Hi Sercan,
In order to control the amplitude of the signal, "OSK enable" should be set in the evaluation software. If this is not set, any value in the ASF registers would be ignored. "External OSK enable" when set, allows you to control the OSK behavior through an external pin. That external pin is issued by a microcontroller and can be controlled via "External OSK pin activate". If this external pin is set, the signal amplitude would be the value of the ASF. If it is clear, the signal amplitude would be zero.
Best Regards
Louijie