SID vs. NMOS tech

From: Hársfalvi Levente (
Date: 2000-10-23 23:41:12

Hi all!

I'd need some help, as usual.

I'm trying to help Dag Lem, author of reSID and Simon White, author of
Sidplay2 to implement realistic SID filters to their emulators. In
short, the SID is a hybrid digital/analog design manufactured in pure
NMOS design (8580 is HMOS). In the filter part, biased NMOS inverters
are used as op-amps and simple NMOS FETs as voltage controlled resistors
:-O. Because of this, the filter doesn't seem to match any known filter
responses ;-). It's indeed very nonlinear; its response highly depends
on the amplitude of the signal that is fed through the filter.

The current implementation models an 'ideal' two-integrator-loop
biquadratic filter with a measured cutoff-to-register_value mapping.

I'm aiming to model the filter part in a simulator, but I'd definitely
need some help for increasing my effectivity ;-). I'm not much familiar
with FETs; I used to know their overall features, but have certainly
never digged deep into FET based analog tech.

At the current standing, my questions are aimed to the following:

- Does anybody know the overall parameters of the FETs used in Commodore
NMOS chips?

They would be needed, since the filter is an analog circuit. It seems
like the FETs get overdriven almost everytime during normal operation;
the filter doesn't act as expected except the lowest signal levels (not
to mention, that the OSCs have DC offsets, helping the FETs going out of
their operating point even easier). Resistance, the typical gate opening
voltage and so on. Typical gate-capacitance (seems like it's
significant) and its behaviour in function of the gate voltage.

- How does a Commodore MOS inverter look like _exactly?

AFAIK, similar NMOS FET is used as pullup, but is it a similar one as
the 'active', controlled pulldown FET or something else, what is its
typical resistance in both states and so on... I'd be interested in all
nuances, since any of them could have a noticeable effect inside the SID

- Is it possible to create real (linear) resistors in NMOS

AFAIK it's quite hard, so it used to be cheated with using NMOS FETs,
but I'm not sure and I must know this, also... If no resistors are
possible, how is it cheated and so on...

- And finally, what major differences are between the different
manufacturing technologies of Commodore? (NMOS-x and HMOS, in

AFAIK, the 8580 was manufactured in HMOS, and it is indeed different to
all 6581s. Similarly, different 6581 revisions should have been
manufactured in different NMOS tech. (They also sound different; R4s
have the strongest filter resonance and said to be the 'best').

I'd be happy if I could solve this modelling. After all, if I'd make a
reasonable model (with also some know Commodore NMOS manufacturing
data), I could also do some measurements on the real thing - both to
gain new data and to compare them to each other.

Also, I'd be glad if someone could point me to a webpage with 'standard'
NMOS FET tech, to become more familiar with this subject (NMOS based
analog example circuits and so on...).

Any help is appreciated, thanks...

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