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include
util
setbaud.h
Go to the documentation of this file.
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/* Copyright (c) 2007 Cliff Lawson
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Copyright (c) 2007 Carlos Lamas
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All rights reserved.
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions are met:
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* Redistributions of source code must retain the above copyright
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notice, this list of conditions and the following disclaimer.
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* Redistributions in binary form must reproduce the above copyright
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notice, this list of conditions and the following disclaimer in
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the documentation and/or other materials provided with the
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distribution.
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* Neither the name of the copyright holders nor the names of
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contributors may be used to endorse or promote products derived
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from this software without specific prior written permission.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
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LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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POSSIBILITY OF SUCH DAMAGE. */
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/* $Id: setbaud_8h_source.html,v 1.1.1.7 2022/01/29 09:21:58 joerg_wunsch Exp $ */
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/**
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\file
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*/
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/**
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\defgroup util_setbaud <util/setbaud.h>: Helper macros for baud rate calculations
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\code
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#define F_CPU 11059200
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#define BAUD 38400
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#include <util/setbaud.h>
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\endcode
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This header file requires that on entry values are already defined
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for F_CPU and BAUD. In addition, the macro BAUD_TOL will define
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the baud rate tolerance (in percent) that is acceptable during
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the calculations. The value of BAUD_TOL will default to 2 %.
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This header file defines macros suitable to setup the UART baud
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rate prescaler registers of an AVR. All calculations are done
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using the C preprocessor. Including this header file causes no
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other side effects so it is possible to include this file more than
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once (supposedly, with different values for the BAUD parameter),
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possibly even within the same function.
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Assuming that the requested BAUD is valid for the given F_CPU then
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the macro UBRR_VALUE is set to the required prescaler value. Two
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additional macros are provided for the low and high bytes of the
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prescaler, respectively: UBRRL_VALUE is set to the lower byte of
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the UBRR_VALUE and UBRRH_VALUE is set to the upper byte. An
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additional macro USE_2X will be defined. Its value is set to 1 if
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the desired BAUD rate within the given tolerance could only be
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achieved by setting the U2X bit in the UART configuration. It will
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be defined to 0 if U2X is not needed.
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Example usage:
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\code
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#include <avr/io.h>
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#define F_CPU 4000000
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static void
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uart_9600(void)
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{
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#define BAUD 9600
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#include <util/setbaud.h>
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UBRRH = UBRRH_VALUE;
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UBRRL = UBRRL_VALUE;
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#if USE_2X
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UCSRA |= (1 << U2X);
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#else
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UCSRA &= ~(1 << U2X);
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#endif
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}
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static void
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uart_38400(void)
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{
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#undef BAUD // avoid compiler warning
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#define BAUD 38400
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#include <util/setbaud.h>
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UBRRH = UBRRH_VALUE;
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UBRRL = UBRRL_VALUE;
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#if USE_2X
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UCSRA |= (1 << U2X);
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#else
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UCSRA &= ~(1 << U2X);
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#endif
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}
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\endcode
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In this example, two functions are defined to setup the UART
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to run at 9600 Bd, and 38400 Bd, respectively. Using a CPU
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clock of 4 MHz, 9600 Bd can be achieved with an acceptable
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tolerance without setting U2X (prescaler 25), while 38400 Bd
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require U2X to be set (prescaler 12).
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*/
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#ifndef F_CPU
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# error "setbaud.h requires F_CPU to be defined"
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#endif
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#ifndef BAUD
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# error "setbaud.h requires BAUD to be defined"
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#endif
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#if !(F_CPU)
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# error "F_CPU must be a constant value"
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#endif
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#if !(BAUD)
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# error "BAUD must be a constant value"
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#endif
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#if defined(__DOXYGEN__)
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/**
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\def BAUD_TOL
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\ingroup util_setbaud
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Input and output macro for <util/setbaud.h>
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Define the acceptable baud rate tolerance in percent. If not set
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on entry, it will be set to its default value of 2.
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*/
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#define BAUD_TOL 2
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/**
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\def UBRR_VALUE
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\ingroup util_setbaud
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Output macro from <util/setbaud.h>
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Contains the calculated baud rate prescaler value for the UBRR
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register.
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*/
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#define UBRR_VALUE
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/**
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\def UBRRL_VALUE
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\ingroup util_setbaud
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Output macro from <util/setbaud.h>
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Contains the lower byte of the calculated prescaler value
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(UBRR_VALUE).
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*/
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#define UBRRL_VALUE
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/**
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\def UBRRH_VALUE
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\ingroup util_setbaud
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Output macro from <util/setbaud.h>
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Contains the upper byte of the calculated prescaler value
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(UBRR_VALUE).
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*/
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#define UBRRH_VALUE
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/**
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\def USE_2X
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\ingroup util_setbaud
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Output macro from <util/setbaud.h>
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Contains the value 1 if the desired baud rate tolerance could only
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be achieved by setting the U2X bit in the UART configuration.
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Contains 0 otherwise.
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*/
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#define USE_2X 0
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#else
/* !__DOXYGEN__ */
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#undef USE_2X
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/* Baud rate tolerance is 2 % unless previously defined */
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#ifndef BAUD_TOL
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# define BAUD_TOL 2
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#endif
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#ifdef __ASSEMBLER__
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#define UBRR_VALUE (((F_CPU) + 8 * (BAUD)) / (16 * (BAUD)) -1)
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#else
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#define UBRR_VALUE (((F_CPU) + 8UL * (BAUD)) / (16UL * (BAUD)) -1UL)
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#endif
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#if 100 * (F_CPU) > \
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(16 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) + (BAUD) * (BAUD_TOL))
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# define USE_2X 1
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#elif 100 * (F_CPU) < \
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(16 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) - (BAUD) * (BAUD_TOL))
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# define USE_2X 1
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#else
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# define USE_2X 0
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#endif
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#if USE_2X
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/* U2X required, recalculate */
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#undef UBRR_VALUE
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#ifdef __ASSEMBLER__
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#define UBRR_VALUE (((F_CPU) + 4 * (BAUD)) / (8 * (BAUD)) -1)
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#else
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#define UBRR_VALUE (((F_CPU) + 4UL * (BAUD)) / (8UL * (BAUD)) -1UL)
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#endif
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#if 100 * (F_CPU) > \
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(8 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) + (BAUD) * (BAUD_TOL))
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# warning "Baud rate achieved is higher than allowed"
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#endif
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#if 100 * (F_CPU) < \
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(8 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) - (BAUD) * (BAUD_TOL))
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# warning "Baud rate achieved is lower than allowed"
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#endif
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#endif
/* USE_U2X */
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#ifdef UBRR_VALUE
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/* Check for overflow */
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# if UBRR_VALUE >= (1 << 12)
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# warning "UBRR value overflow"
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# endif
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# define UBRRL_VALUE (UBRR_VALUE & 0xff)
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# define UBRRH_VALUE (UBRR_VALUE >> 8)
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#endif
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#endif
/* __DOXYGEN__ */
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/* end of util/setbaud.h */
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