When it goes low again, the echo has been received. Since sound travels around 1100 feet per second you don't have to be overly accurate in sampling the pin, although doing it with an interrupt service would induce less error. 20-30uS would be unnoticeable for most hobby robotics uses, as it's less than a half inch error. It uses around a 300-400V pulse burst to ping the transducer, and will sting if you get hit by it. And, no guarantees, YMMV, LTBB, other legal mumbo-jumbo.
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