OFF and ON hook detection for PIC18F

Thread Starter


Joined Oct 31, 2008
Hi all,
I'm new to this kind of stuff. Could you please help me with this?
I'm trying to establish the signaling connection between 2 simple phones which connect to my PIC18F4550.
First, if the phone is OFF hook or ON hook, what does the PIC read from the portA0 (i.e. assume that the phone will have 9V power supply already) so that when the other end phone will hear a busy tone when it wants to reach me?
Second, when I press a number to call the other end phone, how does the PIC understand these numbers?
If someone has experience on this, please give me some help. I'm really appreciate it. Thank you all!


Joined Apr 20, 2004
You might want to search Wikipedia for your information. Also about DTMF signalling and detection (hint: you get a chipset to do the tones instead of using your overworked uC).

Here is a link to a possibly interesting circuit - Telephone Receiver.

With no knowledge of what may be attached to or how, it's very difficult to have any idea of what state port A0 might be in.

Just another idea - there is a very mature technology that allows computers to communicate over phone lines. Google "modem".