Я прочитал в мануале на ATxmega256A3U/ATxmega192A3U/ATxmega128A3U/ATxmega64A3U Таблица в разделе Electrical Characteristics->Clock and Oscillator Characteristics->External clock characteristics
Там и написано, на чистом англицком VCC= 2.7 - 3.6V - 0...32 MHz. И ниже приписочка Note: 1. The maximum frequency vs. supply voltage is linear between 1.8V and 2.7V, and the same applies for all other parameters with supply voltage conditions.
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