UART bluetooth communication problem What is the proper format to send data to the UART (integer values) - bluetooth

Question

I have created my functions to send and receive from the UART, and sending the data does not seem to be a problem. In the data visualizer we can see the values and even plot them.
However when sending these data through the bluetooth, we cannot get the values to plot them in any of many available apps.
I believe there is a problem with the way we are sending data through the UART and to the bluetooth and that is why we cannot then get the values to be plotted.
Being a starter at all this, I would like to someone please advice us if the code below is ok, if there is a mistake and if there is a better way to send the data through the UART so as to make the Bluetooth work properly. Target is to be able to plot (graph) the values on the phone.
Many thanks
#define F_CPU 16000000UL
#include <avr/io.h>
#include <util/delay.h>
#include <avr/interrupt.h>
#include <stdio.h>
#define BAUDRATE 9600
#define BAUD_PRESCALLER (((F_CPU / (BAUDRATE * 16UL))) - 1)
//----------VARIABLES
float V_n,V_nm1,V_measure=0;
volatile int Velo_pulse;
float Exp_fltr_Coeff=0.2;
unsigned int Counter_ADC=0b0001;
unsigned int Value1;
char String[]="";
//----------Functions Definition
//---timers
void Timer1_Control();
void AttachInterrupt();
//---AnalogueRead
void Set_Ports();
void AnalogRead_Setup();
unsigned int AnalogRead();
//---UART
void USART_init(void);
unsigned char USART_receive(void);
void USART_send( unsigned char data);
void USART_putstring(char* StringPtr, unsigned int Value1);
int main(void){
USART_init(); //Call the USART initialization code
Set_Ports();
AnalogRead_Setup();
AttachInterrupt();
Timer1_Control();
sei();
while(1){
_delay_ms(1);
}
return 0;
}
void USART_init(void){
UBRR0H = (unsigned char)(BAUD_PRESCALLER>>8); //UBRR0H = (uint8_t)(BAUD_PRESCALLER>>8);
UBRR0L = (unsigned char)(BAUD_PRESCALLER);
UCSR0B = (1<<RXEN0)|(1<<TXEN0); //Enable receiver / transmitter
UCSR0C = (1<<USBS0)|(3<<UCSZ00); //Set frame format: 8data, 2stop bit
}
unsigned char USART_receive(void){
while(!(UCSR0A & (1<<RXC0))); //Wait for data to be received (buffer RXCn in the UCSRnA register)
return UDR0;
}
void USART_send( unsigned char data){
while(!(UCSR0A & (1<<UDRE0))); //Waiting for empty transmit buffer (buffer UDREn in the UCSRnA register)
UDR0 = data; //Loading Data on the transmit buffer
}
void USART_putstring(char* String, unsigned int Value1){
sprintf(String,"%d\r\n",Value1);
while(*String != 0x00){
USART_send(*String);
String++;}
}
void Set_Ports()
{
DDRD = 0b11111111; //All port is output
DDRD ^= (1 << DDD5); // PD5 is now input
}
ISR(ADC_vect)
{
//ADMUX ^= Counter_ADC; //Swapping between ADC0 an ADC1
}
void AnalogRead_Setup()
{
ADCSRA |= (1 << ADPS2) | (0 << ADPS1) | (0 << ADPS0); // Set ADC prescaler to 16 - 1 MHz sample rate # 16MHz
ADMUX |= (1 << REFS0); // Set ADC reference to AVCC
ADMUX |= (1 << ADLAR); // Left adjust ADC result to allow easy 8 bit reading
ADCSRA |= (1 << ADATE); // Set ADC to Free-Running Mode
ADCSRA |= (1 << ADIE); // Interrupt in Conversion Complete
ADCSRA |= (1 << ADEN); // Enable ADC
}
unsigned int AnalogRead(unsigned int PortVal)
{
if (PortVal==5){
ADMUX |= (0 << MUX3) | (1 << MUX2) | (0 << MUX1) | (1 << MUX0); //sets the pin 0101 sets pin5
} else if (PortVal==4){
ADMUX |= (0 << MUX3) | (1 << MUX2) | (0 << MUX1) | (0 << MUX0); //sets the pin 0101 sets pin4
}
ADCSRA |= (1 << ADSC); // Start A2D Conversions
//while(ADCSRA & (1 << ADSC));
return ADCH;
}
//----------Timer Functions
ISR (TIMER1_COMPA_vect) // Timer1 ISR (compare A vector - Compare Interrupt Mode)
{
cli();
V_measure=(Velo_pulse*60/0.250);
//USART_putstring(String,Velo_pulse);
Velo_pulse=0;
V_n=Exp_fltr_Coeff*V_measure+(1-Exp_fltr_Coeff)*V_nm1;
V_nm1=V_n;
USART_putstring(String,(int)V_n);
sei();
}
ISR (INT0_vect)
{
Velo_pulse++;
//USART_putstring(String,Velo_pulse);
}
void Timer1_Control()
{
TCCR1A=0b00000000; //Clear the timer1 registers
TCCR1B=0b00000000;
TCNT1=0b00000000;
TCCR1B=0b00001101; //Sets prescaler (1024) & Compare mode
OCR1A=2604; // 160ms - 6 Hz
TIMSK1=0b00000010;
}
void AttachInterrupt()
{
DDRD ^= (1 << DDD2); // PD2 (PCINT0 pin) is now an input
PORTD |= (1 << PORTD2); // turn On the Pull-up // PD2 is now an input with pull-up enabled
EICRA = 0b00000011; // set INT0 to trigger on rising edge change
EIMSK = 0b00000001; // Turns on INT0
}

Answer 1

Look at string initialization:
char String[]="";
this allocates an array of chars with a size of 1 item (which is terminating zero).
Then you make a call, passing this array reference as the first parameter:
USART_putstring(String,(int)V_n);
And the USART_putstring is as follows:
void USART_putstring(char* String, unsigned int Value1){
sprintf(String,"%d\r\n",Value1);
while(*String != 0x00){
USART_send(*String);
String++;}
}
Note sprintf(String,"%d\r\n",Value1); it converts numeric value into the char buffer. I.e. the buffer should be large enough to contain the text representation of the number, line feeds \r\n\ and zero - the string terminator.
But since your string buffer has size for only 1 char, it totally depends on luck, what will happen after sprintf: maybe there is some unused memory area, so the whole thing will look as if it working. Maybe there are some other variables, and their value will be overwritten, which makes the program behavior unexpected in the future. Or maybe there is some essential data, and your app will be crashing. Behavior may change after you adding several lines and recompile the code.
The point is: be careful with your buffers. Instead of using constants for initialization, set the exact size for the buffer. The number length is maximum 6 symbols (1 possible sign and 5 digits, assuming you're using AVR-GCC, which has the int 16-bits wide, thus has -32768 as the minimum) + 2 for \r\n\ + 1 for terminating zero. I.e. size of the buffer should be 9 at least.
char String[9];