# setbaud.h

/* Copyright (c) 2007  Cliff Lawson

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* Redistributions in binary form must reproduce the above copyright
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* Neither the name of the copyright holders nor the names of
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from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE. */

/* $Id$ */

/**
\file
*/

/**
\defgroup util_setbaud <util/setbaud.h>: Helper macros for baud rate calculations
\code
#define F_CPU 11059200
#define BAUD 38400
#include <util/setbaud.h>
\endcode

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:

\code
#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
}
\endcode

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).
*/

#ifndef F_CPU
#  error "setbaud.h requires F_CPU to be defined"
#endif

#ifndef BAUD
#  error "setbaud.h requires BAUD to be defined"
#endif

#if !(F_CPU)
#  error "F_CPU must be a constant value"
#endif

#if !(BAUD)
#  error "BAUD must be a constant value"
#endif

#if defined(__DOXYGEN__)
/**
\def BAUD_TOL
\ingroup util_setbaud

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 BAUD_TOL 2

/**
\def UBRR_VALUE
\ingroup util_setbaud

Output macro from <util/setbaud.h>

Contains the calculated baud rate prescaler value for the UBRR
register.
*/
#define UBRR_VALUE

/**
\def UBRRL_VALUE
\ingroup util_setbaud

Output macro from <util/setbaud.h>

Contains the lower byte of the calculated prescaler value
(UBRR_VALUE).
*/
#define UBRRL_VALUE

/**
\def UBRRH_VALUE
\ingroup util_setbaud

Output macro from <util/setbaud.h>

Contains the upper byte of the calculated prescaler value
(UBRR_VALUE).
*/
#define UBRRH_VALUE

/**
\def USE_2X
\ingroup util_setbaud

Output bacro 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.
*/
#define USE_2X 0

#else /* !__DOXYGEN__ */

#undef USE_2X

/* Baud rate tolerance is 2 % unless previously defined */
#ifndef BAUD_TOL
#  define BAUD_TOL 2
#endif

#ifdef __ASSEMBLER__
#define UBRR_VALUE (((F_CPU) + 8 * (BAUD)) / (16 * (BAUD)) -1)
#else
#define UBRR_VALUE (((F_CPU) + 8UL * (BAUD)) / (16UL * (BAUD)) -1UL)
#endif

#if 100 * (F_CPU) > \
(16 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) + (BAUD) * (BAUD_TOL))
#  define USE_2X 1
#elif 100 * (F_CPU) < \
(16 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) - (BAUD) * (BAUD_TOL))
#  define USE_2X 1
#else
#  define USE_2X 0
#endif

#if USE_2X
/* U2X required, recalculate */
#undef UBRR_VALUE

#ifdef __ASSEMBLER__
#define UBRR_VALUE (((F_CPU) + 4 * (BAUD)) / (8 * (BAUD)) -1)
#else
#define UBRR_VALUE (((F_CPU) + 4UL * (BAUD)) / (8UL * (BAUD)) -1UL)
#endif

#if 100 * (F_CPU) > \
(8 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) + (BAUD) * (BAUD_TOL))
#  warning "Baud rate achieved is higher than allowed"
#endif

#if 100 * (F_CPU) < \
(8 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) - (BAUD) * (BAUD_TOL))
#  warning "Baud rate achieved is lower than allowed"
#endif

#endif /* USE_U2X */

#ifdef UBRR_VALUE
/* Check for overflow */
#  if UBRR_VALUE >= (1 << 12)
#    warning "UBRR value overflow"
#  endif

#  define UBRRL_VALUE (UBRR_VALUE & 0xff)
#  define UBRRH_VALUE (UBRR_VALUE >> 8)
#endif

#endif /* __DOXYGEN__ */
/* end of util/setbaud.h */