checksum == 0xFF?

char GetByte(){
    while(!DataRdyUSART());
    return(getcUSART());
}


    
    while(GetByte()!=0xce);     //wait for header byte
    checksum=0xce;              //initialise checksum
    for(i=0;i<4;i++){           //now get next 4 bytes
        c[i]=GetByte();
        checksum+=c[i];
    }
    if(checksum==0xff){

        //got a valid package so send acknowledge
        //and use the data
    }