Datasheet
165
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
• 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 UBRRnH
is written.
• Bit 11:0 – UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRnH contains the four most significant bits, and the
UBRRnL 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 UBRRnL will trigger an immediate update of the baud rate
prescaler.
17.10 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 UBRRn settings in Table 17-9. UBRRn 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 Section 17.7.3 “Asynchronous Operational
Range” on page 159). The error values are calculated using the following equation:
Table 17-9. 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
UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn Error UBRRn 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%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%
76.8k – – 1 –18.6% 1 –25.0% 2 0.0% 1 –18.6% 2 8.5%
115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%
230.4k – – – – – – 0 0.0% – – – –
250k – – – – – – – – – – 0 0.0%
Max.
(1)
62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps
Note: 1. UBRRn = 0, error = 0.0%
Error[%]
BaudRate
Closest Match
BaudRate
--------------------------------------------------
1–
100%•=