Datasheet
177
ATmega325/3250/645/6450
2570K–AVR–04/07
• 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 cor-
rupted if the baud rate is changed. Writing UBRRL will trigger an immediate update of
the baud rate prescaler.
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 77.
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 170). The error values are cal-
culated using the following equation:
Error[%]
BaudRate
Closest Match
BaudRate
-------------------------------------------------------- 1–
⎝⎠
⎛⎞
100%•=
Table 77. Examples of UBRR Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
f
osc
= 1.0000 MHz f
osc
= 1.8432 MHz f
osc
= 2.0000 MHz
U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 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% 70.0%150.0%8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 50.0%110.0%6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 38.5%8-3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 28.5%6-7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 18.5%38.5%
76.8k – – 1 -18.6% 1 -25.0% 20.0%1 -18.6% 2 8.5%
115.2k – – 0 8.5% 0 0.0% 1 0.0% 08.5%18.5%
230.4k – – – – – – 00.0%––––
250k––––––––––00.0%
Max.
(1)
62.5 kbps 125 kbps 115.2 kbps 230.4 kbps 125 kbps 250 kbps
1. UBRR = 0, Error = 0.0%