I would like to power 328p with two coin cells. Together, they give 6.5V. That voltage will eventually go down as they drain up. But, should i put some voltage regulator? I thought to put AMS1117 5.0 but, as i can see, VIN must be 6.5v. Cells gives that voltage now, but later it could be 5V. Not sure how to solve it, or to leave it as is. Atmega can run on 5.5V. It runs with 6.5V as well, but not sure how long. 
Thanks in advance.
I use these: https://smile.amazon.com/gp/product/B0758ZTS61/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1 for my battery projects.
I am sure they can address your concerns, as per the above posting.
Interesting...
I have a few these:
Input 4,5V -5V
Output 1V - 17V
And way cheaper. Should they do the job?
who_took_my_nick:
I
And way cheaper. Should they do the job?
Your decision.
Coin cells are able to provide only limited amount of current. Both peak and total. Using a switching regulator will probably break your budget.
Cheaper/simpler/longer battery life: use a single coin cell & run the ATmega off the internal 8 MHz oscillator.