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3 Video
Video support on FIGnition requires just 2 diodes, 2 resistors and a phono socket to pro-
vide PAL or NTSC monochrome video. It uses the internal serial port to output pixels; a
timer to generate TV synchronisation signals and another timer to manage the top and bot-
tom margins without CPU intervention. FIGnition is capable of a text-based video mode
(with 16 UDGs) and a fully bitmapped video mode using the external SRAM as video
memory.
3.1 Circuit Diagram
Fig 3.1
3.2 Operation
We'll start with the way a TV generates composite video; then look into how the circuit
generates the right signals for it. Then we'll look again in more depth and look into how the
AVR and video firmware provides video efficiently.
3.2.1 Composite Video Basics
Intuitively we may think TV receives video the way we see the outside world, that is, in the
same way countless beams of light from our surroundings continually hit our eyes, broad-
casters send countless (invisible) beams of light as TV signals to TVs. This is not true.
Instead TV uses a property of human vision called "persistence of vision". If you look a
bright scene, then snap your eyes shut, the scene will fade quickly, but not instantly. Video
uses this to create an illusion of movement by showing a rapid sequence of still images,
called frames, at least 25 per second. It makes use of the same principle to break down
each still image into hundreds of thousands, or millions of individual dots, which are dis-
played one at a time in the order we read books: a row of (perhaps thousands) of dots (of
different colours) in a line and repeated for hundreds or thousands of lines (called scans)
for each frame [Fig 3.2]. This is true for traditional 20th century TVs and the basic principle
is true for LCD TVs too.
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