The PV is specified for 4MHz at Vcc=1.8V and 10MHz for Vcc=2.7-5.5V.
The P is specified for 10MHz for Vcc=2.7-5.5V and 20MHz at Vcc=4.5V-5.5V.
I believe that the 644 has a single UART versus two for the 644P.
I am not sure if there are other differences between P and non-P.
Unless you are running at voltage levels below 2.7V I would go with
the ATmega644P.
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