Edit in JSFiddle

angular.module('myApp', [])
 
    .directive('myUpperCase', function() {
	   return {
		 require: 'ngModel',
		 link: function(scope, element, attrs, modelCtrl) {
			var upperCaseFunc = function(inputValue) {
			
				if (typeof inputValue == "undefined") {
					return;
				}
				
				var upperCasedString = inputValue.toUpperCase();
			   
				if(upperCasedString !== inputValue) {
				  modelCtrl.$setViewValue(upperCasedString);
				  modelCtrl.$render();
				}         
				
				return upperCasedString;
			 }
			 
			 modelCtrl.$parsers.push(upperCaseFunc);
			 upperCaseFunc(scope[attrs.ngModel]); 
		}
	  };
	})

<div id="myApp" ng-app="myApp">
<input type="text" ng-model="myInput" my-upper-case />
</div>