At the beginning episode "Treachery" (SE2.EP3), we can hear a monologue from King Ecbert about the late Emperor Charlemagne, further in the episode there is another reference to the Frankish Emperor witnessing a Viking raid.
However Charlemagne was very much alive at the time the episode is taking place, in the early 800s. Ragnar and Horik battle Jarl Borg and Rollo in 796; judging by both Bjorn's and by his half brothers'respective ages, no more than 10 years have passed which places us well before 814, the time of Charlemagne's death.