I'm guessing that very few of you know that USB certifications impose a very strict limit on a USB connected device's inrush current.

What's the inrush current? 

It is basically the current that a device receives when it is connected to a USB port. It is usually high given the numerous decoupling capacitors on a board.

How to measure it?

One of the solutions is to do like I did: put a shunt resistor between your device and power supply, measure the voltage across it. As I'm sure you already know, it will represent the current going through it.

What should I see above?

This is the voltage across the shunt resistor when the device is first plugged to my computer. The USB certification specifies a maximum 50uC charge (Q = I*dt integrated) when the current is above 100mA.

Looking at the oscilloscope you can see a 500mV vertical axis, corresponding to I = V / R = 500/1.5 = 333mA. The horizontal axis is 20us. 

What we are going to do simply count the number of "blocks" for given time bins (red zone). Let's say we have 5 time bins here: 

- first time bin has 4 blocks (1.333A)

- second has 3 blocks (1A)

- third has 2 (0.666A)

- 4th and 5th have 1 (0.333A)

Yes, this is a very approximate measurement ;). Anyway, this gives us: (1.33+1+0.66+0.33+0.33)*20us = 53uC!

This is therefore very close to the norm.... we should reduce one of our capacitors' value then!