Go Down

Topic: Reading signal frequency (Read 5 times) previous topic - next topic

dhenry

Quote
the original code is from here


If you are measuring a signal with sufficient swing (that it will trigger a counter click), all you need is a small resistor + the mcu.

comsat

AWOL, yup I tried and it can read the 490 Hz.

It looks like pin 8, ICP1, the Input Capture pin for timer0/counter0, is open-circuited, and it sees only power frequency noise.  An easy way to test this is to connect a high-value resistor - like maybe 100k - between pin 8 and either ground and VCC, ort turn on the internal pullup resistor for pin 8.  [/quote]

tmd3, so, i should try connect resistor between pin 8 and ground is it and dont i have to feed with any signal? by the way, what is ort?

tmd3

... so, i should try connect resistor between pin 8 and ground is it and dont i have to feed with any signal?

Um, no.  If you want to know something about a signal, like it's frequency, you'll certainly have to connect it.  My recommendation is that you connect a resistor to either 5V or to ground, and connect the other end to ICP1, or pin 8 on an Arduino Uno.  Then connect your signal between ICP1 and ground, as you normally would.

Here's what I suspect:  For some reason - a broken lead, an in correct connection, a switch in the wrong position, or something, there's nothing at all connected to ICP1.  That makes ICP1 a "floating input."  The pin has a very high impedance to both ground and VCC, and it's not referenced to anything.  In that state, it's much more susceptible to electromagnetic interference than it would be if it were connected to something.  I suspect that what you're reading is not your input signal, but the frequency of electromagnetic fields caused by the electric power system.  Your readings suggest that you live in a region where the power frequency 50 Hz.  Tell us if that's correct.

The fact that you can read 490 Hz from a PWM pin supports this theory.  I suspect that you didn't use the leads from your frequency generator to connect the PWM pin to ICP1.  That means that both the frequency generator and its leads were out of the circuit.  I suspect that one of them isn't doing what you think:  either the lead is broken, or improperly connected, or the frequency generator is disconnected from the circuit - probably by a switch on the front panel.

By applying a resistor between ICP1 and a good logic level, like VCC or GND, you make the pin a lot more resistant to the effects of electromagnetic fields.  Pick a high value resistor - like maybe 100K.  Then, connect your signal between ICP1 and ground, and run your program.  I suspect that you'll never get a reading, because the logic level on ICP1 will never change.  If that's what happens, it supports my theory that the input isn't seeing the signal, or anything else.  To see what the frequency measurement library does with an input that never changes - one whose frequency is zero - see your link - http://www.pjrc.com/teensy/td_libs_FreqMeasure.html - under the heading, "Zero Handling."

Quote
... what is ort?

"ort" is a word that I typed by mistake.  I had intended to type "or".

comsat

I try to connect as u suggested but no luck. or is it possible because i connect wrong pin? i looked the code again and there is no mention about pin 8 as an input.

tmd3

... but no luck.
Be more specific.  What were your results?  Did you continue to measure about 50 Hz?  Did you fail to get a reading at all?

Quote
... or is it possible because i connect wrong pin?
I can't see your connections.  If you connected to the wrong pin, that would be an explanation.  Maybe you're asking if pin 8 is the correct pin for this program to use - I'll suggest that you review the site you referenced in your original post - http://www.pjrc.com/teensy/td_libs_FreqMeasure.html - which states that pin 8 is the pin to use for the Arduino Uno.  If you're not using an Uno, maybe you are indeed connected to the wrong pin.  Which Arduino are you using?

Quote
... i looked the code again and there is no mention about pin 8 as an input.
There's no mention of any pin as an input.  INPUT is the default state of the pins, so it's generally not necessary to explicitly set the mode to INPUT.

I don't see that you've answered the question about the power frequency at your location.  Is it 50 Hz?

Go Up