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ATmega48 Datasheet(PDF) 188 Page - ATMEL Corporation |
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ATmega48 Datasheet(HTML) 188 Page - ATMEL Corporation |
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188 / 340 page  188 7530H–AVR–02/09 ATmega48/88/168 Automotive 17.9.5 USART Baud Rate Registers – UBRRnL and UBRRnH • 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 asyn- chronous 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 resis- tance when the error ratings are high, especially for large serial frames (see “Asynchronous Operational Range” on page 181). The error values are calculated using the following equation: Bit 15 14 13121110 9 8 –––– UBRRn[11:8] UBRRnH UBRRn[7:0] UBRRnL 76543210 Read/Write RRRR R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 00000000 00000000 Error[%] BaudRate Closest Match BaudRate -------------------------------------------------------- 1 – ⎝⎠ ⎛⎞ 100% • = Table 17-9. 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 = 1U2Xn = 0U2Xn = 1U2Xn = 0U2Xn = 1 UBRR nError UBRR nError UBRR nError UBRR nError UBRR nError UBRR nError 2400 250.2%510.2%470.0%950.0%510.2% 103 0.2% 4800 120.2%250.2%230.0%470.0%250.2%510.2% 9600 6 -7.0% 120.2%110.0%230.0%120.2%250.2% 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% |
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