I Design A Circuit With 8051 And MT8888.This Circuit Is Working With This Way That Write Below. At First I Initialize MT8888 Upon Its Datasheet With 8051 And Then Initialize It For The State That Wait For A Ringing Of Telephone ,I work with INT0 Of 8051 And Connect Interrupt Pin Of MT8888 To INT0 Pin Of 8051. There Is An Interrupt Routine In 8051 That Works In 2 Way.When Call Progeress Or DTMF Code Has Recieved This Routine Was Invoked.When Interrupt Was Occure This Routine Automatically Invoked, When Telephone Rings,Routine Interrupt That Was Available In 8051 Was Call And After This, Routine Was Ready To Take DTMF Code From Phone Line And Save Them To A Variable.When The Phone Is Ringing The Buzzer That Available In The Circuit Turn On And After A Pause It Turn Off Via The Interrupt Routine That Was Available In 8051, And After 3 Rings Of Telephone 8051 Turn On A RELAY To Take The Phone Line To The OFF-HOOK State .After This Event MT8888 Initialize For Detect DTMF Code That Was Sent To This Circuit From Phone Line.I Also Test This Section Of The Circuit And I Found That Is Work True. But My Problem Was Occure When I Use Serial Transmission To The RS232 Of My PC. In Serial Transmission I Must Connect GND Of My RS232 Port To The GND Of My Circuit ,In This Time My Problem Was Occure And After This Connection When I Turn On My Circuit The Interrupt Routine Automatically Was Invoked And Buzzer Start To Sound With Out Any Call Progress.I Check Interrupt Pin Of MT8888 With One Osiloscope And I Found That MT88888 Cause This Interrupt And Upon To This Signal Of Interrupt ,8051 Invoked The Interrupt Routine And The Buzzer Start To Sound And After A Little Time Relay Is Turn On And Take The Phone To The OFF-HOOK State With Out Any Ringing Progress.I Must Have Serial Transmission In My Circuit. I Try To Reduce The Noise In My Circuit And Use Polygone And Bold Track And Some Capacitives Between VCC And GND In The PCB For Reducing Noise . I Attach the Schematic And Source Code Of My Circuit To This Mail. I Use MicroVision Compiler For Source Code.
Please Answer Me.
By Regards.