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ATMEGA649 Datasheet(PDF) 183 Page - ATMEL Corporation |
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ATMEGA649 Datasheet(HTML) 183 Page - ATMEL Corporation |
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183 / 375 page  183 ATmega329/3290/649/6490 2552H–AVR–11/06 • 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. 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 85. 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 176). The error values are cal- culated using the following equation: Error[%] BaudRate Closest Match BaudRate -------------------------------------------------------- 1 – ⎝⎠ ⎛⎞ 100% • = Table 85. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies Baud Rate (bps) fosc = 1.0000 MHz fosc = 1.8432 MHz fosc = 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% 70.0% 15 0.0% 8 -3.5% 16 2.1% 19.2k 2 8.5% 6 -7.0% 50.0% 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% 20.0% 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 ––– ––– – – – – 0 0.0% Max. (1) 62.5 kbps 125 kbps 115.2 kbps 230.4 kbps 125 kbps 250 kbps 1. UBRR = 0, Error = 0.0% |
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