Hello all,

I have Siemens M35 mobile phone connected directly to the RX and TX pins of an Atmel T89C51rd2. I am using the phone as a GSM modem. If I power up my system without the phone connected and then connect it everything works prefectly, I can communicate with the phone no problem (i.e. send SMS messages and receive responses), and all other pripherals work as they're supposed to.

However if I have the phone connected when I power up strange things start happening. The first thing my code does is initialise an LCD display and write a message to the display. About half the time the display isn't initialised. The rest of the time the display is initialised and the main loop of the program is executed properly but I am unable to communicate with the phone. The serial port and the other interrupts being used are all correctly initialised before the main loop is entered and the int0 and int1 interrupts work correctly.

When I connect an oscilloscope up to the TX pin of the phone I see alot of noise but I thought this wouldn't make a difference before the serial interrupt are enabled (I deal with it in my serial interrupt service routine).

Anybody have any idea what could be causing this to happen because I'm at a loss to explain it?

Ray

P.s. if schematics and code is needed I can post these up but I'm hoping there is something simple that I'm just missing.