I often read that it is good practice to connect VCC with AVCC. Even in the ATMega8 datasheet it says so:
AVCC is the supply voltage pin for the A/D Converter, Port C (3..0),
and ADC (7..6). It should be externally connected to VCC, even if the
ADC is not used. If the ADC is used, it should be connected to VCC
through a low-pass filter. Note that Port C (5..4) use digital supply
voltage, VCC.
But nowhere I can find an explanation as to why they have to be connected.
A simple circuit for blinking a LED works without connecting VCC and AVCC.
Do I just have to accept it, or is there a good reason?
Best Answer
Mainly, it has to be connected because the manufacturer says it should.
Aside from that, they should for full operation of the chip (all ports/pins), to prevent floating pin issues on the AVCC side, to prevent noise on the digital side. There are issues where leaving the AVCC side unpowered causes parasitic power draw and can destablize the internal clock, or can prevent stable startup.
Atmel designers have decided that having a separate Analog VCC and Ground is the best way of allowing relatively noise free analog section, by allowing users to add filtering and separation of the Digital and Analog Planes, even inside the ATmega. It's not just the ATMega8, afaik all ATMegas and even some ATTinys have this design.