Why NRF24LE1 ADC is reading value always as 255?

Question

I am doing a register retention gzll transmit code with ADC. But problem is that the ADC value transmitted is always 255, if I connect ground to ADC it would read 247. Why is there such a problem here. Here is my code: 
 #include"reg24le1.h" // I/O header file for NRF24LE1
//#include"hal_uart.h" // library containing serial communication functions
#include"hal_delay.h" // header file containing delay functions
#include"hal_clk.h"
#include <stdio.h>
#include "nrf24le1.h"
#include "gzll_mcu.h"
#include "gzll.h"
#include "hal_adc.h"

// Repeated putchar to print a string
/*void putstring(char *s)
{
 while(*s != 0)
 hal_uart_putchar(*s++);
}*/
 int gzlinit=0,val=0;
 uint8_t payload[3];
 void wakeup_irq() interrupt INTERRUPT_WUOPIRQ
{
	;//putstring("Hello Ranjan I woke up
");
	//gzlinit=1;
}
// main function
void main()
{
	//hal_clklf_set_source(HAL_CLKLF_XOSC16M_SYNTH);
	//hal_clk_set_input(HAL_CLK_XOSC16_OR_RCOSC16);
 mcu_init();
 gzll_init();
	
	//hal_clk_set_freq(HAL_CLK_125KHZ); //if you put in 125Khz then uart wont work
 WUOPC0 = 0x02;

P01 = 0x00;
P1 = 0x00;
P2 = 0x00;
P3 = 0x00;

P0DIR = 0x02;
//P0DIR = 0xf2;//0x02 only for pin wake up 0xF2 both for uart and pin wake up
P1DIR = 0x00;
P2DIR = 0x00;
P3DIR = 0x00;


// Initializes the UART
 // hal_uart_init(UART_BAUD_9K6);
 hal_adc_set_input_channel(HAL_ADC_INP_AIN6);
 hal_adc_set_reference(HAL_ADC_REF_VDD);
 hal_adc_set_input_mode(HAL_ADC_SINGLE);
 hal_adc_set_conversion_mode(HAL_ADC_CONTINOUS);
	//hal_adc_set_conversion_mode(HAL_ADC_SINGLE_STEP);
 hal_adc_set_sampling_rate(HAL_ADC_2KSPS);
 hal_adc_set_resolution(HAL_ADC_RES_8BIT);
 hal_adc_set_data_just(HAL_ADC_JUST_RIGHT);
// Enable global interrupts
	IEN1 = 0x08;
 EA = 1;
	//MISC=1;
	hal_adc_start();
	PWRDWN = 0x04;
 // Clear power down
	PWRDWN = 0x00;
 //putstring("
Hello Init!!
") ;
// delay_ms(2000);
 
//infinite loop
while(1)
{
// Print "Hello Everyone"
//putstring("
Hello Everyone!!
") ;
//delay_ms(1000); // delay of 1sec
	
	while(P01==1)
 {
 //if(gzlinit==0)
 {
 PWRDWN=0x07;
 //gzll_init();
 
 
 if(gzll_get_state() == GZLL_IDLE)
 {
 while( hal_adc_busy() ) // Wait for the ADC to finish a conversion
 {
 }
 val = hal_adc_read_LSB();
 payload[0] = val;
 // Transmits payload[] to pipe 0 address
 gzll_tx_data(payload, 3, 0);
 //gzlinit=2; 
 }	
 gzll_goto_idle();
	
 //putstring("
Hello Everyone!!
") ;
 //if(gzlinit==2)
 //break;
 }
	}
 gzlinit=0;
 // Register retention mode
 PWRDWN = 0x04;
 // Clear power down
 PWRDWN = 0x00;
	}
}