[Diy_efi] Continuing the chip switcher "problem"

Mon Feb 14 07:45:11 GMT 2005

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Hi Marcello,

You aren't being specific enough when you say your DSO only goes down to 5  
micro seconds.  What does that mean?  I suspect it means the X  timebase sweep 
speed won't go faster than 5 microseconds per centimetre of  screen display.

That's not nearly fast enough.  You need about 50-100 times  faster.  What 
you are getting here I think is garbage traces based on the  fact that you are 
trying to sample something very fast on a very infrequent  basis.

You seem to be hitting what electronics and information tech engineers call  
the Nyquist limit, sometimes also known as the Shannon limit.  Basically,  if 
an event happens x times a second, you have to sample it at least 2x times a  
second to have any chance of seeing it as it is.  If so, what you see on  
screen bears no relation to what is actually happening.

I'm not convinced a PIC is the right approach either to switch banks.   It is 
very over-engineered, even if it is fast enough.  Simple HCMOS  devices like 
a quad latch, triggered off a suitably chosen pulse, should  guarantee clean 
reads every time, and it will overcome the need for  debouncing of the switches 
you use to select different banks.  If somebody  can find me a data sheet for 
the 29C010 I'd be happy to make a few practical  suggestions.  What I need to 
see are the timing diagrams for the read cycle  in particular.

Regards

Will Cowell

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<DIV>Hi Marcello,</DIV>
<DIV>&nbsp;</DIV>
<DIV>You aren't being specific enough when you say your DSO only goes down t=
o 5=20
micro seconds.&nbsp; What does that mean?&nbsp; I suspect it means the X=20
timebase sweep speed won't go faster than 5 microseconds per centimetre of=20
screen display.</DIV>
<DIV>&nbsp;</DIV>
<DIV>That's not nearly fast enough.&nbsp; You need about 50-100 times=20
faster.&nbsp; What you are getting here I think is garbage traces based on t=
he=20
fact that you are trying to sample something very fast on a very infrequent=20
basis.</DIV>
<DIV>&nbsp;</DIV>
<DIV>You seem to be hitting what electronics and information tech engineers=20=
call=20
the Nyquist limit, sometimes also known as the Shannon limit.&nbsp; Basicall=
y,=20
if an event happens x times a second, you have to sample it at least 2x time=
s a=20
second to have any chance of seeing it as it is.&nbsp; If so, what you see o=
n=20
screen bears no relation to what is actually happening.</DIV>
<DIV>&nbsp;</DIV>
<DIV>I'm not convinced a PIC is the right approach either to switch banks.&n=
bsp;=20
It is very over-engineered, even if it is fast enough.&nbsp; Simple HCMOS=20
devices like a quad latch, triggered off a suitably chosen pulse, should=20
guarantee clean reads every time,&nbsp;and it will overcome the need for=20
debouncing of the switches you use to select different banks.&nbsp; If someb=
ody=20
can find me a data sheet for the 29C010 I'd be happy to make a few practical=
=20
suggestions.&nbsp; What I need to see are the timing diagrams for the read c=
ycle=20
in particular.</DIV>
<DIV>&nbsp;</DIV>
<DIV>Regards</DIV>
<DIV>&nbsp;</DIV>
<DIV>Will Cowell</DIV></FONT></BODY></HTML>

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