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nirbar11

UART interface to sim900

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Hello
Im trying to send sms message by sending AT commands from MSP40F5529 to SIM900 (by Simcom).
physical connections: Rx(P3_4) to Tx, Tx(P3_3) to Rx, GND to GND.
I scoped the Rx and Tx pins and they both have constant voltage value of 2.7 (logical 1) 
When I plug the micro-usb cable and turn the serial monitor on ,I can see the commands and messages well.

this is my code (i print int 55 in order to see some changes in the uart pins value ,which didnt happend) :

void setup(){
Serial.begin(9600);
}


void sendsms(){
Serial.println("AT\r");
delay(1000);
Serial.println("AT+CMGF = 1\r");
delay(1000);
Serial.println("AT+CMGS =\"+972544844095\"\r");
delay(1000);
Serial.println("HELLO WORLD");
delay(1000);
Serial.println((char)26);
delay(100);
}


void loop() {
  Serial.println(int(55));
  //delay(200);
  sendsms(); 
}

questions:
1. do i need to configure my UART pins in setup() or its done automatically when I use Serial? because I didn't declare anything
2. When I plug the usb cable - the serial reading and writing from the UART pins are stopped? because i scoped both cases (with and without usb cable) and the results were different
3. Why is the serial monitor shows all fine and when i scope the signals , the voltage value does not change?

 

Looking forward for help.
Thank you
Nir

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