Minimal ATMega328P - common error

Minimal ATMega328P - common error  - won't run at 3V3.

I've started using ATMega328P as a minimal version  of the Arduino Pro Mini operating a 3Vdc.
I have been breadboarding circuits using the various articles published on the Internet as a guide for the minimal wiring required for running an ATMega328P as an Arduino.

The problem I suffered was that the circuits would happily run at 5Vdc, but stop running as the voltage was reduced to 3.3Vdc.

There seems to be a lot of discussion about the problem on forums; with almost none of it relevant to me.

Eventually I realised the silly mistake that I was making, by taking minimal circuits from Internet without checking the details for myself.

Many articles for minimal ATMega circuits leave AVCC open, rather than tying it to VCC.
If AVCC is left open, the ATMega will work at 5Vdc but will stop working at around 3.5Vdc.
Once connected to VCC, the ATMega will work below 3Vdc.

In my case, it worked down to about 2.7Vdc, which is one of the settings for the Brown Out Detector, so I suspect, that is what has come into play.