Re: 8520/21

On 03/05/2015 07:36 PM, Jim Brain wrote:
> On 3/5/2015 12:25 PM, Gerrit Heitsch wrote:
>>
>> Well, the way it looks with MOS means that the lower last number is
>> the chip that was done first, just like the 6520 and the 6522.
>>
>> So they had that 6526 in NMOS and when the Amiga design came around
>> ('85), they wanted a CIA for it as well but decided against the
>> TOD/RTC from the original design, probably wanting the plain counter
>> instead.
> This is what makes no sense.
>
> Hey, we're building a new PC.  Let's see, we have a steady source of CIA
> chips we could use.  Uh, no, let's go to *ALL* the trouble to tape out a
> new IC, drop the TOD clock, go to a new process, and test it all and run
> through 4 revs just to get it working.
>
> That is nuts.  The 6526 variant was speced at 2MHz, just pull it out of
> the parts bin and use it.  Why go to all the trouble to create a new
> IC.  That makes no sense at all.

The 8520 runs at about 700 kHz (E-clock from the 68000 which is, as far 
as I know CLOCK/10) in the Amiga, so speed is not the issue. I can only 
imagine that they didn't need the TOD/RTC but thought a plain counter 
would be more useful.

Remember, Commodore made a lot of different chips just because they 
could... Why a special CPU for the 264 series and the C128 instead of 
just using a different bond out of the 6510? The 8500 is the 6510 in 
HMOS-II and, judging from the number, the earlier design than the 8501 
and 8502. Or why the 6523 and later the 6525 (6523 with IRQs)? And then 
all the 77xx-TTL replacements... Or the 6529...


  Gerrit



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