#124 Rising Edge Detection shows false counts with capacitance on input

[Paul Versteeg]

When a clean edge with a fast (nSec range) rise/fall time is send to the input pins, Falling, Rising and BOTH edge detections work as advertised.

However, by only adding a small amount of capacitance (50pF) to the input, this changes the rise-time so much that false edge recognition is introduced.
I was able to reproduce this by using part of the test suite. I connected an output pin to an input pin by a 10K resistor and added capacitors of different values to the input pin and ground to show the effect.
The table below only shows count decrepancies.

    Rpi.GPIO

Capacitor Counter Falling Rising Both
Value sb

1uF 100 175 258 263
440nF 100 - 200 -
220nF 100 - 200 -
100nF 100 - 200 -
50nF 100 - 200 -
4.7nF 100 - 170 -
1nF 100 - 133 -
470pF 100 - 110 -
100pF 100 - 105 -
50pF 100 - 102 -
none 100 - - -

A detailed analysis of the Rising edges shows that they are recognized correctly, however, the total count is "falsified" by Falling edges, which are also detected.
I could verify this also on a scope.

    Rpi.GPIO        
    Testing Rising Edges

Capacitor Counter Falling Rising Total
Value sb

1uF 100 138 100 238
440nF 100 109 100 209
220nF 100 100 100 200
100nF 100 100 100 200
50nF 100 100 100 200
4.7nF 100 81 100 181
1nF 100 24 100 124
470pF 100 6 100 106
100pF 100 2 100 102
50pF 100 2 100 102
none 100 0 100 100

[Paul Versteeg]

Thanks Ben, sorry for the messed-up tables. I tried to format them, but when
I posted the software took all formatting away.

I think that the major "take-away" is that any capacitance on the input
messed the edge recognition somehow.

Paul

From: Ben Croston [mailto:croston@users.sf.net]
Sent: dinsdag 9 februari 2016 18:00
To: [raspberry-gpio-python:tickets]
Subject: [raspberry-gpio-python:tickets] #124 Rising Edge Detection shows
false counts with capacitance on input

• status: Verified --> Duplicate

---

[tickets:#124] http://sourceforge.net/p/raspberry-gpio-python/tickets/124/
Rising Edge Detection shows false counts with capacitance on input

Status: Duplicate
Created: Tue Feb 09, 2016 12:36 PM UTC by Paul Versteeg
Last Updated: Tue Feb 09, 2016 04:54 PM UTC
Owner: Ben Croston

When a clean edge with a fast (nSec range) rise/fall time is send to the
input pins, Falling, Rising and BOTH edge detections work as advertised.

However, by only adding a small amount of capacitance (50pF) to the input,
this changes the rise-time so much that false edge recognition is
introduced.
I was able to reproduce this by using part of the test suite. I connected an
output pin to an input pin by a 10K resistor and added capacitors of
different values to the input pin and ground to show the effect.
The table below only shows count decrepancies.

Rpi.GPIO

Capacitor Counter Falling Rising Both
Value sb

1uF 100 175 258 263
440nF 100 - 200 -
220nF 100 - 200 -
100nF 100 - 200 -
50nF 100 - 200 -
4.7nF 100 - 170 -
1nF 100 - 133 -
470pF 100 - 110 -
100pF 100 - 105 -
50pF 100 - 102 -
none 100 - - -

A detailed analysis of the Rising edges shows that they are recognized
correctly, however, the total count is "falsified" by Falling edges, which
are also detected.
I could verify this also on a scope.

Rpi.GPIO        
Testing Rising Edges

Capacitor Counter Falling Rising Total
Value sb

1uF 100 138 100 238
440nF 100 109 100 209
220nF 100 100 100 200
100nF 100 100 100 200
50nF 100 100 100 200
4.7nF 100 81 100 181
1nF 100 24 100 124
470pF 100 6 100 106
100pF 100 2 100 102
50pF 100 2 100 102
none 100 0 100 100

---

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[Ben Croston]

Merged into issue 121