Can a frequency be measured on the RB0 interrupt?

DaveM · Guest

Hello,

Does anyone know how to measure a frequency on the RB0 interrupt? I am trying to measure the frequency from a square wave generated from a DTMF chip (MT8889). Normally RB0 is used to read the IRQ generated from the chip but the chip also outputs a square wave on the IRQ pin (when in call progress mode).

I know this can be done with the CCP register or with timer1 but I can't use any of these because...one these pins are connected to something else and two the PIC is already on a PCB.

Thanks

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Does this have to be interrupt driven? What else does the PIC have to do while reading the frequency. How about just using B0 as a normal input pin and read any counter on the B0 edges?
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DaveM · Guest

I am using the interrupt to detect when a valid pin has been received. Nothing else is running except for a timer (used for call duration). But thats the thing... How do I measure the time of the square wave?

The MT8889 outputs a square wave that is proportional to the signal it is analyzing. For instance if a busy signal is detected it will output a square wave that has the same frequency as the busy signal.

RLScott · Joined: 10 Jul 2007 Posts: 465

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

Ttelmah · Guest

Multiple key things apply.

First, critical one, the frequencies you are dealing with. If they are low, then you are 'better' measuring time between edges. If they are high, then you are better counting the edges in asystem time interval.
Second, what is your system frequency?. This affects whether it is possibl to do this at all with the RB0. Basically handling interrupts takes _time_, and the amount depends on the processor involved (18 versus 16 chips), and the system clock. Also, is _any other interrupt in use_?.

Now, with those said:

What are you actually trying to do?. The MT8889, is a DTMF generator/receiver.
When acting as a receiver, the unit _does not_ generate a square wave. It outputs the decoded key pattern, in logic form, on a readable internal register.
The only 'square waves', are the decoder output signal, saying that a tone pair has been 'seen', and latter the interrupt signal saying it has been decoded. Neither contains data designed to allow you to decode the tone pair, but just signals that the chip is seeing legitimate tones.
Measuring the frequency of either signal, will only tell you how often tones have been seen, not the data in the tones.....

Best Wishes

DaveM · Guest

Yes the MT8889 does generate a square wave. The MT8889 has a feature called a Call Progress Filter which allows you to monitor an input during a call.

Here is a quote from the data sheet