前言：

最近在调试STM32ADC采集NTC热敏电阻的温度值，总结下NTC热敏电阻温度值的采集方法。
硬件平台：STM32F205
软件平台：keil V5
函数库：标准库

NTC热敏电阻温度采集方法

　　热敏电阻的相关知识点见百度文库的介绍：传送门。
　　本例中使用的热敏电阻型号为mfh103-3950。其电阻与温度对应表如下：

 附表1NTC热敏电阻R/T对照表T(℃)    R(KΩ)        T(℃)   R(KΩ)       T(℃)    R(KΩ)-20.0   95.3370        20.5   12.2138        61.0    2.3820   
    -19.5   92.6559        21.0   11.9425        61.5    2.3394   
    -19.0   90.0580        21.5   11.6778        62.0    2.2977   
    -18.5   87.5406        22.0   11.4198        62.5    2.2568   
    -18.0   85.1009        22.5   11.1681        63.0    2.2167   
    -17.5   82.7364        23.0   10.9227        63.5    2.1775   
    -17.0   80.4445        23.5   10.6834        64.0    2.1390   
    -16.5   78.2227        24.0   10.4499        64.5    2.1013   
    -16.0   76.0689        24.5   10.2222        65.0    2.0644   
    -15.5   73.9806        25.0   10.0000        65.5    2.0282   
    -15.0   71.9558        25.5    9.7833        66.0    1.9928   
    -14.5   69.9923        26.0    9.5718        66.5    1.9580   
    -14.0   68.0881        26.5    9.3655        67.0    1.9240   
    -13.5   66.2412        27.0    9.1642        67.5    1.8906   
    -13.0   64.4499        27.5    8.9677        68.0    1.8579   
    -12.5   62.7122        28.0    8.7760        68.5    1.8258   
    -12.0   61.0264        28.5    8.5889        69.0    1.7944   
    -11.5   59.3908        29.0    8.4063        69.5    1.7636   
    -11.0   57.8038        29.5    8.2281        70.0    1.7334   
    -10.5   56.2639        30.0    8.0541        70.5    1.7037   
    -10.0   54.7694        30.5    7.8842        71.0    1.6747   
     -9.5   53.3189        31.0    7.7184        71.5    1.6462   
     -9.0   51.9111        31.5    7.5565        72.0    1.6183   
     -8.5   50.5445        32.0    7.3985        72.5    1.5910   
     -8.0   49.2178        32.5    7.2442        73.0    1.5641   
     -7.5   47.9298        33.0    7.0935        73.5    1.5378   
     -7.0   46.6792        33.5    6.9463        74.0    1.5120   
     -6.5   45.4649        34.0    6.8026        74.5    1.4867   
     -6.0   44.2856        34.5    6.6622        75.0    1.4619   
     -5.5   43.1403        35.0    6.5251        75.5    1.4375   
     -5.0   42.0279        35.5    6.3912        76.0    1.4136   
     -4.5   40.9474        36.0    6.2604        76.5    1.3902   
     -4.0   39.8978        36.5    6.1326        77.0    1.3672   
     -3.5   38.8780        37.0    6.0077        77.5    1.3447   
     -3.0   37.8873        37.5    5.8858        78.0    1.3225   
     -2.5   36.9246        38.0    5.7666        78.5    1.3008   
     -2.0   35.9892        38.5    5.6501        79.0    1.2795   
     -1.5   35.0801        39.0    5.5363        79.5    1.2586   
     -1.0   34.1965        39.5    5.4251        80.0    1.2381   
     -0.5   33.3378        40.0    5.3164        80.5    1.2180   
      0.0   32.5030        40.5    5.2102        81.0    1.1983   0.5   31.6915        41.0    5.1064        81.5    1.1789   1.0   30.9026        41.5    5.0049        82.0    1.1599   1.5   30.1355        42.0    4.9057        82.5    1.1412   2.0   29.3896        42.5    4.8088        83.0    1.1229   2.5   28.6644        43.0    4.7140        83.5    1.1050   3.0   27.9590        43.5    4.6213        84.0    1.0873   3.5   27.2730        44.0    4.5307        84.5    1.0700   4.0   26.6058        44.5    4.4421        85.0    1.0530   4.5   25.9567        45.0    4.3554        85.5    1.0363   5.0   25.3254        45.5    4.2707        86.0    1.0199   5.5   24.7111        46.0    4.1878        86.5    1.0038   6.0   24.1135        46.5    4.1068        87.0    0.9880   6.5   23.5320        47.0    4.0275        87.5    0.9725   7.0   22.9661        47.5    3.9500        88.0    0.9573   7.5   22.4154        48.0    3.8742        88.5    0.9424   8.0   21.8795        48.5    3.8000        89.0    0.9277   8.5   21.3579        49.0    3.7275        89.5    0.9133   9.0   20.8502        49.5    3.6565        90.0    0.8991   9.5   20.3559        50.0    3.5870        90.5    0.8852   10.0   19.8747        50.5    3.5190        91.0    0.8715   10.5   19.4063        51.0    3.4525        91.5    0.8581   11.0   18.9502        51.5    3.3875        92.0    0.8450   11.5   18.5060        52.0    3.3238        92.5    0.8320   12.0   18.0735        52.5    3.2615        93.0    0.8193   12.5   17.6523        53.0    3.2005        93.5    0.8068   13.0   17.2421        53.5    3.1408        94.0    0.7945   13.5   16.8426        54.0    3.0824        94.5    0.7825   14.0   16.4534        54.5    3.0252        95.0    0.7707   14.5   16.0743        55.0    2.9692        95.5    0.7590   15.0   15.7049        55.5    2.9144        96.0    0.7476   15.5   15.3450        56.0    2.8608        96.5    0.7364   16.0   14.9944        56.5    2.8082        97.0    0.7253   16.5   14.6528        57.0    2.7568        97.5    0.7145   17.0   14.3198        57.5    2.7065        98.0    0.7038   17.5   13.9954        58.0    2.6572        98.5    0.6933   18.0   13.6792        58.5    2.6089        99.0    0.6831   18.5   13.3710        59.0    2.5616        99.5    0.6729   19.0   13.0705        59.5    2.5153       100.0    0.6630   19.5   12.7777        60.0    2.4700      20.0   12.4922        60.5    2.4255      

热敏电阻的采样电路图如下图：

　　从上图电路原理可知，温度上升，NTC阻值下降，对应的ADC采样电压下降，j进而AD采样值下降。即每对应一个温度值，就对应一个ad采样值，这样便可得到一个温度与ad值的对应表。
　　那么NTC采集温度的原理就是利用采样的ad值与对应表进行比较，得到温度值。温度与ad值的对应表可利用已经做好的表格来自动转化（输入参数，即可得到对应的ad值），ＮＴＣ计算表下载地址见：传送门。
　　本例中VCC是3.3V，电路中ＡＤＣ参考电压是3V，所以转换表格中乘以1.1进行换算，若输入电压和ADC采样的参考电压都是3.3V，删去1.1即可。
　　
　　利用上面的NTC计算表，可得到如下对应表。
　　

//NTC的ad值对应表
static u16 NTCTAB[241]= 
{0xFED   ,0xFE2  ,0xFD6  ,0xFCB  ,0xFBF  ,0xFB3  ,0xFA6  ,0xF9A  ,0xF8D  ,0xF80, 0xF73   ,0xF65  ,0xF58  ,0xF4A  ,0xF3B  ,0xF2D  ,0xF1E  ,0xF0F  ,0xF00  ,0xEF1, 0xEE1   ,0xED1  ,0xEC1  ,0xEB1  ,0xEA0  ,0xE8F  ,0xE7E  ,0xE6C  ,0xE5B  ,0xE49,0xE37   ,0xE24  ,0xE12  ,0xDFF  ,0xDEC  ,0xDD9  ,0xDC5  ,0xDB1  ,0xD9D  ,0xD89, 0xD75   ,0xD60  ,0xD4B  ,0xD36  ,0xD21  ,0xD0B  ,0xCF6  ,0xCE0  ,0xCCA  ,0xCB4,0xC9D   ,0xC87  ,0xC70  ,0xC59  ,0xC42  ,0xC2B  ,0xC14  ,0xBFC  ,0xBE4  ,0xBCD, 0xBB5   ,0xB9D  ,0xB85  ,0xB6C  ,0xB54  ,0xB3C  ,0xB23  ,0xB0A  ,0xAF2  ,0xAD9, 0xAC0   ,0xAA7  ,0xA8E  ,0xA75  ,0xA5C  ,0xA43  ,0xA2A  ,0xA11  ,0x9F8  ,0x9DF, 0x9C6   ,0x9AD  ,0x994  ,0x97B  ,0x962  ,0x949  ,0x930  ,0x917  ,0x8FE  ,0x8E5, 0x8CC   ,0x8B4  ,0x89B  ,0x882  ,0x86A  ,0x852  ,0x839  ,0x821  ,0x809  ,0x7F1, 0x7D9   ,0x7C2  ,0x7AA  ,0x793  ,0x77B  ,0x764  ,0x74D  ,0x736  ,0x720  ,0x709,0x6F3   ,0x6DC  ,0x6C6  ,0x6B0  ,0x69B  ,0x685  ,0x670  ,0x65A  ,0x645  ,0x630, 0x61C   ,0x607  ,0x5F3  ,0x5DE  ,0x5CB  ,0x5B7  ,0x5A3  ,0x590  ,0x57D  ,0x569,0x557   ,0x544  ,0x532  ,0x51F  ,0x50D  ,0x4FB  ,0x4EA  ,0x4D8  ,0x4C7  ,0x4B6,0x4A5   ,0x495  ,0x484  ,0x474  ,0x464  ,0x454  ,0x444  ,0x435  ,0x425  ,0x416, 0x407   ,0x3F9  ,0x3EA  ,0x3DC  ,0x3CD  ,0x3BF  ,0x3B2  ,0x3A4  ,0x397  ,0x389, 0x37C   ,0x36F  ,0x363  ,0x356  ,0x34A  ,0x33D  ,0x331  ,0x325  ,0x31A  ,0x30E, 0x303   ,0x2F8  ,0x2EC  ,0x2E2  ,0x2D7  ,0x2CC  ,0x2C2  ,0x2B7  ,0x2AD  ,0x2A3,0x299   ,0x290  ,0x286  ,0x27D  ,0x273  ,0x26A  ,0x261  ,0x258  ,0x250  ,0x247,0x23F   ,0x236  ,0x22E  ,0x226  ,0x21E  ,0x216  ,0x20E  ,0x206  ,0x1FF  ,0x1F8,0x1F0   ,0x1E9  ,0x1E2  ,0x1DB  ,0x1D4  ,0x1CD  ,0x1C7  ,0x1C0  ,0x1BA  ,0x1B3, 0x1AD   ,0x1A7  ,0x1A1  ,0x19B  ,0x195  ,0x18F  ,0x18A  ,0x184  ,0x17E  ,0x179, 0x174   ,0x16E  ,0x169  ,0x164  ,0x15F  ,0x15A  ,0x155  ,0x150  ,0x14C  ,0x147,0x142   ,0x13E  ,0x139  ,0x135  ,0x131  ,0x12C  ,0x128  ,0x124  ,0x120  ,0x11C, 0x118
};

接下来，编写查表函数，当采集到ad值后，从这个表中查到最接近的ad值。

//查表函数
u8 look_up_table(u16 *a,u8 ArrayLong,u16 data)
{    u16 begin,end,middle ;  u8 i ;  begin = 0 ;  end = ArrayLong-1 ;  i = 0  ;  if(data >= a[begin]) return begin ;  else if(data <= a[end]) return end ;  while(begin < end)  {  middle = (begin+end)/2 ;  if(data == a[middle] ) break ;  if(data < a[middle] && data > a[middle+1]) break ;   if(data > a[middle])  end = middle ;                      else begin = middle ;      if(i++ > ArrayLong) break ;  }  if(begin > end ) return 0 ;   return middle ;  
}

上述函数中参数u16 *a是表头，即表的首地址，u8 ArrayLong是表的元素的个数，u16 data是要查的ad值，函数返回值是查到的ad值在表中的序号值。得到对应的序号值，就可得到对应的温度值。

//输入表的序号值，得到温度值
float num_to_temperature(u8 num)
{float  data;data = 0.5*num-20;return data;
}

上述是查到与采样AD最近的ad值，求出温度值。这个温度值精度不高，可作为初略计算的温度。
下面也可利用得到的这个粗略的温度值t1，结合表中的下一个节点t2,进行线性计算，求的更精确的温度值tx。

简单的计算公式如下：

    if( (data<=NTCTAB[0]) && (data>NTCTAB[240]) ){num=look_up_table(NTCTAB,241,data);t1=num_to_temperature(num);tx=0.5*(data-NTCTAB[num])/(NTCTAB[num+1]-NTCTAB[num])+t1;printf("temp_ntc:%4.2f\r\n",tx);    }

　　ADC的配置采集部分见另外两篇文章：传送门1和传送门2。根据获得的ad采样值和上述查表计算方法，便可得到NTC的温度值。