There are Two Fundamental Types of Frequency Counter
Did
you know there are two types of frequency counter?
The
method you'll probably think of first is to count the input edges of
the signal you want to measure and time a precise delay e.g. 1s then
the frequency you measure will be in Hz.
There is another method called reciprocal counting where you
count the edges of the internal clock instead (used in commercial
counters).
Normal direct frequency counting
The obvious method is to feed the measurement signal into a
counter chip which is turned on for a set period e.g. 1 second.
The counter value is actually the frequency measurement since:
f = events/time =
counter value/1 second = counter value in Hz.
The problem with this method is that the resolution of the counter is
1Hz and the number of digits displayed is dependent on the input
frequency:
A 1Hz input gives 1 digit
A 100Hz input gives 3 digits etc.
The resolution is in Hz (dependent on the gate time -which must be a
multiple (or sub-multiple) of a second) - this is why the reciprocal
counting method is better as it gives a resolution in terms of the
master clock frequency inside the unit that is not dependent on gate
time and the number of digits is also not dependent on the input
frequency.
Reciprocal frequency counting
The second method is reciprocal counting where instead of counting the
input signal you count periods of a master clock and instead of
counting the input signal edges you let the input signal start and stop
the counter.
For example the counter is started at the rising edge of the input
signal and stopped at the next rising edge. Now the counter
reading is actually the period of the input signal in
multiples of the master clock.
Its called reciprocal counting since you have to work out:
f= 1.0 / period
...to work out the frequency.
A simple reciprocal counting example.
Lets say your internal counter runs at 1 MHz and you have an
input signal that has a frequency of 1345Hz.
The number of 1MHz periods in 1345Hz is:
Tsig/Tmeas = (1/1345)/(1/1e6)
or 1e6/1345 = 743.49
We lose the decimal point to get
743 counts of our 1MHz
counter.
So in the microcontroller we have a value of 743 and to show
the frequency on a display we need to calculate the following equation:
1/(743*(1.0/1e6)) = 1345.895
Hz
which can also be written as:
1e6/743 = 1345.895 Hz
...and the easiest way to do that is using a floating point
library.
Note: Remember
that floating point
libraries are slow, slow, slow so
consider using fixed point maths.
The reason for using reciprocal counting
Reciprocal counting is more difficult as you have to use floating point
routines to work out the frequency but because the counter counts edges
of a master clock the resolution is fixed in multiples of that master
clock and not dependent on the input signal i.e. the frequency counter
will show all digits regardless of the frequency of the input.
The resolution of the measurement is better since in a normal counter
the count will be out by ±1 Hz count whereas the reciprocal
counter will be out by ±1 master clock cycle.
Note: This does not mean
that it is more accurate - accuracy depends on the clock stability of
the unit.
Reciprocal counters are useful for period measurements and you will
find for commercial counters that they usually have two inputs labeled
A and B. You can set up the frequency counter to just measure
A frequency on A or on B but you can also do signal period measurements
e.g. trigger on the rising edge of A to get the high period etc.
In addition you can do measurements between different signals e.g.
rising edge of A and only after that has triggered stop the count on
the rising edge of B - that would be useful for measuring timing
characteristics for a peripheral e.g. RAM access time.
You can find
a project for a normal
Direct
frequency counter here.
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