source: softGlue_examples/source/debouncer/index.rst @ 974

.. $Id: index.rst 922 2012-06-13 15:58:36Z jemian $

========================
Debouncer
========================

.. index:: !Debouncer;using time-delay
.. index:: !Low-pass filter

A signal debouncer, which can be used to remove high-frequency ringing from an
input signal.  The circuit can also be used as a low-pass filter, though the
criterion on which it discriminates is not precisely a signal's frequency, but
rather the time interval between rising edges.

Circuit and documentation: Tim Mooney

Circuit
------------

.. image:: debouncer.gif
        :width: 100%

This circuit shows a debouncer, which accepts as input the signal named
``input``, and produces the output signal ``output``.  As configured here, the
circuit discriminates against symmetric square waves of frequency higher than
around 1 kHz, transmitting nothing higher than around 2 kHz.


Theory of operation
-------------------

``DivByN-4`` produces narrow (125 :math:`\mu`\ s) positive-going pulses at the
frequency :math:`f_{threshold} = 8 \times 10^6 / N`, where :math:`N` is the
``N`` input.  This signal (``filterClock``) initializes the ``accept`` signal
(``DFF-4``) to 1, and its trailing edge samples the input signal, ``input``, by
clocking ``DFF-1``.  If no additional rising edges of ``input`` are received
before the leading edge of the next ``filterClock`` pulse, the sampled input
value is clocked into ``DFF-2``, causing ``output`` to follow ``input``.  But if
an additional rising edge is received during that time, it will clock ``DFF-4``,
driving ``accept`` to 0, and immediately clearing ``DFF-2``, so that ``output``
goes to 0 regardless of the value of ``input``.

        .. note::  A somewhat less robust debouncer can be made with only two flip
                           flops, by connecting ``input`` directly to the ``D`` input of
                           ``DFF-2``, freeing ``DFF-1`` for other uses.  The resulting
                           circuit has a race condition, and its performance may depend on
                           FPGA layout.

Usage
-----

To load the circuit into softGlue:

1)      Download this `BURT
        <http://www.aps.anl.gov/epics/extensions/burt/index.php>`_ snapshot file
        :download:`debouncer.snap <debouncer.snap>`

2)      Edit the file to replace all occurrences of the string ``xxx:softGlue:``
        with whatever value you specified for ``$(P)$(H)`` when you loaded softGlue
        into your IOC.

3)      Load the file with the command ``burtwb -f debouncer.snap``

To use the circuit:

1) Set the threshold frequency of the circuit as described above.

2) Connect ``input`` and ``output`` signals as desired.

Performance
--------------

Here's a demonstration circuit with which the debouncer's performance as a
low-pass filter can be measured:

.. image:: debouncer_demo.gif
        :width: 100%

In this demonstration, ``DivByN-3`` and ``DFF-3`` constitute a :index:`!square
wave generator` with which to exercise the filter.  ``UpCntr-1`` and ``UpCntr-2``
count output and input edges, respectively.  ``BUF-2`` and ``BUF-3`` fan out
reset and counter-enable signals, respectively.

``xxx:softGlue:sseq1`` drives ``demoReset`` and ``demoPause`` to count input and
output pulses for one second, then processes the up-counter value records, so
that their values will be available promptly to ``xxx:userCalcOut9``, which
calculates the filter's pass fraction.  ``xxx:scan1`` programs filter and
input-signal frequencies, using ``xxx:userTran7`` to set the filter frequency
and to translate frequencies into down-counter load values.  The plot shows the
fraction of the input rising edges that produce output rising edges, as a
function of input frequency.  For this case of a symmetric square wave, the
circuit passes everything below :math:`f_{threshold}`, and nothing above
:math:`2 f_{threshold}`.

The BURT file for this demonstration circuit, :download:`debouncer_demo.snap
<debouncer_demo.snap>`, programs only the records included in a standard
softGlue installation, not the records ``xxx:scan1``, ``xxx:userCalcOut9``, or
``xxx:userTran7``.