Hi list.
 
What is the reason that the TXC bit is not cleared when you write a byte into 
UDR?
I am using a mega169, but the other devices do probably behave similar.
 
The TXC bit is set when the last bit in the UART shift register is transmitted 
and UDR does not contain a byte to transmit. The bit is cleared then the 
transmit complete interrupts is executed and can be cleared manually in 
software. 
Why is it not cleared also when starting transmission of a new byte?
 
The datasheet does not mention this but states in the section on baud-rates 
that if you want to change baudrate you should manually clear the txc before 
writing to udr.
 
We had a bug on this which made me curious about the reasons for this design 
choice.
 
Best regards
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