Usb; Circuit Diagram; Hardware Operation - FIGnition FUZE Hardware Reference Manual

7. USB

FIGnition contains a USB interface, circuitry and firmware which allows the computer to
receive firmware upgrades to its internal Flash memory and transfer data (slowly) through
its internal EEPROM. The USB connection is managed entirely in software using the V-
USB bootloader written by Objective Development [http://www.obdev.at].

7.1 Circuit Diagram

7.2 Hardware Operation

FIGnition's hardware interface is one of the standard circuits described on the Objective
Development's website
make it support slow speed USB in Firmware on an AVR microcontroller:
• The D- signal must be connected to a dedicated external interrupt pin, in this case:
INT0. There is more flexibility on the D+ signal.
• The Zener diodes are diodes with the unusual property that when they are reversed
biased they block current flow unless the reverse bias voltage is beyond their 'break-
down voltage'. If the voltage exceeds it they essentially limit the input voltage to the
break-down voltage. Here they limit the voltage on the D+ and D- pins to 3.6v.
• The R1 resistor is needed to limit D+.
4 http://www.obdev.at/products/vusb/index.html
4 . The hardware design contains a number of subtle techniques to
Fig 7.1