I was rather referring to our earlier discussion about why CBM didn't simply put two crystals rather than presumably more expensive stuff in the first place and in the second they even decided to make a custom IC. It seems that the desired result can actually be relatively easily achieved with two crystals and some lightweight semis around.

Until we know the actual circuit we can only guess, but with 2 Crystals, 
the 2 frequencies are not in sync as they are with the old PLL circuit 
based on the 74LS629. This should result in a slightly unstable picture 
on the monitor, especially if you use FBAS/CVBS.

>> Ok, if you had a 8701 around, you could use that one.
> ? I am not sure I understand.

The 8701 is the little MOS IC (16pin) used in the C64 starting with the 
250425 board and also the C128. It has a pin indicating PAL (LOW) or 
NTSC (HIGH), you add the proper crystal (14.318 or 17.73447 MHz) and it 
will supply this frequency as TTL signal and also derive the dot clock 
from it. MOS made it so Commodore would no longer have to buy the 
74LS629 from TI, the only one who makes it.

Using the 8701 it would be simple enough to make a small PCB as a 
replacement for the clock board for the SX64.

There is also a little circuit to replace the 8701, it uses a 74HC7046 
and some extra logic.


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Received on 2013-05-14 12:00:51

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