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setbaud.h
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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.h 2134 2010-06-08 11:19:48Z joerg_wunsch $ */
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 bacro 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 # define UBRRL_VALUE (UBRR_VALUE & 0xff)
234 # define UBRRH_VALUE (UBRR_VALUE >> 8)
235 #endif
236
237 #endif /* __DOXYGEN__ */
238 /* end of util/setbaud.h */

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