Re: Weird /KERNAL line on old boards

Den Wed, 11 Jul 2018 01:46:48 +0300 skrev Nejat Dilek
<imruon@gmail.com>:
> On Tue, Jul 10, 2018 at 9:06 PM, Mia Magnusson <mia@plea.se> wrote:
> > Den Mon, 9 Jul 2018 01:59:35 +0300 skrev Nejat Dilek
> > <imruon@gmail.com>:
> >
> > If the CPU is fast enough to reliable sample the input not longer
> > than 500nS and not shorter than 41nS after an interrupt has been
> > triggered, a software solution would work. If the input isn't
> > active after 41nS but before 500nS, just ignore the interrupt and
> > be sure to return to a state accepting interrupts fast enough so it
> > can trigger on a real pulse.
> 
> I'm using 16mhz internal clock with Attiny85, sure it's fast but it
> has a big interrupt latency.
> I also have another timer interrupt going on. Making the two
> interrupts behave in this scenario would be a bit problematic.

Is there any way to disable the timer interrupt and replace it by
polling the timer?

> >> Any other hardware means to filter out 41nsec pulses from 500nsec
> >> pulses by the way? (Hmm, maybe using some logic gates with >41nsec
> >> propagation delays)
> >
> > A simple low pass filter will do. 500nS is more than 10 times longer
> > than 41nS so even a simple 6dB/octave RC filter should do the trick.
> 
> Tested this with 10pf capactior and 1.5K resistor, used an online
> calculator for 10mhz cutoff frequency. Signal looked clean on the
> logic analyzer but it didn't work.
> Is this cutoff frequency reasonable?

The wanted cutoff frequency depends on what's the lowest signal levels
that's treated as a real digital signal by the input the filter drives.
You'd have to experiment with a trimmer or calculate fall/rise times
for the voltage change required for the signal to be treated as a
logic level change.

> > Depending on the signal quality requirements on the interrupt input
> > you might need a schmidt trigger (a 74xx14, 4093 or similar)
> > between the low pass filter and the interrupt input.
> 
> I'll test this too just for the sake of it. If simple RC worked though
> it would be a better candidate than using a 74hc1g32 2 input or gate.

Look at the data sheet for specifications of the input you're using.

Worst case it has a rather large voltage span of unspecified behavior,
then you'd need a schmidt trigger for the behavior to be repeatable if
you "mass produce" your circuit. You might get away with not using a
schmidt trigger if you buy enough IC's and are sure that they are from
the same batch, but that's relying on unspecified behavior.

If you are lucky, the behavior is well-specified though.


-- 
(\_/) Copy the bunny to your mails to help
(O.o) him achieve world domination.
(> <) Come join the dark side.
/_|_\ We have cookies.