Is it possible to measure the current by ATtiny13 ?

Other Hardware General Electronics
1

I would like to using ATtiny13A or 85 to measure how much current pass through the USB adaptor to USB power bank.

Aim: cut off by trisistor when USB current below 100mA(or 50mA).

2

Use a current sensor.

3

So if you cut it off when the current falls below 100 mA, how does it turn on again?

(Sort of XY problem. The question is why would you want to do this? It's what "power banks" do, but you have to have a push button to start them up)