# Safe clock speeds for ATmega328P at various VCC voltages

Every time somebody asks how fast they can clock an ATmega328P with VCC at 3v3 (or whatever) I return to figure 28-1 in the data sheet only to find that it's impossible to really say because there are no lines for the various values.

So I updated the diagram.

The most common non-5v voltage is of course 3v3 and as you can see at 3v3 the chip is only spec'd at just over 13MHz, people do often run them at 16MHz but that is well out of spec. That's ok if you are playing around, making a widget for yourself etc, but it's not ok for a commercial product, especially if it will be subject to large temperature variations.

Rob

Nice. Thanks.

Is that for an ATmega328 or an ATmega328P ?

P

Title updated.

Rob

Datasheet sez, “As shown in Figure 29-1, the Maximum Frequency vs. VCC curve is linear between 1.8V < VCC < 2.7V and between 2.7V < VCC < 4.5V.”

So two straight lines that intersect at (2.7V, 10MHz). When operating between 1.8V and 2.7V, fmax = (20/3)v - 8 and for 2.7V to 4.5V, fmax = (50/9)v - 5. For Vcc ? 4.5V, fmax = 20.

I figured someone would come up with the formulas :)

I'm more a visual kind of guy myself, but either way hopefully this is covered if people do a search in future.

Rob