Hello Folks ..yup its me Back Again...

Hmmm...the other way is simply you can write a ASM code which CPL a bits 1/400 = time and you will get a square wave on that bit which will be same like DIAL TONE..

OKies...i think its a case of PABX or something related... well buddy the simple way to generate the DIAL TONE..., is using PHASE SHIFT TYPE OSCILLATOR ..so go through your basic electronics and you will find that its much much easier to generate dial tone using this theory also read SALLY KEY THEOREM...

Also remember my friend that if you are doing all this for PABX projects its better to make a separate circuit for DIAL TONE since your controller will be busy doing other things... using the DIAL TONE you can simply genarate the Busy and engage tone ..connecting and disconnected the switch you are using.

Now after all this you will be saying well this isn't helping me... so here is the datasheet of a IC which you can use to genarate DIAL tone ...

www.fairchildsemi.com/ds/KA/KA1458.pdf

hope this helps