var keys;
var timeStamp;
timeStamp = new Date();

function dd_onkeypress(DropDownList1) 
{
    var key = event.keyCode;
    event.returnValue=false;

    //a-z, A-Z, 0-9
    if ((key>=97 && key<=122) || (key>=65 && key<=90) || (key>=48 && key<=57)) 
    {
        key = String.fromCharCode(key);
        var now = new Date();
        var diff = (now.getTime() - timeStamp.getTime());
        timeStamp = new Date();

        //1 seconds = 1000 milliseconds<BR>   
        if (diff > 1000) 
        {
            keys = key;    
        } else {
            keys = keys + key;
        }

        var cnt;

        for (cnt=0;cnt<DropDownList1.options.length;cnt++) 
        { 
            var itm = DropDownList1.options[cnt].text; 
            if (itm.substring(0,keys.length).toLowerCase()==keys.toLowerCase()) {
                DropDownList1.selectedIndex = cnt;
                break;
            }
        }
    }
}