Datasheet
227
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
22.9.5 UBRRnL and UBRRnH – USART Baud Rate Registers
• 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 – 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.
22.10 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 22-9 to Table 22-12
on page 231. UBRR values which yield an actual baud rate differing less than 0.5% from the tar-
get 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 220). The error values are calculated using the fol-
lowing equation:
Bit 151413121110 9 8
– – – – UBRR[11:8] UBRRHn
UBRR[7:0] UBRRLn
76543210
Read/Write RRRRR/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
00000000
Error[%]
BaudRate
Closest Match
BaudRate
-------------------------------------------------------- 1–
100%=
Table 22-9. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
f
osc
= 1.0000MHz f
osc
= 1.8432MHz f
osc
= 2.0000MHz
U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4K 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%
19.2K 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%
28.8K 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%
38.4K 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%