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'[EE]: DTMF Tx/Rx'
2001\02\25@103608 by pang khong lin

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Hi,

1. Have anyone had any experience in using this DTMF transciever
  MC145740 ?
It just does not responds to any DTMF.., i guess it's the transformer side
that i screwed up, but basically i follow exactly what is stated in the
datasheet. In addition i use a Gyrator circuit to create the off hook
condition. The whole thing shouldn't be that difficult since i am only
testing on receiving. Wonder what's wrong? Even if the impedance does not
match, the only effect would be the strength of the signal. By putting a
large resistor at the transformer secondary should do the job right?

2. What is the web address that David  mention for HT9200?

Thanks.
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2001\02\25@165155 by Chris Carr

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Question
How are you testing this?
Without that vital piece of information any reply you get is total guesswork
and not a reasoned response. There are too many variables.

Regards
Chris

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2001\02\25@165818 by David VanHorn

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>
> > 1. Have anyone had any experience in using this DTMF transciever
> >    MC145740 ?
> > It just does not responds to any DTMF.., i guess it's the transformer side
> > that i screwed up, but basically i follow exactly what is stated in the
> > datasheet. In addition i use a Gyrator circuit to create the off hook
> > condition. The whole thing shouldn't be that difficult since i am only
> > testing on receiving. Wonder what's wrong? Even if the impedance does not
> > match, the only effect would be the strength of the signal. By putting a
> > large resistor at the transformer secondary should do the job right?

You need enough signal, in amplitude and time.
Amplitude can be too hot, but they will take a lot of loss.

You also need not too much "twist", which could be aggravated by an
impedance mismatch.


> > 2. What is the web address that David  mention for HT9200?

http://www.dvanhorn.org


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2001\02\26@095824 by pang khong lin

picon face
I managed to grab an oscilloscope and these are what i found : -

1. The output from the transformer secondary produce the correct DTMF
signal. So my transformer is fine.
2. The DTMF transciever has an additional output pin whereby any DTMF
received ( through the receiver input pin )are amplified accordingly and
output through this pin. This output pin is meant to be used by other
devices.
I tap the signal on this pin just to determine if the DTMF goes through the
receiver pin, got amplified and output through this pin.
It does. The output signal is much cleaner and stronger.

So in your opinion what could be the problem here?
FIY, this is a tranciever IC, so it needs to be configured. Configuration is
done using four pins,
1. ENABLE
2. CLOCK
3. DATA
4. R/W
I use a PIC16F84 to configure it. I believe the fault should not be in wrong
configuration. because it is quite straight forward. Configuration data is
clk in serially.

Is there any way to know if the signal is too weak? What is meant by
'twist'?
The signal at the transformer secondary has an amplitude of around 40 mV
peak. Is this considered fine?

Thanks.

>From:  Chris Carr <.....nyedKILLspamspam.....BTINTERNET.COM>
>Subject: Re: [EE]: DTMF Tx/Rx
>Date:    Sun, 25 Feb 2001 21:50:07 -0000

>Question
>How are you testing this?
>Without that vital piece of information any reply you get is total
> >guesswork and not a reasoned response. There are too many variables.

Regards
Chris

{Quote hidden}

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2001\02\26@102303 by Dan Michaels
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pang khong lin wrote:

>
>Is there any way to know if the signal is too weak? What is meant by
>'twist'?
>The signal at the transformer secondary has an amplitude of around 40 mV
>peak. Is this considered fine?


Your signal level sounds like it is on the "low" side - probably
near the minimum allowable for the chip. Possibly due to an impedance
mismatch.

Twist refers to the fact that the row and column DTMF tones have
slightly different amplitudes.

You might do well to read up some more on background information.
I have a several good links to this. Tomi Engdahl has a lot of
circuit info. The cisco.com article is quite helpful:

http://www.oricomtech.com/teklink2.htm#Tele2

- dan michaels

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2001\02\26@110930 by David VanHorn

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>
>Is there any way to know if the signal is too weak? What is meant by
>'twist'?

Relative amplitude between the low and high tones.
You need a spectrum analyzer to see that.



>The signal at the transformer secondary has an amplitude of around 40 mV
>peak. Is this considered fine?

Should be ok, you need to check the chip spec though.

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2001\02\26@135910 by Chris Carr

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OK lets take a quick look at this with some rough calculations.
The Datasheet says that the input level is 0dBm max, -48dBm min.
Assuming that this power level is into a 600 ohm resistance then that
equates to 0.775 volts and 0.0038 volts minimum (actually a bit less but I'm
too lazy to get out a calculator) so your input level is low but well in
limits.

"twist" is the difference in level between the two tones you are receiving.

You say you are getting a good signal out of the AGC out (pin 4). have you
looked at the filter test pins with you scope (pins 6 & 7). If you are
seeing signals there and they are roughly of the same amplitude then your
problem must lie in the configuration.

I assume that you have checked to see if you are controlling the chip, the
easiest way would appear to be to switch it to transmit mode and change the
transmit tones. If you can do that then check you are setting up the
receiver correctly according to tables 1,2 & 3.
Try putting it into Analogue Loopback Mode Does DV (pin 12) go low. Are you
sequencing the R/not W (pin 14) and EN (pin 16) correctly?

Regards
Chris
> I managed to grab an oscilloscope and these are what i found : -
>
> 1. The output from the transformer secondary produce the correct DTMF
> signal. So my transformer is fine.
> 2. The DTMF transciever has an additional output pin whereby any DTMF
> received ( through the receiver input pin )are amplified accordingly and
> output through this pin. This output pin is meant to be used by other
> devices.
> I tap the signal on this pin just to determine if the DTMF goes through
the
> receiver pin, got amplified and output through this pin.
> It does. The output signal is much cleaner and stronger.
>
> So in your opinion what could be the problem here?
> FIY, this is a tranciever IC, so it needs to be configured. Configuration
is
> done using four pins,
> 1. ENABLE
> 2. CLOCK
> 3. DATA
> 4. R/W
> I use a PIC16F84 to configure it. I believe the fault should not be in
wrong
{Quote hidden}

side
{Quote hidden}

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2001\02\27@204319 by pang khong lin

picon face
Hi,

Thanks for all the PICListers help, especially Chris. The circuit now
detects DTMF signal and will output a Low at both the DATA VALID and
TONE DETECT.

Below is just some observation -
During the trouble shooting period I have performed a loopback test
as suggested by Chris and what i get is an occasional short high
pulse at the Data Valid pin. My code is very direct -

In analog Loopback mode
=======================
Loop until forever
       Configure to transmit Tone 1
       Delay 2 sec
       Configure to transmit Tone 2
End loop

I connect to the phone line and try to listen through another phone.
I seems that the moment the MC145740 is configured to transmit tone 1,
then it will not stop doing so. I thought there should be some internal
clock to determine how long a tone is transmitted. The short high pulse at
Data Valid which i observed happens at the transition
of Tone 1 to the next Tone. I guess to create a delay between tone,
one need to configure for Receiving just to tell it to stop transmitting.

Finally, the error which i think is causing the problem is the
pin Vref ( Reference Analog Ground ). The application shows that
this pin is connected to a symbol that resembles a ground. But it is
name as Analog Ground. This pin should not be connected to system ground and
should be decoupled according to the spec.

Once again thanks everyone.



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