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Summary: Microcontrollers have a limited amount of memory in which variable data can be stored. In some cases, it is necessary to store and access large quantities of data, such as a lookup table, for use within a progam. If this data will not be changed by the program during run time, it can be stored in the read only section of memory.
To locate an array in read only memory rather than in RAM, declare and assign it as a constant.
const int DegFRTD[] = {-500, -40, -39, -38, -37, -36, -35, -34, -33,
-32, -31, -30, -29, -28, -27, -26, -25, -24, -23, -22, -21, -20,
-19, -18, -17, -16, -15, -14, -13, -12, -11, -10, -9, -8, -7, -6,
-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44,
45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76,
77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92,
93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106,
107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132,
133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145,
146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158,
159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171,
172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184,
185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197,
198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210,
211, 212, 999, 999};
const long OhmRTD[] = {0,8427, 8450, 8471, 8493, 8515, 8537, 8559, 8581,
8603, 8625, 8647, 8669, 8691, 8713, 8735, 8757, 8779, 8801, 8822, 8844, 8866,
8888, 8910, 8932, 8954, 8976, 8998, 9019, 9041, 9063, 9085, 9107, 9129, 9151,
9172, 9194, 9216, 9238, 9260, 9282, 9303, 9325, 9347, 9369, 9391, 9413, 9434,
9456, 9478, 9500, 9522, 9543, 9565, 9587, 9609, 9630, 9652, 9674, 9696, 9718,
9739, 9761, 9783, 9805, 9826, 9848, 9870, 9891, 9913, 9935, 9957, 9978, 10000,
10022, 10043, 10065, 10087, 10109, 10130, 10152, 10174, 10195, 10217, 10239,
10260, 10282, 10304, 10325, 10347, 10369, 10390, 10412, 10434, 10455, 10477,
10498, 10520, 10542, 10563, 10585, 10607, 10628, 10650, 10671, 10693, 10715,
10736, 10758, 10779, 10801, 10822, 10844, 10866, 10887, 10909, 10930, 10952,
10973, 10995, 11017, 11038, 11060, 11081, 11103, 11124, 11146, 11167, 11189,
11210, 11232, 11253, 11275, 11296, 11318, 11339, 11361, 11382, 11404, 11425,
11447, 11468, 11490, 11511, 11533, 11554, 11575, 11597, 11618, 11640, 11661,
11683, 11704, 11725, 11747, 11768, 11790, 11811, 11833, 11854, 11875, 11897,
11918, 11940, 11961, 11982, 12004, 12025, 12046, 12068, 12089, 12111, 12132,
12153, 12175, 12196, 12217, 12239, 12260, 12281, 12303, 12324, 12345, 12367,
12388, 12409, 12431, 12452, 12473, 12494, 12516, 12537, 12558, 12580, 12601,
12622, 12643, 12665, 12686, 12707, 12728, 12750, 12771, 12792, 12813, 12835,
12856, 12877, 12898, 12920, 12941, 12962, 12983, 13005, 13026, 13047, 13068,
13089, 13111, 13132, 13153, 13174, 13195, 13216, 13238, 13259, 13280, 13301,
13322, 13343, 13365, 13386, 13407, 13428, 13449, 13470, 13491, 13513, 13534,
13555, 13576, 13597, 13618, 13639, 13660, 13681, 13702, 13724, 13745, 13766,
13767, 13808, 13829, 13850, 1000000, 1000000};
Data constants that do not need to be indexed can be stored as a constant, but they can also be defined by a preprocessor directive so that the preprocessor simply substitutes in the number where the name of the constant is used before the code is compiled. The value can be changed in each location it is used by simply changing the preprocessor directive in one place, saving time and reducing errors.
#define pi (3.14159) //preprocessor directive
float circumference;
float diameter;
void main (void)
{
circumference = pi*diameter; // The preprocessor translates this into circumference = 3.14159 * diameter;
}
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This work is licensed by Trent Kelly under a Creative Commons Attribution License (CC-BY 2.0), and is an Open Educational Resource.
Last edited by Trent Kelly on Sep 10, 2008 11:18 pm -0500.