The Start-Of-Frame (SOF) signal, physically transmitted over the AXI4-Stream TUSER0 signal, marks the first pixel of a
video frame. The SOF pulse is 1 valid transaction wide, and must coincide with the
first pixel of the frame, as seen in the previous timing diagram. The SOF signal
serves as a frame synchronization signal, which allows downstream cores to
re-initialize, and detect the first pixel of a frame. The SOF signal can be asserted
an arbitrary number of AP_CLK cycles before the
first pixel value is presented on DATA, as long as a VALID is not asserted.