Re: 8520/21

From: Gerrit Heitsch <>
Date: Sat, 07 Mar 2015 12:22:42 +0100
Message-ID: <>
On 03/07/2015 12:01 PM, wrote:
> What I tend to speculatively believe is that the rational approach was
> something like: "We need to move HMOS so let's make the new CIA in HMOS
> but since our 8-bit line is passe, let's strip the new chip from
> unneeded baggage". That's how the 8520 came to the world. The fact is
> though that the C64 didn't want to die and continued to outsell all
> other machines despite the launch of both Amiga and the C128 (and the
> 16/116/+4). The management was faced with a dilemma "to kill or not to
> kill the only hen that lays golden eggs". At some point then a decision
> was made to redesign the C64 (into C64C) rather than phase it out as
> previously planned. But at that point the 8520 didn't have what was
> absolutely needed to keep full compatibility with the previous C64 and
> all of its software (something they learned the hard way was important
> and possibly became - potentially unnecessarily - obsessed with). Hence
> an "afterthought" - "we need the BCD based TOD back NOW!". The 8521 came
> as the answer…

Might not be a 'we need it now' but more along the lines 'We're moving 
away from NMOS (The 8xxx-Chips started in 1984 after all)'. Keeping the 
NMOS line running besides the HMOS line would soon become too costly. 
Well, what do you do then? Do another process change and port the 6526 
to HMOS or take the 8520 which was already there and put the few extra 
transistors back in to make the counter behave as a clock again? The 
latter seems to be the more rational approach. Since the 8521R0 works it 
was the better way, chip revisions, meaning mask changes cost quite a 
bit of money, even at MOS/CSG.

Another reason to go to HMOS was money savings in the long run. You were 
able to get rid of the costly +12V supply for VIC, everything ran cooler 
and used less power. Only SID still needed a higher voltage with very 
low current.

Judging by the numbers, the 8565 in the C64C was started _before_ the 
8566 for the C128, suggesting that the move to HMOS was an ongoing 
process for MOS.


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Received on 2015-03-07 12:01:07

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