What limited the C64's CPU to ~ 1 MHz? , Vol 17, Issue 23
SVCatITC at aol.com
SVCatITC at aol.com
Thu Mar 23 20:52:28 EDT 2017
Hi Dan, John;
Thanks for the enlightenment on legacy high speed chip design.
Boy does this bring back memories!
Back in the early 80's, had a similar problem designing a quiet 5mHz main
with dividers, to run test fixture registers, on component hardware from
(Working in the test fixture / model shop.)
In a macro sense, like your 'micro' illustration, I had harmonic splash
the fixture and DUT circuits..... Way too noisy for testing the low level
circuits on the hybrid DUT PC board.
I finally ended up taking the main clock components off the main breadboard
(oscillator and the like), and enclosing them in a total metal capacitor
(forget the alloy) with ceramic feedthroughs for power and clock out.
With the total metal shield around the 'heartbeat'..... quieted everything
We all, in the normal thoughts in our brains, think only digital when we
are in front of such
devices, however, when you are designing main 'heartbeats' in these things,
to think 'analog' and design around the analog component of the
before moving on to 'clocking' signals and gate chasing around the PC
In devices that have an analog component (Data Acquisition), have seen
PC traces totally circling around the 'heartbeat' circuits, for shielding.
So like the 12 items you identified in rudimentary CPU design, when you are
that level, one must think basic analog and physics for high speed design
and compensate for all the parasitic effects.
As you noted, they all had trade-offs. It comes down to best bang for the
Thank you so much for the refresher course!
----------- Original Message -------------
Date: Thu, 23 Mar 2017 11:01:04 -0400
From: Dan Roganti <ragooman at gmail.com>
<vcf-midatlantic at lists.vintagecomputerfederation.org>
Subject: Re: [vcf-midatlantic] What limited the C64's CPU to ~ 1 MHz?
<CADo=F9Eai8W_OvDosnWSm1vq9tbF+yQHKcB179t5cEBg05HzLA at mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Thu, Mar 23, 2017 at 8:47 AM, John Heritage via vcf-midatlantic <
vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
>> Does anyone know the reason why the C64 only ran at ~1 MHz? Was it to
>> keep RAM costs or system cost as low as possible or was there a
>> engineering reason?
>> (The Atari and BBC machines managed higher clocks, but were very
>> compared to the C= devices, although the 1977 Atari 2600 managed ~1.19
>> out of it's 6507 (6502 with low pin count/less addressing)..
> The various clock circuits inside the different home computers was the
> least of their problems. All of those different combinations used between
> the various home computers were just methods they found to minimize their
> Bill of Materials - because every penny was crucial to their bottom line
> when making thousands of them in the consumer market.
> There were still real and significant design challenges present in the
> early VLSI designs of the 1970s to make these faster. But they slowly
> marched on and made improvements every year to where we had the 8086
> running at 5mhz and 68000 running at 8mhz already by 1979. This list
> is just some of design problems and they are all interrelated. Making
> offs between each of these was a constant battle. This list is mostly
> ordered by significance, but some of this could be a matter of opinion.
> 1. BJT, NMOS, PMOS, or CMOS logic families contribute to speed,
> eg. depletion-load NMOS is faster than BJT
> 2. Miller Effect [capacitance] prsent even on the silicon die
> 3. VLSI design geometries were limited to about 5um
> 4. Clock distribution across the silicon die [eg, too much clock skew]
> 5. Higher Clock speed required more power dissipation
> 6. Negative Bias voltage was a burden to compensate for Miller Effect
> [eg. 8080 ]
> 7. Lead Inductance present even on the silicon die
> 8. Slew Rate,
> eg, ECL being faster ran on smaller voltages, slew rate was less,
> giving faster clock speed
> 9. TTL signals are above the noise floor
> eg, ECL being faster was below noise floor, ie power is negative
> 10. TTL operate in saturation for logic signals, [eg. ECL being faster
> operate in Linear region]
> 11. Square wave logic signals have more odd numbered harmonics
> than a linear analog device, reducing the possible bandwidth
> 12. Pin Capacitance limited not only the driver characteristics but also
> the frequency bandwidth
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