Codeigniter ajax post csrf The action you have requested is not allowed - php

Question

i have the ajax request for fetching the information if user is still logged in or not let's say checking its session, and also i have enabled the csrf protection also and the GET is working fine in all aspects but the problem is only with POST from ajax, i have also turned off the re-generation of token and also did pass the new token to every ajax request, but still it gives me an error :
The action you have requested is not allowed.
fetching token :
var getToekn = function(){
return $('meta[name=csrf_token]').attr('content');
}
call to userLog every 1 minute:
var userLog = function () {
var formdata = new FormData();
formdata.append('csrf_',getToken);
$.ajax({
url: site_url + 'action=userCheck',
type: 'post',
dataType: 'json',
data:formdata,
processData:false,
success: function (d) {
if (d.login === 'true') {
jQuery('meta[name="csrf_token"]').attr('content', d.csrf);
console.log('loggedin: True');
} else if (d.login === 'false') {
if (jQuery('#js_login_again').legnth) {
popup('js_login_again', 'o');
jQuery('meta[name="csrf_token"]').attr('content', d.csrf);
} else {
$.ajax({
url: site_url + '?pop=loginagain',
type: 'post',
dataType: 'html',
success: function (data) {
var popup = $(data).filter('#js_login_again');
jQuery('body').append(popup);
popup('js_login_again', 'o');
loading('h');
}
});
}
}
}
});
}
i did check if the sent token is correct or not and it is a correct token but the ajax post request still results to an error The action you have requested is not allowed.
The thing is i'm already giving the csrf token to ajax request and the token is being also added to the parameters sent to the url, so it's not the case of token , i'm just lost about why it does so.
also did check the $this->input->post('csrf_'); it's empty.