Re: Counting 2ms long pulses on several input pins

From	pozz <pozzugno@gmail.com>
Newsgroups	comp.arch.embedded
Subject	Re: Counting 2ms long pulses on several input pins
Date	2013-09-25 00:33 +0200
Organization	A noiseless patient Spider
Message-ID	<l1t40d$sma$3@dont-email.me> (permalink)
References	<l1qck1$9dd$4@dont-email.me>

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Il 23/09/2013 23:42, pozz ha scritto:
> Do you have better ideas?

Ok, this is the full story. I want to interface a microswitch-based
sensor (http://www.utk.it/img/img_prodotti/00279.jpg) that is normally
mounted to a rolling shutter
(http://www.pakuya.com/upload/20111128/China_luhaitian_Aluminum_insulation_roller_shutter.jpg)
in order to detect its movement and prevent unauthorized access by thiefs.

When the shutter moves vertically, the thin rope attached to it creates
the circular movement of the cam inside the sensor. I have seen sensors
with 6 cams per rotation, but there are so many low- and high-quality
sensors on the market that I expect some differences among them.

The final goal of the project is to detect movement of the shutter
caused by humans (i.e., not caused by wind). This can be achieved if
I'm able to count the debounced pulses of the microswitch. For example,
I can raise the alarm when I see 10 pulses in a second (low sensitivity)
or just 2 pulses in 5 seconds (high sensitivity). Depending on the
sensor used, I can change the sensitivity of the alarm detector by
changing the number of the pulses and the period of counting.

So I connected the two wires at the output of the sensor (connected
internally to the microswitch) at a microcontroller-based circuit: one
pole is connected to 0V and the other to a pulled-up input of the
microcontroller. Normally the switch is closed so the level is 0V when
the shutter isn't moving (it's impossible to stop the shutter when the
microswitch is open).

I made some tests pulling/releasing manually the rope of the sensor and
the pulses frequency, width and bouncing changes dramatically.
Here two pictures with a high rotation speed:
http://imageshack.us/a/img191/6538/5v63.jpg
http://imageshack.us/a/img545/2902/nrw5.jpg
Here a picture with a low rotation speed (just one pulse):
http://imageshack.us/a/img843/4931/2ebz.jpg
As you can see, the frequency of the pulses changes from 0 to about
300Hz, width (including final bouncing) from 2ms to 11ms, final bouncing
from 1ms to 3.4ms. The positive edge is usually sufficiently clean
(without bouncing).

As I wrote in my previous email, the best solution I found that minimize
the not-detected and false pulses is to sample the switch line at 1ms
rate and count the pulse when I see the 0-1 sequence. After this
sequence, I wait for 1-0-0 sequence to consider the pulse finished and
start waiting for the next pulse (sequence 0-1).

Of course there's no problem with low rotation speed: the pulse is
sufficiently long to detect it correctly. The problems come with short
pulses that have a very small stable interval (where the signal is
stable high, so the switch is open without bouncing).

I know I should sample at a higher frequency (maybe 100us) to increase
reliability, but I prefer to avoid stressing the microcontroller too
much, because it has other tasks (consider I have 8 similar inputs) to run.

I know the pulses are very noisy and an optical/magnetic sensor could
solve many problems, but I can't change the sensor. Moreover for this
application I could tolerate a few errors in the counting process.

Thread

Counting 2ms long pulses on several input pins pozz <pozzugno@gmail.com> - 2013-09-23 23:42 +0200
  Re: Counting 2ms long pulses on several input pins mike <ham789@netzero.net> - 2013-09-23 15:00 -0700
  Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-23 16:12 -0700
  Re: Counting 2ms long pulses on several input pins Paul Rubin <no.email@nospam.invalid> - 2013-09-23 18:45 -0700
    Re: Counting 2ms long pulses on several input pins Arlet Ottens <usenet+5@c-scape.nl> - 2013-09-24 08:06 +0200
      Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 00:37 -0700
        Re: Counting 2ms long pulses on several input pins Arlet Ottens <usenet+5@c-scape.nl> - 2013-09-24 10:24 +0200
          Re: Counting 2ms long pulses on several input pins Paul Rubin <no.email@nospam.invalid> - 2013-09-24 02:05 -0700
            Re: Counting 2ms long pulses on several input pins Arlet Ottens <usenet+5@c-scape.nl> - 2013-09-24 11:01 +0200
          Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 09:55 -0700
            Re: Counting 2ms long pulses on several input pins Tom Gardner <spamjunk@blueyonder.co.uk> - 2013-09-24 18:13 +0100
              Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 11:03 -0700
            Re: Counting 2ms long pulses on several input pins Arlet Ottens <usenet+5@c-scape.nl> - 2013-09-24 19:38 +0200
              Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 11:13 -0700
                Re: Counting 2ms long pulses on several input pins Arlet Ottens <usenet+5@c-scape.nl> - 2013-09-24 20:30 +0200
                Re: Counting 2ms long pulses on several input pins Paul E Bennett <Paul_E.Bennett@topmail.co.uk> - 2013-09-24 21:20 +0100
                Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 16:08 -0700
    Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 00:34 -0700
  Re: Counting 2ms long pulses on several input pins Andrew Smallshaw <andrews@sdf.lonestar.org> - 2013-09-24 04:31 +0000
  Re: Counting 2ms long pulses on several input pins Paul E Bennett <Paul_E.Bennett@topmail.co.uk> - 2013-09-24 11:15 +0100
    Re: Counting 2ms long pulses on several input pins Paul <paul@pcserviceselectronics.co.uk> - 2013-09-24 15:51 +0100
  Re: Counting 2ms long pulses on several input pins pozz <pozzugno@gmail.com> - 2013-09-25 00:33 +0200
    Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 16:36 -0700
      Re: Counting 2ms long pulses on several input pins pozz <pozzugno@gmail.com> - 2013-09-25 07:39 +0200
        Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-24 23:37 -0700
          Re: Counting 2ms long pulses on several input pins pozz <pozzugno@gmail.com> - 2013-09-27 15:54 +0200
            Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-27 12:35 -0700
    Re: Counting 2ms long pulses on several input pins mike <ham789@netzero.net> - 2013-09-24 16:43 -0700
  Re: Counting 2ms long pulses on several input pins j.m.granville@gmail.com - 2013-09-24 17:32 -0700
    Re: Counting 2ms long pulses on several input pins Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2013-09-25 09:52 +0300
      Re: Counting 2ms long pulses on several input pins Don Y <this@isnotme.com> - 2013-09-25 00:09 -0700
        Re: Counting 2ms long pulses on several input pins j.m.granville@gmail.com - 2013-09-25 16:53 -0700

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