Does ATTiny85 reset when power is disconnected and then reapplied?

Hi, I am making a PCB for the ATTiny85 microcontroller for a small project. I am wondering if the ATTiny85 resets when power is disconnected and then reapplied? If not, how do I reset the it?

Yes, the ATtiny automatically performs a reset when the operating voltage is applied.
Important, so that the ATtiny starts up at all, a pull-up resistor from 10 kOhm to VCC (+ 5V) must be set from Pin1 (Real Pin1 on the IC). The real pin 1 is the reset pin, which must NOT hang in the air, but must be HIGH during operation. A LOW pulse on Pin1 also triggers a reset.

All AVR parts perform a power-on reset (POR) when the supply voltage rises above the power-on reset threshold (typ 1.4V for recent production t85's, 1V for old ones), after having been lower than the power on reset activation/rearm voltage (typ 1.3V for recent production, 0.9V for old ones - they switched the t85 on Rev. C and later; note that there is a typo in the datasheet, which states that the old values apply to Rev. B and newer, that should be Rev. B and older, which should be apparent from context). This is described in the System and Reset Characteristics section in Electrical Characteristics chapter of the datasheet. If enabled, BrownOut Detection (BOD) can hold the chip ins reset up to 1.8, 2.7 or 4.3 volts, which correspond to the specified minimum voltages for operation at up to 4, 10 or 20 MHz.

The 10k pullup on reset is best practice; it is not required, as the internal 30k pullup is always enabled on the reset pin when it is acting as reset. The reason for a stronger pullup is for improved noise rejection (ie, preventing unwanted resets from electromagnetic noise picked up, which is most likely when there are long wires tied to the reset pin (though a 0.1uF capacitor to ground, or both together, is even better).

I'm not sure how the "reset pin must not be left floating" myth has become so common here - yet people recommend the 10k resistor and not the 0.1uF cap - the cap of course can't be used if you've got an autoreset circuit to reset upon opening a serial port, but that's almost unheard of on a t85 since it doesn't have a proper serial port (software serial is possible, but rarely used on parts with so few pins anyway)

@DrAzzy, thanks for the info.

Ok, I haven't thought of that now, but I'm still installing the 10K in principle. If I have additional IC's on a board that also need a power on reset, of course the capacitor to GND is also added.