Hi all,
I really worked hard for last few months in order to clear my concepts about programming with controller and asm.
really interrupts make life easy and they are really easy to use....

I have made few circuits like 7-segment clock , running LEDS with 1 minute intervals etc...

Now i really did lots of searching and when i did not get any thing from any book or any thing esle i thought let go to the best forum on the internet for getting help on programming...

I hope you people are not agree at me for what i did in the past.

Here is what i want to do:-

Iam planning to design the most economical 8 Lines Telephone Exchange. Here the thing to note is that iam not planning to go for a PBX , i want to design a simple 8 lines Exchange not involving any external telephone service line.

Suppose you have 8 Tone Dialing Telephone Sets.

Each Telephone Set is connected to a central exchange using single pair cable.

Each Telephone user can communicate with other by dialing the desired extension number. from 10 to
18. Two digit dialing.

Now for this i have selected following basic components:-

1) Microcontroller At89c52 or 8051.

2) Using Assembly Language asm.

3) 24 Volts Dc , 5 volts Dc and 60 Volts Ac supply is used.

4) Analog Switch MT8816 or CMOS4066 which ever i find easy and economical to use.

5) Most Important is the SLIC subcriber Line Interface circuit. For this :-

( Each SLIC is for Four Telephone Users )

a) One LM3900N,
b) Four A673 Transistors
c) Four C828 Transistors
4) Four 5 Volts Relays.

I want to design this in such a way that the same circuit theory can be implemented for more than 8 lines.

In order to practical start the project i want to do an experiment using two telephone sets only.

Telephone No.1 ( Dialing No.10 )
Telephone No.2 ( Dialing No.20 )


When ever any user lets suppose User of Telephone No.1 goes off-hook, at this stage controller do the following:-

1) Firstly sents a Dial tone for 20 seconds and if the user did not dial any thing upto 20 seconds then controller sents a Busy Tone.

2) if the user dial digit "20" then the controller :-

a) sents a Ring Back tone to the user of
telephone no.1 ,
b) Sents a Signal to Ac 60 Supply so that ring can occur on telephone no.2.
c) When the user of telephone no.2 picks up the handset then the controller sents the signal to analog switch top create communication between the two telephone sets.

similarly the same function happens for telephone no.2


Now i have gathered lots of information in past 1 year in how to design such systems but practical iam standing at zero level and really need some help from you...i have found websites which contain some information but all have incomplete information:-


http://wandel.ca/homepage/pbx.html


My Main Problem is :-

1) I want to micrcontroller to count the incoming signals from Tone Dialing Telephone Sets , compare the frequency generated by the digits pressed and if the dialed number is equal to number stored in the controller's memory than perform the desired function...can controller be use to do this... as in case of DTMF when a digit is pressed it causes electrical sum of frequencies of high and low band, how i can make controller to do the JOb of DTMF decorder IC...i have seen few local exchange in which telephone of DTMF are connected but these exchange don't have DTMF Receiver or any thing like this...any idea how controller can do this ?

2) Next how i can make controller to generate different TONES like :-

a) DIAL TONE which requires 350+440 Hz continuous on time frequenct upto 20 seconds....we can go for some rounf figure like 440 Hz..but how to program the controller using delay interrupts or any idea?

b) similary in case of Busy Tone 480+620Hz 0.5 seconds on and 0.5 seconds off

c) Ring Back Tone involves 440 Hz + 480 Hz 2 second ON and 2 Seconds OFF frequency..

we can use 440 Hz frequency and generates a square waves from controller and involve ON and OFF conditions for ring back and busy tones...

there are call progress IC like M 991 but i don't want to use these , i want to make use of controller programming?

i have seen the frequency counter code in the library but it does not fit in my requirement.

Please help me out how to get going with this project.


Thanks