200
7679H–CAN–08/08
AT90CAN32/64/128
• 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 – UBRRn11:0: USARTn Baud Rate Register
This is a 12-bit register which contains the USARTn baud rate. The UBRRnH contains the four
most significant bits, and the UBRRnL contains the eight least significant bits of the USARTn
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.
17.12 Examples of Baud Rate Setting
For standard crystal, resonator and external oscillator frequencies, the most commonly used
baud rates for asynchronous operation can be generated by using the UBRRn settings in Table
17-9 up to Table 17-12. 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 “Asynchronous Operational Range” on page 192). The error values are calcu-
lated using the following equation:
Note: 1. UBRRn = 0, Error = 0.0%
Error[%] 1
BaudRate
Closest Match
BaudRate
--------------------------------------------------------–
⎝⎠
⎛⎞
100%•=
Table 17-9. Examples of UBRRn Settings for Commonly Frequencies
Baud
Rate
(bps)
fclk
io
= 1.0000 MHz fclk
io
= 1.8432 MHz fclk
io
= 2.0000 MHz
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––––––00.0%––––
250k ––––––––––––
500k ––––––––––––
1M ––––––––––––
Max.
(1)
62.5 Kbps 125 Kbps 115.2 Kbps 230.4 Kbps 125 Kbps 250 Kbps
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