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AVR Baud Rate Calculations

Posted by Bob Paddock on January 30, 2005

http://www.freertos.org/a00098.html

Says:

"RTOS Demo application serial port driver

The serial port driver included with the demo application uses the calculation detailed in the AVR ATMega323 manual to set the baud rate registers. At some baud rates I have found it necessary to adjust the calculated setting slightly. I suspect this is due to an inaccuracy in the 8MHz crystal installed
in my prototyping board."

The error my not be your crystal, but an issue of rather the math is truncated or rounded. I find that when using a 3.6864 MHz the baud rate calculations would frequently be off by a count of one. Neil Johnsons on the AVR-GCC list had the solution that I have been using, and has seemed to work
with all CPU frequencies and baud rates that I have needed so far:

/*
* #define UART_BAUD_SELECT (F_CPU/(UART_BAUD_RATE*16L)-1)
*
* When doing integer divide it is usually better to round to the nearest
* integer, rather than to the lowest (which the above expression does).
*
* Add 0.5 (i.e. half the value of the denominator)
* to the numerator before the division. This can be achieved in the
* following way:
*
* #define UART_BAUD_SELECT ((F_CPU + UART_BAUD_RATE * 8L) /\
* (UART_BAUD_RATE * 16L) - 1)
* - Neil Johnson AVR-GCC List
*/

#define UART_BAUD_CALCULATE(UART_BAUD_RATE) ((F_CPU + UART_BAUD_RATE * 8L) / (UART_BAUD_RATE * 16L) - 1)

#define BAUD_RATE UART_BAUD_CALCULATE( 9600 )



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