I'm planning to assemble a circuit this weekend; it is basically an attiny85 with a USB interface. The circuit is well known on the internet; see for instance this schematic:
(Image taken from http://codeandlife.com/2012/03/03/diy-usb-password-generator/)
Now, currently I don't have any zener at home; I hope the local supply has them, but couple of times they didn't have even the most basic components, so there is some risk I will not be able to have them by the WE. So, here comes the question.
Can I substitute the two zeners with two blue leds? I know that the zeners have a steeper curve, so I will not have stable 3.6V (or better, a slightly lower voltage) on the lines, but… Is this really a problem for USB lines?
I read question LED as Zener: is this a good way to lower the BOM cost?, and there the main concerns were about the stability of the voltage (and in that case they were not an issue). Do you think there can be issues with USB? Did you already try that?
Thank you
Best Answer
Yes, for a USB Low-Speed applications 3-V LEDs can be used in this schematics.
Actually, in this case these Zeners serve some USB functionality, and their main purpose is not ESD protection but voltage limiting. Reason is that this MCU is powered from +5V supply directly, without any 3.3V regulator. At 5V the ATtiny I/O drivers have about 25 Ohms, and would drive hard to 5V, which is not what USB needs nor likes. The signals should be limited to 3.3 - 3.6 maximum in order to not to fry the USB host. That's why the schematics uses the insane 68 Ohms in-series resistors, and 3.6V Zeners. The data signal clipping don't need to be precise, so 3-V LEDs will be okay, and even add some fun illuminating USB traffic.
In the past the USB interface used to be tolerant to direct contact with +5V. With modernization in silicon technology and integration of USB physical interfaces (PHYs) into low-voltage 22 and 14 nm silicon (where transistors are barely 1-V tolerant), the requirement to hold 5V on data lines was eliminated. So the USB lines better be protected from DIY devices like this ATTiny projects.