So I got a circuit and a regulator for atmega 328p but I got a 6v battery, the minimum input voltage of the regulator is 6.5v and with the 6v battery I use i get from the 5v regulator only 4.2~4.5v output. As for the amtega328 i use a 16Mhz crystal which as i understand needs 5v, I use as well few leds (5x leds) just for blinking and a temperature sensor a4,a5. Is there any problem that my atmega get's lower than 5v input voltage ? or everything is fine?
The regulator is not functioning within its limits.
I have placed a silicon diode (.6 or .7 volts) in series with a 6 volt battery to get it to 6 - .6 = 5.4v in the past.
Then I connected it to the 5 volt pin.
You may want to look at a switching DC to DC power supply.
example:
Here
Domino60:
i use a 16Mhz crystal which as i understand needs 5v
Section 29.3 on page 303 of the datasheet shows that there's a relationship between input voltage and maximum clock speed so you can calculate the minimum voltage for any particular crystal.
Between 2.7V (10MHz) and 4.5V (20MHz) the relationship is linear, so you can work out the safe minimum voltage for 16Mhz from there. 16MHz minimum safe voltage should be 6/10 the way between 2.7V and 4.5V.
(6 * 0.18V) + 2.7V = 3.78V
So according to the datasheet and my pre-caffiene (therefore risky!) maths, using Larry's suggestion above you can start with fresh cells on 5.4V (still under the max 5.5V operating Voltage for the ATmega328P) and your power can sag all the way to 3.78V before it's out of spec. Of course by then the rest of your circuit might not be operating as you expect, but the ATmega will still be fine.
Cheers!
Geoff
So you guys mean that I can work fine with my regulator giving to atmega approx 4.2~4.5v instead of stable 5v ? Well that's cool for me, that's what I wanted to hear.
Of course by then the rest of your circuit might not be operating as you expect, but the ATmega will still be fine.
What exactly will not work? because I have tons of components 
You may want to look at a switching DC to DC power supply.
of curse i would like one and i have one but it's a step down i will order one in the future but
for the exact circuit im working with I got only 3days to finish my project. So there is no time for orders right now
Domino60:
What exactly will not work? because I have tons of components
Components that require the full 5v and won't work at 4.2 or w/e it works out to.
Go check the datasheets for operating voltage of every device that has a Vcc and Gnd pin, make sure they'll be in spec, and also check the gate voltage spec of any MOSFETs.
You know your device better than we do, so this is really your job to figure out. If you have a bench supply, I suggest running it off a worst case and best case voltage during testing.
Also - 1117-series regulator? ZLDO1117 is a little better than the one you're using I think, in terms of dropout. And better LDO regulators exist, with lower dropout still - you can get 5 from 6 with a good LDO no problem. Though I think we've already established that better parts which you don't have time to obtain would solve your problem.