ATmega8U2/16U2/32U2
18.11.5 UCSRnD – USART Control and Status Register n D
Bit
7
-
6
-
5
-
4
-
3
-
2
-
1
CTSEN
R/W
0
0
RTSEN
R/W
0
UCSRnD
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
• Bits 1 – CTSEN : USART CTS Enable
Set this bit to one by firmware to enable the transmission flow control (CTS). Transmission is
allowed if CTS = 0.
Set this bit to zero by firmware to disable the transmission flow control (CTS). Transmission is
always allowed.
• Bits 0 – RTSEN : USART RTS Enable
Set this bit to one by firmware to enable the receive flow control (RTS).
Set this bit to zero by firmware to disable the receive flow control (RTS).
18.11.6 UBRRnL and UBRRnH – USART Baud Rate Registers
Bit
15
14
13
12
11
10
9
8
–
–
–
–
UBRR[11:8]
UBRRnH
UBRRnL
UBRR[7:0]
7
R
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
R/W
0
0
0
0
0
0
0
0
0
• Bit 15:12 – 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 – UBRR[11:0]: 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.
18.12 Examples of Baud Rate Setting
For standard crystal and resonator frequencies, the most commonly used baud rates for asyn-
chronous operation can be generated by using the UBRR settings in Table 18-9 to Table 18-12.
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 Receiver will have less noise
resistance when the error ratings are high, especially for large serial frames (see “Asynchronous
Operational Range” on page 163). The error values are calculated using the following equation:
BaudRateClosest Match
Error[%] = ------------------------------------------------------- – 1 100%
BaudRate
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7799D–AVR–11/10
ATMEL [ ATMEL ]
