ADS1256: ADS1256 is not outputting the data

/*
 * ADS1256.cpp
 *
 * Created: 6/13/2018 3:47:12 PM
 * Author : Ajay
 */ 

#include <avr/io.h>
#include <avr/interrupt.h>

#define MYUBRR 51
#define F_CPU 8000000UL
#include <util/delay.h>

unsigned char spi_tranceiver (unsigned char data);
void USART0_Init(void);
void transmit (unsigned char DATA);

int a = 0;
unsigned long adc_val = 0;

ISR(INT0_vect)
{
if (a == 0)
{
	spi_tranceiver(0x03);
	_delay_us(25);
	a = 1;
}	
adc_val = spi_tranceiver(0);
adc_val <<= 8;
adc_val = spi_tranceiver(0);
adc_val <<= 8;
adc_val = spi_tranceiver(0);	
}

int main(void)
{	
	DDRB |= 0b10110000; //Setting the MOSI, SS & SCK as outputs and MISO as input
	
	SPCR |= (1<<SPE)|(0<<DORD)|(1<<MSTR)|(0<<CPOL)|(1<<CPHA)|(1<<SPR0)|(0<<SPR1); //Enabling SPI and setting freq to 2 MH. Also, setting MSB transferred first
	SPSR |= (1<<SPI2X);
	
	EIMSK |= (1<<INT0); //Enables INT0
	EICRA |= (1<<ISC01)|(0<<ISC00); //Interrupt INT0 on falling edge
	
	sei(); //Enable the interrupts
	
    while (1) 
    {
		USART0_Init();	

		transmit((adc_val >>16));
		transmit((adc_val >>8));
		transmit((adc_val));
	}
}

unsigned char spi_tranceiver (unsigned char data)
{
	SPDR = data;
	while(!(SPSR & (1<<SPIF)));
	return(SPDR);
}

void USART0_Init(void)
{
	UBRR0H = (unsigned char)(MYUBRR>>8);
	UBRR0L = (unsigned char)(MYUBRR);
	UCSR0B = (1<<RXEN0)|(1<<TXEN0);
	UCSR0C = (0<<USBS0)|(1<<UCSZ00)|(1<<UCSZ01)|(0<<UCSZ02)|(0<<UMSEL00);
}

void transmit (unsigned char DATA)
{
	while ( !(UCSR0A & (1<<UDRE0)) );
	UDR0 = DATA;
}