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SIM800L using TM4C123GXL and Energia

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Hello everyone! I am using the SIM800L module to send SMS through a TivaC TM4C123GXL board, but its clock is 80MHz, and the library only accepts 20, 16 or 8 MHz. In Arduino it worked perfectly, this code found on a website. How do I reduce the TM4C123 clock in Power software? Is there another solution? Thanks in advance!

The code:

#include <SoftwareSerial.h>
SoftwareSerial sim(3, 4);
int _timeout;
String _buffer;
String number = ""; //here I inserted my number phone
void setup() {
  delay(4000); //delay for 7 seconds to make sure the modules get the signal
  Serial.println("Sistem Started...");
void loop() {
void SendMessage()
  //Serial.println ("Sending Message");
  sim.println("AT+CMGF=1");    //Sets the GSM Module in Text Mode
  //Serial.println ("Set SMS Number");
  sim.println("AT+CMGS=\"" + number + "\"\r"); //Mobile phone number to send message
  String SMS = "Testing GSM module";
  sim.println((char)26);// ASCII code of CTRL+Z
  _buffer = _readSerial();
String _readSerial() {
  _timeout = 0;
  while  (!sim.available() && _timeout < 12000  )
  if (sim.available()) {
    return sim.readString();

The error: 

 #error This version of SoftwareSerial supports only 20, 16 and 8MHz processors 

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Thank you, Fmilburn! I was wrong when I was trying to use the library SoftwareSerial.h. Now just using Serial3.begin and etc and looking the arm's pin map, the program is working. Thank you one more time! 

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