mt8870

lboghy · Joined: 04 Jun 2008 Posts: 23

Hi,

I want to use MT8870 IC to decode DTMF tones. This circuit need 3.5795MHz XTAL. This signal can be generated by PWM of a micro?
I want to use PIC18F25k20.

Thank you.

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

Usually on the datasheet of your PIC you will see the Frequency range for the PWM.

There might be other ways... maybe using regular I/O ... you will have to time things carefully.

G
_________________
CCS PCM 5.078 & CCS PCH 5.093

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

temtronic · Posted: Fri Jul 13, 2012 12:56 pm

one option...

You could use the 3.5795 xtal as the 'master' clock for the PIC and use the PLL ( if the PIC has it) to internally run the PIC faster.

Since I don't know what else the PIC has to do it is a viable option. I use 4MHz xtals all the time on 4550 projects...
...and my trusty Model III runs at 2Mhz!
It all depends on the scope of the project.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny

PCM and Temtronic are steering you in a good direction.

If you use an external 3.3579545 mhz xtal and the PLL in the 18F family PIC
it will be the osc source for the DTMF decoder -

You have a #use delay of (14318181)
that results in low enough RS232 clock errors with BRG16 asserted.

Then you just feed the osc out pin of your pic to the osc IN pin of the DTMF detector chip. all in all - not so bad a way to handle it.

As to the timer periods that this provides - it is less sweet, in that when divided down from Fosc - you are stuck with (multiples &&|| fractions) of appx 7millisecs.