Serial.print start prints unreadable character without any reason

Question

I wrote the following code. The idea is to execute some AT command on my ESP-01 module. The problem is that first command is execute without problem, the second sometimes but the reply on AT+GMR is the following: AT

OK
AT+GMR

AT version:1.1/0.0(May 11 3016 18:09:57)
SDK version:1.5.ar.0
AR)�WS�TZ�j
C�j�L�AZCA�O�C�

FINE

The problem is that, after some thing that I don't understand, the formatting (maybe the baud change?) gone wrong. Then the reply of the following commands became unreadable. Any idea why it happens?

#include <SoftwareSerial.h>
#define DEBUG true
SoftwareSerial wifi(2,3);


void setup() {
   Serial.begin(115200);
   wifi.begin(115200);
   sendCommand("AT\r\n", 2000);
   sendCommand("AT+GMR\r\n",2000);
   sendCommand("AT+CWMODE=2\r\n", 2000);
   sendCommand("AT\r\n", 2000);
   Serial.print("\n\nFINE");
}

void loop() {}

String sendCommand(String cmd, int timeout){
   wifi.print(cmd);
   return readSerial(timeout);
 }

 String readSerial(int timeout){
   long int time = millis();
   String res = "";
   while( (time+timeout) > millis() ){
        while(wifi.available()){
          char c = wifi.read();
           res += c;
        }  
    }
    if(DEBUG)
       Serial.print(res);

    return res;
 }

Answer 1

The problem was about high baud for SoftwareSerial. Changing it to 9600 the problem disappears.

Answer 2

Take a look at the connections check the all the connections properly i also had the same problem