Char Count with Tinymce

I’m creating a plugin that has multiple instances of tinymce in a form. I just want to update a textbox with the character count onkeyup.
As it stands, I have a button that will update the count and it’s working with this code activated with an onclick event:

function getStats(id,mytbid) {
    var body = tinymce.get(id).getBody(), text = tinymce.trim(body.innerText || body.textContent);
	var amhtextbox = document.getElementById(mytbid);
	var ed = tinyMCE.activeEditor;
	amhtextbox.value=text.length;
	ed.focus();
    return {
        chars: text.length,
        words: text.split(/[\w\u2019\'-]+/).length
    };
}

I also found this code for the keyup event. I tried putting it in my theme’s function file, but can’t get it to work.

setup: function(ed) {
    ed.on('keyup', function(e) {
        console.log('Editor contents was modified. Contents: ' + ed.getContent());
        check_submit();
    });
}

Any help/suggestions/tips would be appreciated!