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AT90USB646 Datasheet(PDF) 204 Page - ATMEL Corporation |
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AT90USB646 Datasheet(HTML) 204 Page - ATMEL Corporation |
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204 / 434 page  204 7593A–AVR–02/06 AT90USB64/128 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. 18.10 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 18-9 to Table 18-12. 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 197). The error values are calculated using the following equation: Error[%] BaudRate Closest Match BaudRate -------------------------------------------------------- 1 – ⎝⎠ ⎛⎞ 100% • = Table 18-9. 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 = 0U2Xn = 1 U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1 UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error 2400 250.2%510.2%470.0% 95 0.0%510.2% 103 0.2% 4800 120.2%250.2%230.0% 47 0.0%250.2%510.2% 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) 62.5 kbps 125 kbps 115.2 kbps 230.4 kbps 125 kbps 250 kbps 1. UBRR = 0, Error = 0.0% |
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