Datasheet
147
ATmega8A [DATASHEET]
8159E–AVR–02/2013
• Bit 0 – UCPOL: Clock Polarity
This bit is used for Synchronous mode only. Write this bit to zero when Asynchronous mode is used. The UCPOL
bit sets the relationship between data output change and data input sample, and the synchronous clock (XCK).
20.10.5 UBRRL and UBRRH – USART Baud Rate Registers
The UBRRH Register shares the same I/O location as the UCSRC Register. See the “Accessing UBRRH/UCSRC
Registers” on page 142 section which describes how to access this register.
• Bit 15 – URSEL: Register Select
This bit selects between accessing the UBRRH or the UCSRC Register. It is read as zero when reading UBRRH.
The URSEL must be zero when writing the UBRRH.
• Bit 14:12 – Reserved Bits
These bits are reserved for future use. For compatibility with future devices, these bit must be written to zero when
UBRRH is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four most significant bits,
and the UBRRL contains the eight least significant bits of the USART baud rate. Ongoing transmissions by the
Transmitter and Receiver will be corrupted if the baud rate is changed. Writing UBRRL will trigger an immediate
update of the baud rate prescaler.
20.11 Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous operation
can be generated by using the UBRR settings in Table 20-9. UBRR values which yield an actual baud rate differing
less than 0.5% from the target baud rate, are bold in the table. Higher error ratings are acceptable, but the
100Reserved
101Reserved
110Reserved
1119-bit
Table 20-8. UCPOL Bit Settings
UCPOL Transmitted Data Changed (Output of TxD Pin) Received Data Sampled (Input on RxD Pin)
0 Rising XCK Edge Falling XCK Edge
1 Falling XCK Edge Rising XCK Edge
Table 20-7. UCSZ Bits Settings (Continued)
UCSZ2 UCSZ1 UCSZ0 Character Size
Bit 151413121110 9 8
URSEL – – – UBRR[11:8] UBRRH
UBRR[7:0] UBRRL
76543210
Read/Write R/W R R R R/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
00000000