The post is valid for the ADV7182, ADV7280, ADV7280-M, ADV7281, ADV7281-M, ADV7281-MA, ADV7282, ADV7282-M and ADV7283.
How Free-run Works
Free-run mode is a mode where the ADV728x ignores in incomming analog video input and outputs a test pattern. The ADV728x device has a line based FIFO memory block in the digital core of the part. Usually this store about 3 to 5 lines of active video ( i.e. video that's been output by the ADC in the ADV728x).
However there are also a few test patterns stored in the FIFO memory block. In free-run mode the active video lines are ignored and the test patterns are output instead. The test patterns consist of a single line that is repeated over an over again to create a full field.
By default the ADV728x will output a blue field however It is possible to modify the output color or select a number of different test patterns. See the "FREE-RUN OPERATION" section of the ADV7182 datasheet and ADV728x hardware manual for more information.
When ADV728x Enters Free Run
By default the ADV728x will enter free-run mode if it is not able to connect to an analog video source. Using standard scripts, after an analog video source is disconnected it can take the ADV728x upto 1 second to enter free-run mode. Using Fast Swtiching scripts it can take the ADV728x under 250 ms to enter free-run mode. For automotive applications analog devices advises using fast swithcing script. See this post for more information on Fast Switching script. Fast Switch Mode- ADV728x
It is also possible to force the ADV728x not to enter free-run mode if a source is disconnected. Setting the "DEF_VAL_AUTO_EN" bit (user map register 0x0C bit ) will force the ADV728x not to enter free-run mode if a source is disconnected. This mode is not advised as the ADV728x will output noise until a black noisy video source is connected.
What Format will the ADV728x output in Free-Run Mode
See this post for more details What Video Output Standard does the ADV728x Output in Free-Run Mode
Non-standard field length
It is possible for the ADV728x to output fields that are too long/ short. See this post for more details.