Define your EditText in your xml this way:
<EditText
android:id="@+id/txtOppCodigoCliente"
android:inputType="textVisiblePassword"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:maxLength="11"
android:hint="@string/strCodigoCliente"/>
output:
android:inputType=”textVisiblePassword”
A row of numbers and the rest of letters.