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From: "Frank Bemelman"
Subject: Re: Questions regarding the PIC16C77X and its built in ADC
Date: Sun, 22 Sep 2002 11:50:51 +0200
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Organization: EuroNet Internet
NNTP-Posting-Date: 22 Sep 2002 09:53:18 GMT
schreef in bericht news:firstname.lastname@example.org...
> I have a few questions in regards to ADC capable PICs (in regards to the
> project mentioned below). Unfortunately, my knowledge is lacking (although
> am trying to correct that), so bare with me.
> What I would like to do is sample the signal from a telephone, using the
> built in ADC in a Microchip PIC. Since human speech usually is below 4000
> hz, the minimum sample rate I would need is 8,000 samples per second.
> the signal itself's highest frequency should be around 4000 hz, a 12 bit
> resolution (4096 possible values) should work. Is this correct?
> I have been looking at the PIC16C77X MCUs.. I was also looking at the PDF
> datasheet at:
> So here are some more somewhat basic questions:
> The pinout diagram shows pins 11 as AVdd, pin 12 as AVss, pin 4 as VRef-
> pin 5 as VRef+. These are all of the pins specifically for the ADC part of
> things, correct? I wish to have a telephone connected via a standard
> telephone cord to an RJ11 jack on a standalone unit (with among other
> the PIC in it). A 9V power supply will be used to power the phone line.
> Since 9v is way too much to connect to the AD on the PIC (correct?), a
> resistor will need to be used to get it below the 4.096V level (which is
> max voltage level for AVDD,AVSS right?).
> So, basically I was thinking the negative terminal on the battery can be
> connected to the RING pin on the RJ11 jack, and the TIP pin on the jack
> be connected to the Avdd pin on the PIC, and have the AVss pin connected
> the positive terminal on the battery... I would need a resistor inbetween
> the RJ11 TIP pin and the AVDD pin (correct?)..
> I have the bad feeling that I have something (lots of things?) backward
> however (hence the reason for this post).. I also don't know what to use
> VREF + or VREF -...
> If someone would care to point out the mistakes I made above, point me in
> the right direction etc.. that would be great!
The sampling rate of 8KHz looks allright. You don't need 12 bit conversion,
8 bits is more than enough. Use the 8 MSB of the ADC.
What are you going to do with all those samples? The PIC does not have much
ram to store them. The only thing you can do (IMO) is measuring the time for
each period of the input signal.
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