The minimum system circuit of a microcontroller is a series of minimal so the microcontroller can work.
ATmegga AVR chip is equipped with an internal oscillator and thus no need external crystal for CPU clock source in order to save costs.
AVR microcontroller minimum system is very simple, just connect the VCC and AVCC to the DC +5V power supply voltage. Then GND and AGND with the ground. No need an additional crystal and reset pin is not connected to anything. With this circuit, AVR microcontroller has been able to work normally, but you can not reset the microcontroller manually. If you want to activate the reset pin with a button to trigger it, then connect the reset pin on your microcontroller with pull circuit made up of a 10k resistor and capacitor 10uF.