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All things pertaining to wireless and RF links
By geometrixeng
Hey Everybody,

I am currently using the MG2639 for a project. I want to use this to transmit SMS text messages ONLY. No receiving of text messages, no phone calls, etc.

I am using a PIC24F host microcontroller. I am using 3 out of the 4 available UARTS on the PIC24F and the other 2 peripherals (4D Systems uLCD-43PT and Sparkfun SEN-11096 magnetic card reader)seem to work great, so I am extremely confident that my UART TX and RX code work just fine.

I have a few questions regarding the MG2639 Cellular Shield:

1. POWER BUTTON (PIN 7 on cellular shield) - When I power up my the MG2639, the red LED turns on. But this does not necessarily mean that the GSM module is active...or does it? When I press the BOOT button for 2s-5s, I get 3 Hz blinks of the green LED. When I press the BOOT button again for 2s-5s, I get no green blinks and the red LED is still on. I suppose that I have turned off the GSM module while the overall cellular shield still has power. So I control PIN 7 of the cellular shield with the PIC24F. I have a few lines of code in that initialize the MG2639. The code latches PIN 7 HIGH and then unlatches the pin LOW to turn the GSM module on/off. I see the green LED blink 3 Hz. But every other time that I compile and run my code, it seems to toggle the GSM module from OFF to ON and then from ON to OFF. How do I test whether or not the device is on or off in the first place????? When my project is complete, It will be powered up OR powered down. During development however, the power supply is always on during compilation and running so it seems to toggle. And when the MG2639 is idle, I get no blink at all...should be 1 Hz.

2. LEVEL SHIFTER (VDDIO 5V default) - I believe that the purpose of this is to accommodate either 5V microcontrollers OR 3.3V microcontrollers. Please confirm. I believe that the TX and RX pins on my PIC24F(3.3V) host are 5V tolerant. So it may not matter what I do with the jumper on the cellular shield. Please confirm.

3. VBAT - I am powering the device from a wall wart and I have connected the wall wart to a 5V voltage regulator (LM7805) to provide power to VIN on the cellular shield. Do I need to cut or short any of the VBAT jumpers (SJ2 and/or SJ3)???

4. When I transmit any AT command, shouldn't I receive an "OK" reply from the MG2639 cellular shield. Well I am NOT receiving "OK" I sent "AT+CMGF=1\r" to the cellular shield. I have both TX and RX pins on my PIC24F connected to a scope and to a signal analyzer. I have confirmed that I am sending exactly "AT+CMGF=1\r". But I see no "OK" reply on the RX pin of my PIC24F host.