Disable RDRF assertion due to partially filled FIFO when receiver is idle.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 1 character.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 2 characters.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 4 characters.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 8 characters.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 16 characters.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 32 characters.
Enable RDRF assertion due to partially filled FIFO when receiver is idle for 64 characters.