Please check your dscr.c file again to make sure everything is correct. After that you could use oscilloscope to check if the image sensor outputs data and if FV/HV works fine.
The necessary changes to add a different resolution/frame rate is mentioned in the below link. Please make sure that you have followed the instructions.
The page is no more available. Can you please send a copy of the same or provide another link where i can find it?
Please provide the OV7670 project folder?
I hope will help me.
You said that :
"Also, the frame sync vsync signal output from OV7670 is active low."
Did the active low really work? since no matter i choose active high or low, the result are the same
because of this, I have to change the sensor vsync polarity when i initiateing it.