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ATmega644PA Datasheet(PDF) 176 Page - ATMEL Corporation |
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ATmega644PA Datasheet(HTML) 176 Page - ATMEL Corporation |
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176 / 468 page  176 8152G–AVR–11/09 ATmega164PA/324PA/644PA/1284P f OSC System Oscillator clock frequency UBRRn Contents of the UBRRHn and UBRRLn Registers, (0-4095) Some examples of UBRRn values for some system clock frequencies are found in Table 18-9 on page 196. 18.4.2 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. 18.4.3 External Clock External clocking is used by the synchronous slave modes of operation. The description in this section refers to Figure 18-2 on page 175 for details. External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance of meta-stability. The output from the synchronization register must then pass through an edge detector before it can be used by the Transmitter and Receiver. This process intro- Table 18-1. Equations for Calculating Baud Rate Register Setting Operating Mode Equation for Calculating Baud Rate(1) Equation for Calculating UBRR Value Asynchronous Normal mode (U2Xn = 0) Asynchronous Double Speed mode (U2Xn = 1) Synchronous Master mode BAUD f OSC 16 UBRRn 1 + () ------------------------------------------ = UBRRn f OSC 16BAUD ------------------------ 1 – = BAUD f OSC 8 UBRRn 1 + () --------------------------------------- = UBRRn f OSC 8BAUD -------------------- 1 – = BAUD f OSC 2 UBRRn 1 + () --------------------------------------- = UBRRn f OSC 2BAUD -------------------- 1 – = |
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