• 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).
Table 67. UCPOL Bit Settings
Transmitted Data Changed (Output of
UCPOL TxD Pin)
Received Data Sampled (Input on
RxD Pin)
0
1
Rising XCK Edge
Falling XCK Edge
Falling XCK Edge
Rising XCK Edge
USART Baud Rate Registers –
UBRRL and UBRRH
Bit
15
14
13
12
11
10
9
8
URSEL
–
–
–
UBRR[11:8]
UBRRH
UBRRL
UBRR[7:0]
7
R/W
R/W
0
6
R
5
R
4
R
3
R/W
R/W
0
2
R/W
R/W
0
1
R/W
R/W
0
0
R/W
R/W
0
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
0
0
0
0
0
0
0
0
The UBRRH Register shares the same I/O location as the UCSRC Register. See the
“Accessing UBRRH/ UCSRC Registers” on page 158 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 8 least significant bits of the
USART baud rate. Ongoing transmissions by the transmitter and receiver will be cor-
rupted if the baud rate is changed. Writing UBRRL will trigger an immediate update of
the baud rate prescaler.
164
ATmega32(L)
2503J–AVR–10/06
ATMEL [ ATMEL ]
