avr-libc  2.0.0
Standard C library for AVR-GCC

AVR Libc Home Page

AVRs

AVR Libc Development Pages

Main Page

User Manual

Library Reference

FAQ

Example Projects

setbaud.h
Go to the documentation of this file.
1 /* Copyright (c) 2007 Cliff Lawson
2  Copyright (c) 2007 Carlos Lamas
3  All rights reserved.
4 
5  Redistribution and use in source and binary forms, with or without
6  modification, are permitted provided that the following conditions are met:
7 
8  * Redistributions of source code must retain the above copyright
9  notice, this list of conditions and the following disclaimer.
10 
11  * Redistributions in binary form must reproduce the above copyright
12  notice, this list of conditions and the following disclaimer in
13  the documentation and/or other materials provided with the
14  distribution.
15 
16  * Neither the name of the copyright holders nor the names of
17  contributors may be used to endorse or promote products derived
18  from this software without specific prior written permission.
19 
20  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
21  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
24  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
25  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
26  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
27  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
28  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
29  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
30  POSSIBILITY OF SUCH DAMAGE. */
31 
32 /* $Id: setbaud_8h_source.html,v 1.1.1.6 2016/02/09 07:13:53 joerg_wunsch Exp $ */
33 
34 /**
35  \file
36 */
37 
38 /**
39  \defgroup util_setbaud <util/setbaud.h>: Helper macros for baud rate calculations
40  \code
41  #define F_CPU 11059200
42  #define BAUD 38400
43  #include <util/setbaud.h>
44  \endcode
45 
46  This header file requires that on entry values are already defined
47  for F_CPU and BAUD. In addition, the macro BAUD_TOL will define
48  the baud rate tolerance (in percent) that is acceptable during
49  the calculations. The value of BAUD_TOL will default to 2 %.
50 
51  This header file defines macros suitable to setup the UART baud
52  rate prescaler registers of an AVR. All calculations are done
53  using the C preprocessor. Including this header file causes no
54  other side effects so it is possible to include this file more than
55  once (supposedly, with different values for the BAUD parameter),
56  possibly even within the same function.
57 
58  Assuming that the requested BAUD is valid for the given F_CPU then
59  the macro UBRR_VALUE is set to the required prescaler value. Two
60  additional macros are provided for the low and high bytes of the
61  prescaler, respectively: UBRRL_VALUE is set to the lower byte of
62  the UBRR_VALUE and UBRRH_VALUE is set to the upper byte. An
63  additional macro USE_2X will be defined. Its value is set to 1 if
64  the desired BAUD rate within the given tolerance could only be
65  achieved by setting the U2X bit in the UART configuration. It will
66  be defined to 0 if U2X is not needed.
67 
68  Example usage:
69 
70  \code
71  #include <avr/io.h>
72 
73  #define F_CPU 4000000
74 
75  static void
76  uart_9600(void)
77  {
78  #define BAUD 9600
79  #include <util/setbaud.h>
80  UBRRH = UBRRH_VALUE;
81  UBRRL = UBRRL_VALUE;
82  #if USE_2X
83  UCSRA |= (1 << U2X);
84  #else
85  UCSRA &= ~(1 << U2X);
86  #endif
87  }
88 
89  static void
90  uart_38400(void)
91  {
92  #undef BAUD // avoid compiler warning
93  #define BAUD 38400
94  #include <util/setbaud.h>
95  UBRRH = UBRRH_VALUE;
96  UBRRL = UBRRL_VALUE;
97  #if USE_2X
98  UCSRA |= (1 << U2X);
99  #else
100  UCSRA &= ~(1 << U2X);
101  #endif
102  }
103  \endcode
104 
105  In this example, two functions are defined to setup the UART
106  to run at 9600 Bd, and 38400 Bd, respectively. Using a CPU
107  clock of 4 MHz, 9600 Bd can be achieved with an acceptable
108  tolerance without setting U2X (prescaler 25), while 38400 Bd
109  require U2X to be set (prescaler 12).
110 */
111 
112 #ifndef F_CPU
113 # error "setbaud.h requires F_CPU to be defined"
114 #endif
115 
116 #ifndef BAUD
117 # error "setbaud.h requires BAUD to be defined"
118 #endif
119 
120 #if !(F_CPU)
121 # error "F_CPU must be a constant value"
122 #endif
123 
124 #if !(BAUD)
125 # error "BAUD must be a constant value"
126 #endif
127 
128 #if defined(__DOXYGEN__)
129 /**
130  \def BAUD_TOL
131  \ingroup util_setbaud
132 
133  Input and output macro for <util/setbaud.h>
134 
135  Define the acceptable baud rate tolerance in percent. If not set
136  on entry, it will be set to its default value of 2.
137 */
138 #define BAUD_TOL 2
139 
140 /**
141  \def UBRR_VALUE
142  \ingroup util_setbaud
143 
144  Output macro from <util/setbaud.h>
145 
146  Contains the calculated baud rate prescaler value for the UBRR
147  register.
148 */
149 #define UBRR_VALUE
150 
151 /**
152  \def UBRRL_VALUE
153  \ingroup util_setbaud
154 
155  Output macro from <util/setbaud.h>
156 
157  Contains the lower byte of the calculated prescaler value
158  (UBRR_VALUE).
159 */
160 #define UBRRL_VALUE
161 
162 /**
163  \def UBRRH_VALUE
164  \ingroup util_setbaud
165 
166  Output macro from <util/setbaud.h>
167 
168  Contains the upper byte of the calculated prescaler value
169  (UBRR_VALUE).
170 */
171 #define UBRRH_VALUE
172 
173 /**
174  \def USE_2X
175  \ingroup util_setbaud
176 
177  Output macro from <util/setbaud.h>
178 
179  Contains the value 1 if the desired baud rate tolerance could only
180  be achieved by setting the U2X bit in the UART configuration.
181  Contains 0 otherwise.
182 */
183 #define USE_2X 0
184 
185 #else /* !__DOXYGEN__ */
186 
187 #undef USE_2X
188 
189 /* Baud rate tolerance is 2 % unless previously defined */
190 #ifndef BAUD_TOL
191 # define BAUD_TOL 2
192 #endif
193 
194 #ifdef __ASSEMBLER__
195 #define UBRR_VALUE (((F_CPU) + 8 * (BAUD)) / (16 * (BAUD)) -1)
196 #else
197 #define UBRR_VALUE (((F_CPU) + 8UL * (BAUD)) / (16UL * (BAUD)) -1UL)
198 #endif
199 
200 #if 100 * (F_CPU) > \
201  (16 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) + (BAUD) * (BAUD_TOL))
202 # define USE_2X 1
203 #elif 100 * (F_CPU) < \
204  (16 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) - (BAUD) * (BAUD_TOL))
205 # define USE_2X 1
206 #else
207 # define USE_2X 0
208 #endif
209 
210 #if USE_2X
211 /* U2X required, recalculate */
212 #undef UBRR_VALUE
213 
214 #ifdef __ASSEMBLER__
215 #define UBRR_VALUE (((F_CPU) + 4 * (BAUD)) / (8 * (BAUD)) -1)
216 #else
217 #define UBRR_VALUE (((F_CPU) + 4UL * (BAUD)) / (8UL * (BAUD)) -1UL)
218 #endif
219 
220 #if 100 * (F_CPU) > \
221  (8 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) + (BAUD) * (BAUD_TOL))
222 # warning "Baud rate achieved is higher than allowed"
223 #endif
224 
225 #if 100 * (F_CPU) < \
226  (8 * ((UBRR_VALUE) + 1)) * (100 * (BAUD) - (BAUD) * (BAUD_TOL))
227 # warning "Baud rate achieved is lower than allowed"
228 #endif
229 
230 #endif /* USE_U2X */
231 
232 #ifdef UBRR_VALUE
233  /* Check for overflow */
234 # if UBRR_VALUE >= (1 << 12)
235 # warning "UBRR value overflow"
236 # endif
237 
238 # define UBRRL_VALUE (UBRR_VALUE & 0xff)
239 # define UBRRH_VALUE (UBRR_VALUE >> 8)
240 #endif
241 
242 #endif /* __DOXYGEN__ */
243 /* end of util/setbaud.h */