Hi Quang,
Before, when it was my first time to use the OSK function in the evaluation board, I also had the same confusion with you. I read the datasheet many times about the OSK function, but I only cleared myself with the confusion when I set-up the evaluation board and explored most of the configurations about the OSK function.
There are three bits involved in setting up the modes of the OSK function (AD9914 and AD9915):
Function Name Function Control Eval Board GUI
OSK enable Register Addr: 0x00 [8] OSK enable
External OSK enable Register Addr: 0x00 [9] External OSK enable
OSK Pin 66 External OSK pin activate
The Amplitude Scaling Factor (ASF) is the value entered to control the amplitude of the output signal. This can be found on the Profile modes and DRG modes in the evaluation board. Take note: ASF values are ignored when OSK enable = 0 be it in Profile or DRG. Likewise, ASF values are latched when OSK enable = 1.
External OSK enable acts as a mask to the OSK/External OSK pin activate. So if it is set(1) then OSK/External OSK pin activate is enabled. Otherwise, it is ignored.
If OSK/External OSK pin activate = 1, The amplitude of the signal is equal to ASF. Otherwise, the amplitude is zero.
The table summarizes the behaviour of the OSK functionality
| OSK Enable | External OSK enable | OSK/External OSK pin activate | Mode |
| 0 | X | X | ASF Registers Ignored |
| 1 | 0 | X | ASF Registers Enabled, Amplitude = ASF |
| 1 | 1 | 0 | Amplitude = 0 |
| 1 | 1 | 1 | Amplitude = ASF |
Best Regards
Louijie