some weird effect of Telephone DTMF MT8870CE !!!

My project (DTMF Teleremote Control) spinning around a lot of strange problems. Every now and then it occurs and after some thought
if cannot figure out, I go for Forum Thread posting. I optimize no one get bitterer by checking indistinguishable subject.
But I anticipate this would be an open place to clear any doubts (basic to advanced) of a common man around the world.
If someone getting bored, please refuse it as an unrequited topic.But I think every new post would contain some challenging bottlenecks.
Entirely different issues on same project, so I use different threads.

Now come to the point,
Look up the attachment ( many members may easily recognized it as of previous threads ).

When power up the circuit all LED s 1 to 4 turned-on (I expect all turn-off).
The problem is that, When I calling from remote end all the LED s blinking randomly for 1 sec (undesired result) and then
comes to a stable state.

But this blinking effect doesn't exist when I manually turn-on the circuit after passing first ring.
Now all LED s directly comes to the stable state without blinking as narrated below in the table.

The surprise attack is that depending upon the service provider of telephoning remote person the
stable state varies as indicated below:

OPERATOR(India) LED4 LED3 LED2 LED1

BSNL OFF ON OFF OFF

VODAFONE OFF ON OFF ON

IDEA ON OFF ON OFF

RELIANCE ON OFF OFF ON

AIRTEL OFF OFF ON OFF

Local land-line end working on BSNL line.

I doesn't believe in the supernatural effects, but there should be an evil influence.

I also tried the TOE in of DTMF IC, but after enabling DTMF Outs (after passing first ring) the
first state would be the stable state described above without blinking problems. PWDN pin also not working.

At a time subsequent to this stable state, DTMF IC obeys the normal truth table specified in the MITEL data sheet
upon different key events on remote end after off-hooking local end.

This is the third IC I have replaced, first two ones- MT8870CE,35P050C9,9004 AE
didn't work at all. Third one- MT8870CE,12579.2,9125 AE- have the above issues. Next time I will choose KT3170 !!!

Without solving these issues I can implement my project by,
1. Designing a ring counter which turn on the circuit only after passing first ring, thus no blinking problems at all.
2. Designing a tri-state buffer with decoder at output of DTMF IC. Thus #<decimal> key combination can be designed.
Circuit designed to turn on trisate for 5 sec when # key presses. During this interval I can press a decimal key,
which is assigned to a particular house-hold device. Thus turn on-off corresponding devices. One of 16 outputs of
Decoder IC will turn on at a time depends on DTMF BCD outs.

First time I didn't notice this stable state changes (as indicated in previous threads) between different operators as
in my workbench I have been using my personnel mobile phone as remote end and land line in my home as local end.
When I switched to my friends' mobile having different connection I noticed this change.

 

The 8870 has no power on reset. If you connect pin 10 to +5v, to enable the output latches, then they will be in an unknown state until the device has received a valid DTMF code pair.

The easy way to prevent this is to create a power on reset (POR), so that the latches are not enabled until a valid code is received.

Attached sketch will do what you need

 

Dear Rogs,
My predictions are as follows,
1. You use 4093 Schmitt NAND to resist noise effects
2. 4093 is configured as a latch circuit.
See the new attachment with S,R, & Q are marked.
3. Upon power up R=S=1, Q=0. 8870 outputs not enabled.i.e; all LED s OFF.
4. When key press occurs pin 15 turns on for time determined by the time
for which key press lasts.S=0,R=1,Q=1.8870 outs get enabled. But this condition exists only
for small time determined by physical key press duration.
That is after this short time 8870 outs goes into all OFF state.
We cannot use latching facility of DTMF IC.
( Actually in my application one-time-on facility is enough )
5. 100K & IO uF act as power on reset circuit (POR).
6. On detecting very first ring all LED s, including 15 pin LED,turn on
-off randomly for 1 sec. So there may be possibility for S=0,R=1,Q=1.
8870 outs get enabled on ring detection without detecting any DTMF key events.

 

1 -Yes
2- Yes
3 - Yes, except R starts low, and goes high after r/c time constant.
4 -No - what you say is correct except that Q stays high on key release. Q does not go low on key release, only on new 'Power On Reset' -when unit is powered down, and then up again.
5 - Yes
6 - That shouldn't happen. The 8870 should not be in circuit until after line is looped, and ringer stopped.
If you overdrive the 8870 input with ringing current, then you might get false decoding , and you will set the S/R POR, and activate the output latches.
Avoid this condition. Only put the 8870 in circuit after ringing current is disabled.

 

Initial 1 sec blinking is due to caller id signaling.
Exactly, in India CLIP signals are in DTMF format and occurs between
first and second rings for 1 sec duration.

 

???????????????? ----- no, sorry, you've lost me there!

Glad I was able to offer some suggestions to the parts of your project I understood!

I'm sure your idea to post this project in many different threads, to every electronics forum you can find, will eventually help you to sort it all out!

10 out of 10 for persistence!!

 

Dear Rogs,
Are you bored with my continuous doubts (many members getting bored with continuous dealing on same topic) ?

See the again modified attachment. The latch can be treated as Active-LOW input S/-R/ latch.
Its truth table is also given on the diagram.

Assume unit is powered down, and then up again. At this small instant only we want TOE input to be zero.
During this interval capacitor is charging but not fully charged (R/=0). So the condition can be considered as
S/=1,R/=0 thus according to truth table Q=0, disabling 8870 outs on power up instant.

After this small instant (based on RC time constant), capacitor get fully charged R/=1.
S/=1,R/=1 thus no change in previous state. That is outputs disabled.

When any key press event occurs, S/=0,R/=1,Q=1 Outputs get enabled.
When the remote person remove his finger from key press, S/ turns to 1. S/=1,R/=1 thus no change for
previous state. That is outputs get still enabled.(This is the latching result I wanted).

Capacitor once charged, only discharge on power-off of entire circuit. So S/=0,R/=0 inhibit mode never occurs.
Only first 3 states of truth table are used by us.

Thank you for familiarizing me with this valuable circuit.

 

Avoid this condition. Only put the 8870 in circuit after ringing current is disabled.[/QUOTE]

Can I use pin 6 (PWDN) of 8870 to do this work.
It is an active HIGH pin. So I will connect it to active HIGH to turn-off 8870 until off-hook or until then passing first two rings (which contains CLIP signals causing initial blinking problem) and return back to LOW to power up 8870 after occurring first two rings or after off-hook.

 

6 - That shouldn't happen. The 8870 should not be in circuit until after line is looped, and ringer stopped.
If you overdrive the 8870 input with ringing current, then you might get false decoding , and you will set the S/R POR, and activate the output latches.
Avoid this condition. Only put the 8870 in circuit after ringing current is disabled.[/QUOTE]

Dear Rogs,

Can I use pin 6 (PWDN) of 8870 to do this work ?
It is an active HIGH pin. So I will connect it to active HIGH to turn-off 8870
until off-hook or until then passing first two rings (which contains CLIP signals causing initial blinking problem)
and return back to LOW to power up 8870 after occurring first two rings or after off-hook.
By this technique, no need for separating both ring detector and DTMF IC.

 

I've not tried the power down feature on the 8870, but what you say sounds as if it would work.

I don't understand why you don't just delay the line loop relay until after 3 or 4 rings?

In you original schematic, the 8870 was the other side of the transformer, and therefore not connected, until after the line was looped. I should just stick with that idea.

 

I think your country is 25 years behind our countries.
We don't use DTMF for caller ID, we use digital.