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From: Richard Atkinson
Date: 1998-09-14
Subject: Re: FF30, timers, FF11

> > would you share your experience with us in some usable format? Eg. a
> > nice tablet of analogue or digital values for each value of $FF11? (I
> > need only values between $80 and $ff...)
>
> Rachy:
>
> You'll indeed, need values between 00 and $ff. Else, no digital
> converters of Doky, maybe NST and ofcoz' mine will be emulated
> correctly.

I see the importance of this... okay, I will investigate high frequencies of TED. Someone tell me, does $ff produce the highest frequency of all, or no signal?

> Richard:
>
> Please also measure the values in my table (I mean, setting the TED
> Frq regs to those high values, and then also checking $10...$18,
> $20...$28 and $30...$38 like $90...$98 and the others.). ...It would
> give us (...me) an answer for this weird behaviour of the TED.

This is complicated by the problem of synchronising the two oscillators.
If they are sync-ed (by setting bit 7 high then low, AFTER setting the frequency) then it should be fairly easy to determine the results.
However, if they are running asynchronously, then cycle-exact (note: maybe even 17MHz clock-exact) emulation is necessary.

> Ps. A lowpass filter is quite like an integrater, huh? Then, how about
> feeding the TED's output through a strong lowpass filter, then measuring
> the filter output's DC level? ...I mean, after disconnecting everything
> else from the TED's output (except our lowpass filter) to avoid parasite
> currents.

OK, Tibor sent a mail also about this. I'll try to clarify what I'm doing:

The output of TED itself only has two states: high and low. In every way it is a digital (1 bit) output. Therefore, if we know when it is high and when it is low (or to be precise, the ratio of high to low) we can predict what the analogue voltage will be at the output after filtering,
without having to measure it (and incur inaccuracies), and thus emulate the "final" analogue voltage based on what we know the digital one is doing.

I am paying no attention whatsoever to actual analogue voltages, only observing how long the signal is high and how long low under different settings of $ff11 etc. I should really use a digital storage oscilloscope,
though :-)

(thinks - perhaps my company can help me with this!)

Richard.