Thank you Jon @jpeyron, I have tried it several times. I am sure that i had connected the ov7670 with the same configuration in its constraint file. However, still, I got the black screen on monitor. I have tested the fpga and monitor with some vga programs and they worked. I presume that my ov7670 might be broke. Is there a way to know if it is really broke? or maybe external electronic circuit should be added? Thank you very much for your reply.
I am new to FPGA. I have several problems in connecting OV7670 to Nexys2, so i decide to test wether it is broke. I have tried to connect XCLK with 25MHz output from scaling 50MHz Nexys2 clock, then i connect the pixel clock PCLK to oscilloscope. Unfortunately, i cant see any clock signal on PCLK. I connect the 3.3V and the ground pin properly. Any other pins remain unconnected. Did i miss something?