Jon

Possibly a stupid question, but is your multimeter from a reputable manufacturer?

What is the output impedance of the voltage you are measuring?

@Andyaka The volt-turn thing comes from Faraday's law of induction. That's what sets the number of secondary turns for a given core flux, area and freq.

Note that the USART won't start transmitting until a clock edge, and the clock it uses is pre-scaled down significantly from what the CPU uses. So depending where it is in it's internal cycle you'll expect a variable delay between when the CPU asserts PA.4 and the USART starts sending the byte.