when data is received serially in which register it is saved
Which interface? U(S)ART, TWI, SPI?
You need to provide a little more detail on the problem if you want to get help
Subject line says USART, Axos.
Still, no mention of AVR model AFAICS, and as register names vary between models we a more or less stuck anyway.
What did the data sheet tell you?
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Oh right... Wasn't paying attention.
If there is only one USART, it is usually named UDR (Usart Data Register), but you better check the datasheets USART section.
AVR Datasheets are a gem, very good, and clear documentation. So don't be afraid, you won't have to read all the 700 pages, just the ones related. Also check the Register Table at the end.
If only every datasheet were just like that...
If there is only one USART, it is usually named UDR
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I stand corrected
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