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ATmega48 Datasheet(PDF) 168 Page - ATMEL Corporation |
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ATmega48 Datasheet(HTML) 168 Page - ATMEL Corporation |
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168 / 340 page  168 7530H–AVR–02/09 ATmega48/88/168 Automotive Table 17-1 contains equations for calculating the baud rate (in bits per second) and for calculat- ing the UBRRn value for each mode of operation using an internally generated clock source. Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps) BAUD Baud rate (in bits per second, bps) f OSC System Oscillator clock frequency UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095) Some examples of UBRRn values for some system clock frequencies are found in Table 17-9 (see page 188). 17.2.2 Double Speed Operation (U2Xn) The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect for the asynchronous operation. Set this bit to zero when using synchronous operation. Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer rate for asynchronous communication. Note however that the Receiver will in this case only use half the number of samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate baud rate setting and system clock are required when this mode is used. For the Transmitter, there are no downsides. Table 17-1. Equations for Calculating Baud Rate Register Setting Operating Mode Equation for Calculating Baud Rate(1) Equation for Calculating UBRRn Value Asynchronous Normal mode (U2Xn = 0) Asynchronous Double Speed mode (U2Xn = 1) Synchronous Master mode BAUD fOSC 16 UBRRn 1 + () ------------------------------------------ = UBRRn fOSC 16 BAUD ------------------------ 1 – = BAUD fOSC 8 UBRRn 1 + () --------------------------------------- = UBRRn fOSC 8 BAUD -------------------- 1 – = BAUD fOSC 2 UBRRn 1 + () --------------------------------------- = UBRRn fOSC 2 BAUD -------------------- 1 – = |
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