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• The small resistors R2 and R3 limit the current flow to PD4 and PD2, because the
current flow on these signals may still be higher than most of FIGnition's signals, high
current rated resistors are needed for R2 and R3.
• The C1 capacitor is the main decoupling capacitor for the whole computer, which is
why it's the largest capacitor in FIGnition.
USB communicates through serial, differential signals operating between 0 and 3v [Fig
7.2]
Differential signals are used for reliability. Data is sent on both data wires, but sent with
opposite voltages. Mostly this is so that momentary electrical currents a signal may en-
counter between the host (a conventional computer) and the device (FIGnition) won't af-
fect the data signal. It works because these electrical currents will affect both signal wires
at the same time in the same way, but the difference between them will still be the same
(Fig 7.3].
USB requires signal voltages to be between 0 and 3v, but FIGnition's AVR chip needs to
operate at 5v to run at 20MHz. The standard V-USB circuit contains two 3v 500mA Zener
diodes to clamp voltages to between 0 to 3v. Zener diodes are described in Appendix B.
Fig 7.2
Fig 7.3
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