I'm trying to connect up an ATTiny85 to usb and have been led to this schematic of the digispark ATTiny usb board.
From the diagram, I can see the 5v input of the controller going through a reverse biased schottky diode; a similar diagram is also present on this site. This may be a noobie question but wouldn't the diode limit current to something ridiculously low, or am I missing something?
Please explain how it is reversed biased? From what I see it supplies 5V - Vf of the diode to the USB connector and prevents the USB from powering itself. If you turn it around it will supply 5V -VF to the board from the USB.
Hi, thanks for replying.
The USB powers the board in this design so that's why I thought it was weird.
Apparently there have already been plenty of forum posts on the D3 diode, and the schematic is indeed drawn incorrectly.
best threads are here:
and
I think this person has the best answer, using a PCB diagram and photos of the board:
Digispark and Adafruit's Trinket are all based upon prior work V-USB on AVR microcontrollers.
https://www.obdev.at/products/vusb/index.html
The link gives schematics and the zener diode specs ... some sublinks gone unfortunately.
The zeners are somewhat critical as they drop the AVR's 5V to USB 3.3V signaling.
The diode in question is sometimes included when the PC powersupply 5V is not used for the external microcontroller. Unnecessary if USB 5V is used.
Ray
Thought so when you first posted it, so I just couldn't answer! It made no sense at all. 
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