dht222 shows check sum error

[mshh]

this code works well for dht11 but for dht22 it shows checksum error in the range from 49-51%.
I tried to put it in a code #but there is a problem

Chip type : ATmega8
Program type : Application
AVR Core Clock frequency: 8,000000 MHz
Memory model : Small
External RAM size : 0
Data Stack size : 256
*****************************************************/

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Code C - [expand]
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#include mega8.h
#include stdint.h
#include stdio.h
#include delay.h
#include stdlib.h
 
#define DHT22       //define DHT22 or DHT11
#define DHT_PORT        PORTD
#define DHT_DDR         DDRD
#define DHT_PIN         PIND
#define DHT11_PIN 1
 
 
 
// Alphanumeric LCD Module functions
#include <alcd.h>
uint8_t c=0,I_RH,D_RH,I_Temp,D_Temp,CheckSum;
 int q;
     float RH;
   unsigned char data [5];  
 
void Request()                /* Microcontroller send start pulse/request */
{
    DDRD |= (1<<DHT11_PIN);
    PORTD &= ~(1<<DHT11_PIN);    /* set to low pin */
    delay_ms(20);            /* wait for 20ms */
    PORTD |= (1<<DHT11_PIN);    /* set to high pin */
}
 
void Response()             /* receive response from DHT11 */
{
    DDRD &= ~(1<<DHT11_PIN);
    while(PIND & (1<<DHT11_PIN));
    while((PIND & (1<<DHT11_PIN))==0);
    while(PIND & (1<<DHT11_PIN));
}
 
uint8_t Receive_data()          /* receive data */
{   
    for (q=0; q<8; q++)
    {
        while((PIND & (1<<DHT11_PIN)) == 0);  /* check received bit 0 or 1 */
        delay_us(30);
        if(PIND & (1<<DHT11_PIN))/* if high pulse is greater than 30ms */
        c = (c<<1)|(0x01);  /* then its logic HIGH */
        else            /* otherwise its logic LOW */
        c = (c<<1);
        while(PIND & (1<<DHT11_PIN));
    }
    return c;
}
 
 
void main(void)
{
PORTB=0x00;
DDRB=0xFF;
 
PORTC=0x00;
DDRC=0x00;
 
PORTD=0x00;
DDRD=0xFF;
 
lcd_init(16);
 
while (1)
      {
       Request();       /* send start pulse */
        Response();     /* receive response */
        I_RH=Receive_data();    /* store first eight bit in I_RH */
        D_RH=Receive_data();    /* store next eight bit in D_RH */
        I_Temp=Receive_data();  /* store next eight bit in I_Temp */
        D_Temp=Receive_data();  /* store next eight bit in D_Temp */
        CheckSum=Receive_data();/* store next eight bit in CheckSum */
        
        if ((I_RH + D_RH + I_Temp + D_Temp) != CheckSum)
        {
            lcd_gotoxy(0,1);
            lcd_putsf("Error");
        }
        
        else
        {  
          
        
            lcd_gotoxy(0,0);
            lcd_putsf("Humidity ="); 
            lcd_gotoxy(0,10); 
    
           #ifdef DHT22
            RH= (float) I_RH * 256.0 + D_RH ;
            RH=RH/10.0;
            
            #else
            RH =  I_RH;
             #endif 
             sprintf(data,"%.1f%%  ",RH);
           lcd_puts(data);
   
   
            } 
                delay_ms(2000);
 
            }
}

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[andre_luis]

if ((I_RH + D_RH + I_Temp + D_Temp) != CheckSum)

In some cases, it is necessary to invert the calculated value and add 1 (or alternatively, add a minus signal); moreover, the method you are using do not seem a good choice, once you are not checking when the calculated value equals to the one received, but you are checking if it differs. I presume you should have to do something like this:

if ((I_RH + D_RH + I_Temp + D_Temp) == - CheckSum)

I'm assuming that compiler will not perform any kind of type casting to 16 bit on the above sum.