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Fig 3.5
White is the most complex. To output white, PD3 (Sync) and PD1 both output a 1. The
diode on PD3 and PD1 both lower their voltage by 0.7v (taking their output voltage to
4.3v). Then, because the 1KΩ and 470Ω resistors are in parallel, they have a combined
resistance of 1/(1/1000+1/470) = 320Ω.
Finally, this combined resistance and the 75Ω resistor acts as a potential divider generat-
ing an output voltage of: 4.3x75/(320+75) = 0.817v; which is close to 1v.
This means 'white' on FIGnition is only 75% as bright as it could be. The ideal resistor
would be 328.9Ω instead of 470Ω, but I chose 470Ω, because it is much easier to get hold
of than a 330Ω resistor (which is the closest normal resistor to 470Ω).
3.2.3 Video Firmware Details
A text-mode FIGnition image consists of 25 characters x 24 characters on the screen and
each character has 8 rows of 8 pixels. This means the firmware must generate 200 pixels
across and generate an image of 192 scans.
Generating A Frame. We can see there are 192 scans that contain an image, but 312.5
scans in a frame. The first 7.5 scans of a Frame are the vertical sync. Then there is a top
margin of 56 scans, which are just blank. Then there are 192 image scans and finally there
are another 56 scans for the bottom margin. FIGnition handles this using another state
machine (See section 2.2.4 for an introduction to state machines), which it runs through in
a simple sequence. It uses 4 states to manage the vertical sync, then a state for the top
margin, two states per image scan line and finally one more state for the bottom margin
[Fig 3.6].
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