var xmlhttp;

function envia(){

    if(document.getElementById('cep').value.length>=8){
        var cep=document.getElementById('cep').value;
        xmlhttp=GetXmlHttpObject();
        if (xmlhttp==null){
            alert ('Your browser does not support AJAX!');
            return;
        }
        
        var url='inc/busca_cep.php';
        url=url+'?cep='+cep;
        xmlhttp.onreadystatechange=stateChanged;
        xmlhttp.open('GET',url,true);
        xmlhttp.send(null);
        return false;
    }
}

function stateChanged(){
    if (xmlhttp.readyState==4){
        document.getElementById('retornoCep').innerHTML=xmlhttp.responseText;
    }
}

function GetXmlHttpObject(){
    if (window.XMLHttpRequest){
        // code for IE7+, Firefox, Chrome, Opera, Safari
        return new XMLHttpRequest();
    }
    if (window.ActiveXObject){
        // code for IE6, IE5
        return new ActiveXObject('Microsoft.XMLHTTP');
    }
    return null;
}MLHTTP');
    }
    return null;
}