Composite Video Details - FIGnition FUZE Hardware Reference Manual

Fig 3.7
Generating A Text Screen. FIGnition FUZE uses 25x24 = 600 bytes of internal RAM for
display memory, and each byte contains a character not a bit pattern. The bit patterns are
stored in the AVR's internal Flash memory starting at the location kChrSet. There are 8
bit-pattern bytes per character. To display each bit pattern the firmware looks up the char-
acter code byte in video memory according to the current row and column; then looks up
the bit pattern at kChrSet+8*TheCharacterCode+(ScanLine mod 8). If the charac-
ter code was <16, then it looks up the bit pattern from the UDG memory. If bit 7 of the
character code was set, the bit pattern is inverted [Fig 3.7, top half]. After all this, the bit
pattern is sent to the serial port, by storing the bit pattern at location $C6.

3.2.4 Composite Video Details

Each scan line is almost as simple as described earlier except that after the HSync pulse
there is a 4µs gap for what is called the colour burst, which FIGnition ignores.
Vertical sync however, is complex; complex enough for most websites to get it wrong. Fig
3.4 shows the true standard for PAL Vertical Sync; shown as a single sequence from the
end of one frame to the beginning of the next for even frames.
Vertical Sync is composed of 15 x 32µs periods. The First 5 contain 2µs pulses in each
32µs period. The next 5 contain 30µs pulses in each 32µs period and finally the last 5 con-
tain 2µs pulses in each 32µs period.
This description of composite video provides 312.5 scan lines at 50Hz, but a full imple-
mentation of the PAL video standard provides 625 lines at 25Hz. It does this using a tech-
nique called 'interlacing'. The video is divided into odd and even frames; and the scan lines
of the even frames are shown between those of the odd frames. To distinguish the two, the