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AT90USB646 Datasheet(PDF) 198 Page - ATMEL Corporation |
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AT90USB646 Datasheet(HTML) 198 Page - ATMEL Corporation |
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198 / 456 page  198 7593L–AVR–09/12 AT90USB64/128 • 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 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. 19.10 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 19-9 to Table 19-12 on page 201. UBRR values which yield an actual baud rate differing less than 0.5% from the tar- get 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 191). The error values are calculated using the fol- lowing equation: Error[%] BaudRate Closest Match BaudRate -------------------------------------------------------- 1 – ⎝⎠ ⎛⎞ 100% • = Table 19-9. 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 = 0U2Xn = 1 U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1 UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error 2400 250.2%510.2%47 0.0% 950.0%510.2% 103 0.2% 4800 120.2%250.2%23 0.0% 470.0%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) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps 1. UBRR = 0, Error = 0.0%. |
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