Question about AD5932 with external increment

Hello! 

I am using AD5932 to make a sine wave starting from 1kHz to 3.5MHz. The auto-increment mode worked fine but when I tried to use the external-increment mode, I met some problems.

My control register is now 0x07FF and delta frequency is 17.5kHz with 200 increments. But every time I toggle the CTRL pin with my Arduino, the frequency does not change. The wave stays at 1kHz when CTRL pin is low and it returns to 0 when the CTRL pin is high. 

If I simply put the wire connected with CTRL to GND/VCC, the signal does increment. But the frequency interval is not the expected 17.5kHz, and the resulting frequency seems to be random.

I'm wondering what I have done wrongly. 

Here is part of my code:

void loop() {
digitalWrite(CTRL,LOW);
delay(1000);
digitalWrite(CTRL,HIGH);
delay(10);
}

Thank you!

Best regards