WPF绑定到文本框不更新

I have am writing a WPF application, and I have a textbox for the user to enter a frames per second value for video playback. The value of this textbox is bound to a dependency property in the code behind (trying to follow MVVM like a good designer). My problem is that the textbox is not updating automatically when the FPS value is changed externally. For example, the user can control the value using a slider. The dependency properties value is changed correctly by the slider, but the textboxes text never updates, unless, of course, i do it manualy using GetBindingExpression(..).UpdateTarget() which is what I have implemented pending a better solution. Does anyone know if this is intended functionality or am I setting something up wrong?I have am writing a WPF application, and I have