The maximum output frequency vs. supply voltage is linear between 1.8V and 2.7V, and can never be higher than four times the maximum CPU frequency.
33.4.13.6 External clock characteristics
Figure 33-24.External clock drive waveform.
Table 33-111.External clock used as system clock without prescaling.
Note:
1.
The maximum frequency vs. supply voltage is linear between 1.6V and 2.7V, and the same applies for all other parameters with supply voltage conditions.