As discussed in Product Specification, the AXI4-Stream video
streaming interface for frame buffer read and write cores includes two flags,
tlast and tuser, which are used to identify
specific pixels in the video stream.
The tlast signal, also known as the end of line (EOL), marks the last
valid pixel of each line. This EOL pulse is one valid transaction wide and must align
with the last pixel of a scanline. The tuser signal, referred to as the
start of frame (SOF), designates the first valid pixel of a frame. The SOF pulse is also
one valid transaction wide and must coincide with the first pixel of the frame, serving
as a frame synchronization signal. These EOL and SOF flags are essential for identifying
pixel locations on the AXI4-Stream interface, as there are no sync or
blank signals. The frame buffer write core maintains internal registers to track the
pixels received per line and the lines received per video frame. The behavior of frame
buffer write in case of early or late EOL and SOF flags is explained below, assuming
tvalid is high when these events occur.
- Early EOL
- When the EOL signal is asserted prematurely, the core disregards the EOL and continues to receive data until the pixel count per line reaches the frame width specified in the configuration register at offset 0x0010. After receiving the next EOL pulse, the core will reset the pixel count register one cycle later.
- Late EOL
- If the EOL signal is delayed, the core will ignore any data received after the pixel count per line reaches the frame width set in the configuration register, until the EOL signal is asserted.
- Early SOF
- When the SOF signal is asserted early, the core initiates the internal line count and pixel count per line, and begins to receive any data arriving on the data bus.
- Late SOF
- If the SOF signal is asserted late, the core will ignore any data received on the data bus once the internal line count reaches the frame height specified in the configuration register at offset 0x0018, until the SOF signal is asserted.