FPGA Central

Hallo,
I should measure the frequency of an input signal.
Max frequency of signal is 100 KHz.

I have made a simple system to detect rising edge of input signal using a
flip flop.
It samples the input signal every system clock cycle.
To measure the input frequency I count the number of system clock periods
between two rising edges of input signal.

To test the system I' using a function generator.

Reading the data acquired (using a fixed frequency) I have watched that
every 5-10 datas there is an error.
The number aquired is about 50% or 75% less than the others.

Using a pull up resistor will be solved the error?

Otherwise I need a system that verify glitches?

--
Marco

On a sunny day (Mon, 17 Apr 2006 16:59:07 +0200) it happened "Marco T."
<[email protected]> wrote in <[email protected]>:

>Hallo,
>I should measure the frequency of an input signal.
>Max frequency of signal is 100 KHz.
>
>I have made a simple system to detect rising edge of input signal using a
>flip flop.
>It samples the input signal every system clock cycle.
>To measure the input frequency I count the number of system clock periods
>between two rising edges of input signal.
>
>To test the system I' using a function generator.
>
>Reading the data acquired (using a fixed frequency) I have watched that
>every 5-10 datas there is an error.
>The number aquired is about 50% or 75% less than the others.
>
>Using a pull up resistor will be solved the error?
>
>Otherwise I need a system that verify glitches?
>
Hard to tell what your error source is.
But in all cases you need to make a fast rise clean pulse.
You need a Smitt-trigger.
Basically an opamp with some positive feedback.

If you want to measure 100 kHz with 1% accuracy, you need to count the
incoming pulses for at least 1 millisecond.
Or you can measure the 10 microsecond period length, if you have a
clock available that is faster than 10 MHz.

Measuring for a longer time reduces the impact of signal rise time and
jitter, but requires a counter to establish the ms time base.

This is all covered in hundreds of text books...
Peter Alfke

Marco T. wrote:

> Hallo,
> I should measure the frequency of an input signal.
> Max frequency of signal is 100 KHz.
>
> I have made a simple system to detect rising edge of input signal using a
> flip flop.
> It samples the input signal every system clock cycle.
> To measure the input frequency I count the number of system clock periods
> between two rising edges of input signal.

For more precsiion, you can divide the Fu, before doing the time
capture.
Or, you can set up to capture a wide-timebase counter, and NOT reset the
counter between captures - then you have the option to choose
Cycles in SW, and the time is done by subtract, and you do not loose
any time-ticks, so have best precision.

>
> To test the system I' using a function generator.
>
> Reading the data acquired (using a fixed frequency) I have watched that
> every 5-10 datas there is an error.
> The number aquired is about 50% or 75% less than the others.

50% less is explained by false triggers on the wrong edge,
75% less is harder to explain, unless you are over more than one cycle ?

>
> Using a pull up resistor will be solved the error?
>
> Otherwise I need a system that verify glitches?

Hardware solution is to add a Schmitt trigger, and maybe also a RC filter.

SW patch is to read multiple captures, and reject ones that suddenly
'step' from previous readings.

-jg

Marco T. wrote:
> Hallo,
> I should measure the frequency of an input signal.
> Max frequency of signal is 100 KHz.

[... snip ...]

There is a frequency coutner reference design for Spartan-3E Starter
Kit board.
http://www.xilinx.com/products/board...quency_counter

I'm not sure how low in frequency that you want to go, but it does
measure from ~50 kHz to 100s of MHz. It also uses the PicoBlaze 8-bit
embedded controller macro.

[ADV]: Spartan-3E Starter Kit board
http://www.xilinx.com/s3estarter

[ADV]: PicoBlaze Controller
http://www.xilinx.com/picoblaze

---------------------------------
Steven K. Knapp
Applications Manager, Xilinx Inc.
General Products Division
Spartan-3/-3E FPGAs
http://www.xilinx.com/spartan3e
---------------------------------
The Spartan(tm)-3 Generation: The World's Lowest-Cost FPGAs.

Many Thanks to Everyone!

Marco