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i need to get a c code for interfacing the microcontroller to a GSM Modem only to send sms, i have read all the AT COMMANDS of the sms and the data sheet shows me how to use the modem through the hyper terminal of the windows but it doesn't show how to interface with a micro ? can any one describe what are the steps of this ? and i need the hardware interfacing scheme to do this ? if any one have a usefull link please co-operate . thank you
There are a number of follow-up chips based on the legendary MAX232 chip. Some of them has better voltage range. Some have better ESD protection. Some of them may work with smaller capacitors.
One of the similars might be the MAX3232, which supports 3-5.5V.
The important thing to remember is that electrically, the uC doesn't care what external equipment that is connected on a serial port. If you find a schematics for a physical serial port with an LPC2119 (or a number of other chips in the LPC series), you can extrapolate the information.
The things you must know is if the modem is using TTL levels (and in such case which voltage) or true RS232 levels, and which signals are RX, TX, GND, ... If the modem supports external power, it helps to check if you can use a common power source for ARM and modem.
Since you know how to connect the modem to a PC, my assumption is that the modem is using true RS232 signal levels. Hence my reference to the need of a MAX232 or similar to adapt between RS232 levsls and the voltage levels of the ARM chip.
"The things you must know is if the modem is using TTL levels (and in such case which voltage)"
Note that many GSM modules (as opposed to true modems) do not use standard logic voltage levels; ie, not 3.3V and not 5V - so be sure to read the Datasheet very carefully!
Again, this has been discussed recently in other threads here - use the 'Search'