NAME

       util_setbaud - <util/setbaud.h>: Helper macros for baud rate calculations

SYNOPSIS

   Macros
       #define BAUD_TOL   2
       #define UBRR_VALUE
       #define UBRRL_VALUE
       #define UBRRH_VALUE
       #define USE_2X   0

Detailed Description

       #define F_CPU 11059200
       #define BAUD 38400
       #include <util/setbaud.h>

       This header file requires that on entry values are already defined for F_CPU and BAUD. In addition, the
       macro BAUD_TOL will define the baud rate tolerance (in percent) that is acceptable during the
       calculations. The value of BAUD_TOL will default to 2 %.

       This header file defines macros suitable to setup the UART baud rate prescaler registers of an AVR. All
       calculations are done using the C preprocessor. Including this header file causes no other side effects
       so it is possible to include this file more than once (supposedly, with different values for the BAUD
       parameter), possibly even within the same function.

       Assuming that the requested BAUD is valid for the given F_CPU then the macro UBRR_VALUE is set to the
       required prescaler value. Two additional macros are provided for the low and high bytes of the prescaler,
       respectively: UBRRL_VALUE is set to the lower byte of the UBRR_VALUE and UBRRH_VALUE is set to the upper
       byte. An additional macro USE_2X will be defined. Its value is set to 1 if the desired BAUD rate within
       the given tolerance could only be achieved by setting the U2X bit in the UART configuration. It will be
       defined to 0 if U2X is not needed.

       Example usage:

       #include <avr/io.h>

       #define F_CPU 4000000

       static void
       uart_9600(void)
       {
       #define BAUD 9600
       #include <util/setbaud.h>
       UBRRH = UBRRH_VALUE;
       UBRRL = UBRRL_VALUE;
       #if USE_2X
       UCSRA |= (1 << U2X);
       #else
       UCSRA &= ~(1 << U2X);
       #endif
       }

       static void
       uart_38400(void)
       {
       #undef BAUD  // avoid compiler warning
       #define BAUD 38400
       #include <util/setbaud.h>
       UBRRH = UBRRH_VALUE;
       UBRRL = UBRRL_VALUE;
       #if USE_2X
       UCSRA |= (1 << U2X);
       #else
       UCSRA &= ~(1 << U2X);
       #endif
       }

       In this example, two functions are defined to setup the UART to run at 9600 Bd, and 38400 Bd,
       respectively. Using a CPU clock of 4 MHz, 9600 Bd can be achieved with an acceptable tolerance without
       setting U2X (prescaler 25), while 38400 Bd require U2X to be set (prescaler 12).

Macro Definition Documentation

   #define BAUD_TOL   2
       Input and output macro for <util/setbaud.h>

       Define the acceptable baud rate tolerance in percent. If not set on entry, it will be set to its default
       value of 2.

   #define UBRR_VALUE
       Output macro from <util/setbaud.h>

       Contains the calculated baud rate prescaler value for the UBRR register.

   #define UBRRH_VALUE
       Output macro from <util/setbaud.h>

       Contains the upper byte of the calculated prescaler value (UBRR_VALUE).

   #define UBRRL_VALUE
       Output macro from <util/setbaud.h>

       Contains the lower byte of the calculated prescaler value (UBRR_VALUE).

   #define USE_2X   0
       Output macro from <util/setbaud.h>

       Contains the value 1 if the desired baud rate tolerance could only be achieved by setting the U2X bit in
       the UART configuration. Contains 0 otherwise.

Author

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