ATMega32U4 USB design considerations

Question

I'm desigining a board with ATMega32U4 microcontroller.

The board will be minimal, and will feed the microcontroller from USB of an M.2 socket (E type) on a PC mainboard:

Now, checking Leonardo's schematic, they are using varistors to protect USB's D+ and D- pins:

This is perfectly understanable, as Leonardo can be plugged into any computer's USB port.

But in my case, this AVR will be powered from an internal M.2 socket soldered onto the PC's mainboard.

My assumption is that I can leave out these varistors, as when voltage on D+ and D- pins are above the varistors' limits (20V), my biggest problem probably will not be this AVR, but the whole computer.

Can you verify this assumption? Can I skip these varistors?

Answer 1

It is likely that the TVS diodes can be omitted.

If you want, you could draw them in and choose at manufacturing stage to mount them or not.

But then the question is, if you need them because the board will be handled in an ESD environment, would all IO need protection. If you don't add protection to other pins, there is little reason to add protection to USB either.

Answer 2

If you can make the ground and Vdd gold edge pads slightly longer than the others, then they will make contact first. That means that any static electricity coupled from this board to the computer will meet a low-impedance path first and tend to be less able to damage anything.

In a high-static-electricity environment (very dry air), just resting the free hand on the computer's chassis before and while inserting the card is sufficient to prevent ESD. Wrist straps are suggested and used because it is easy to forget. In very humid environments, static electricity can barely form.

Back in the 90's I witnessed a computer tech remove a defective ISA modem and replace it with an identical one while the PC was running, successfully, with no wrist strap. They got away with this because of a) good free-hand grounding technique, b) humid summer months, and c) the power and ground fingers of the card were longer than the others, which also reduced the chance of latch-up. Ill-advised though.