No output for long time

I found that didOutputSampleBuffer would not be called for long time when the screen is no change. It sometimes make me confused for if there something wrong.

In my design, it will change to other screen shot method, such as CGWindowListCreateImage, when long time no data. But this is not what I expected. I set the minimumFrameInterval to 30 but it seems no work. [config setMinimumFrameInterval:CMTimeMake(1, 30)];

Is there any settings that can let me get a didOutputSampleBuffer, even given a CMSampleBufferRef with status SCFrameStatusIdle, called atlest one time per second? Which would make me think it works fine without any exception.