Can someone explain how this V-USB circuit works?

Hi everyone,

http://www.practicalarduino.com/pics/virtual-usb-keyboard-schematic.jpg

I'm attempting to learn how to communicate with Atmel chips such as the Attiny85 without having to use a serial-to-USB converter. I'm fairly new to electronics circuit design, and I am having trouble understanding what role the zener diodes and resistors have in the communication between the 5V MCU and USB. If you can, please explain why certain resistances and zener diode ratings are used in this situation(and why zener diodes work and regular diodes don't)

Thanks in advance,

Tom

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D_Hersey over 11 years ago

The designer would have been better served to split his input resistors and place his Zeners at the taps.
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michaelkellett over 11 years ago in reply to D_Hersey

Not really !

USB chips vary a little but usually need series resistors between zero and 22 ohms. Splitting these doesn't help much - 100V on the zener and 10ohms from that to the chip still makes a dead chip. A big series R would protect the chip nicely but the USB spec can't be met with much more R than the chip supplier recommends and it won't work at all with much more than 100R. You can see one of many better ways to do it here: http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf
(But you won't easily find the devices they refer to so you would need to poke around to find substitutes.)

I would use one of these, you need a board and it's surface mount but ESD protection doesn't really work any other way so nothing is lost:

STMICROELECTRONICS - USBLC6-2P6 - DIODE ARRAY, TVS, USB2

Image is for illustrative purposes only.
Please refer to product description

Manufacturer:
STMICROELECTRONICS
Order Code:
1295310
Manufacturer Part No:
USBLC6-2P6

Technical Data Sheet (219.00KB) EN

MK
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michaelkellett over 11 years ago in reply to D_Hersey

Not really !

USB chips vary a little but usually need series resistors between zero and 22 ohms. Splitting these doesn't help much - 100V on the zener and 10ohms from that to the chip still makes a dead chip. A big series R would protect the chip nicely but the USB spec can't be met with much more R than the chip supplier recommends and it won't work at all with much more than 100R. You can see one of many better ways to do it here: http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf
(But you won't easily find the devices they refer to so you would need to poke around to find substitutes.)

I would use one of these, you need a board and it's surface mount but ESD protection doesn't really work any other way so nothing is lost:

STMICROELECTRONICS - USBLC6-2P6 - DIODE ARRAY, TVS, USB2

Image is for illustrative purposes only.
Please refer to product description

Manufacturer:
STMICROELECTRONICS
Order Code:
1295310
Manufacturer Part No:
USBLC6-2P6

Technical Data Sheet (219.00KB) EN

MK
Cancel
Vote Up 0 Vote Down

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Can someone explain how this V-USB circuit works?

STMICROELECTRONICS - USBLC6-2P6 - DIODE ARRAY, TVS, USB2

STMICROELECTRONICS - USBLC6-2P6 - DIODE ARRAY, TVS, USB2