# Understanding schematics: resistors attached to Attiny85 pins V-USB

I'm currently trying to create my own Attiny85 usb thumb(to mimic an joystick). I understand that I have to limit the voltage on the attiny's pins that are connected to the D+/- pins from the usb. In various schematics there are some pull-up resistors used with some zener diodes(that will drop the voltage that exceeds 3.3 V).(taken from insidegadgets)

# #1

I am sure most computers these days have a voltage regulator built-in them(everyone can just plug the usb into the 220v socket...).Can then I just avoid an regulator?(as other schematics incorporate in them)

# #2

Why is there one pull-up resistor on pin2 only(pin2 from usb)? That is D- pin.

# #3

An resistor can already limit the voltage.Why need zener diodes?(eg.: attiny85 draws ~7mA at 16.5 MH and 5v. And we need a resistance that would drop the 5v from one pin-considered in HIGH state- to 3.3v...that is a 1.2v drop.So R=1.2/0.007=171 ohms.That resistor attached to the attiny would produce a 1.2 drop).

# #4

There are two zener diodes attached to D+/- pins. What's with the resistors?(R2/R1) They limit..the current(didn't know the pc is so sensitive..)?

• While 1.2/0.7 ~ 1.71, 7 mA is not 0.7 amps but 0.007, giving 171Ω rather than 1.71. Also, please edit your question and provide an attribution for the schematic. I'm wondering why the zeners are on the USB side of the 68Ω resistors instead of on the '85 side. – James Waldby - jwpat7 May 11 '17 at 15:05
• @JamesWaldby-jwpat7 The zeners are on the USB side so the current from the GPIO pin is limited before being sunk through a zener. You don't want to overload the current from the GPIO pins, do you? – Majenko May 11 '17 at 15:30
• @Majenko, ok, I see that now – James Waldby - jwpat7 May 11 '17 at 15:45

I am sure most computers these days have a voltage regulator built-in them(everyone can just plug the usb into the 220v socket...).Can then I just avoid the regulator?

If you have a 5v supply, as in a USB connection, and you want to run off 5v, then you do not need any regulator.

Why is there one pull-up resistor on pin2 only(pin2 from usb)? That is D- pin.

Because only that pin needs pulling up to indicate the presence of a device on the USB bus.

An resistor can already limit the voltage.Why need zener diodes?(eg.: attiny85 draws ~7mA at 16.5 MH and 5v. And we need a resistance that would drop the 5v from one pin-considered in HIGH state- to 3.3v...that is a 1.2v drop.So R=1.2/0.7=1.71 ohms).

You can only use a resistor to drop a fixed voltage of you have a fixed current. The current you quote is the chip's supply current, not the GPIO current, which you cannot know or predict. Also, if you did use a resistor it would operate both ways and the 3.3V coming in would also be reduced so the MCU will not be able to read the data.

Even if you did know the current you would find it was very small so a huge resistor would be needed, which would induce intolerable amounts of noise. It would also violate the USB specifications.

There are two zener diodes attached to D+/- pins. What's with the resistors?(R2/R1) They limit..the current(didn't know the pc is so sensitive..)?

They are to protect the ATTiny's IO pins. Without those the zener diodes will sink way too much current from the IO pins and damage them. They also act partially as termination resistors for the USB bus (which should be 20Ω).