development environment

This paper is based on the following development environment, if the development environment is different, it may not achieve the desired results.

IDE: AtmelStudio 7
toochain: avr-gcc 
MCU  : atmega128A

There are two ways to collect temperature by thermistor:

1. Formula calculation method, according to the conversion formula, the temperature can be calculated directly by the resistance value measured.
2. Look-up table method, which is also the algorithm used in this example, is used.
Because the computing power of embedded MCU is relatively weak (floating point operation is slow), table lookup method is usually used.

The function of this algorithm is to obtain the corresponding temperature according to the resistance value.
This algorithm can be categorized under the dichotomy algorithm.
Difficulty: The object of searching and comparing is floating point number, and the resistance value is discontinuous, so we should consider the problem between the two typical resistance values.

A Complete Implementation of Table Lookup Method

The following coding is implemented in avr-gcc and tested.

#include "b3950.h"
#include <avr/pgmspace.h>

typedef struct 
{
	int16 t;
	float r;
}sToR;

const sToR tr_array[] PROGMEM = 
{
{-20,97.8396f},
{-19,92.3020},
{-18,87.1124},
{-17,82.2471},
{-16,77.6837},
{-15,73.4018},
{-14,69.3823},
{-13,65.6077},
{-12,62.0616},
{-11,58.7288},
{-10,55.5953},
{-9	,52.6480},
{-8	,49.8747},
{-7	,47.2643},
{-6	,44.8062},
{-5	,42.4906},
{-4	,40.3086},
{-3	,38.2516},
{-2	,36.3117},
{-1	,34.4817},
{0	,32.7547},
{1	,31.1243},
{2	,29.5847},
{3	,28.1301},
{4	,26.7556},
{5	,25.4562},
{6	,24.2274},
{7	,23.0650},
{8	,21.9650},
{9	,20.9239},
{10	,19.9380},
{11	,19.0041},
{12	,18.1193},
{13	,17.2807},
{14	,16.4857},
{15	,15.7317},
{16	,15.0164},
{17	,14.3376},
{18	,13.6933},
{19	,13.0816},
{20	,12.5005},
{21	,11.9485},
{22	,11.4239},
{23	,10.9252},
{24	,10.4510},
{25	,10.0000},
{26	,9.5709 },
{27	,9.1626 },
{28	,8.7738 },
{29	,8.4037 },
{30	,8.0512 },
{31	,7.7154 },
{32	,7.3953 },
{33	,7.0903 },
{34	,6.7995 },
{35	,6.5221 },
{36	,6.2576 },
{37	,6.0051 },
{38	,5.7642 },
{39	,5.5342 },
{40	,5.3146 },
{41	,5.1049 },
{42	,4.9045 },
{43	,4.7130 },
{44	,4.5300 },
{45	,4.3551 },
{46	,4.1878 },
{47	,4.0278 },
{48	,3.8748 },
{49	,3.7283 },
{50	,3.5882 },
{51	,3.4540 },
{52	,3.3255 },
{53	,3.2025 },
{54	,3.0846 },
{55	,2.9717 },
{56	,2.8635 },
{57	,2.7597 },
{58	,2.6603 },
{59	,2.5649 },
{60	,2.4734 },
{61	,2.3856 },
{62	,2.3014 },
{63	,2.2206 },
{64	,2.1431 },
{65	,2.0686 },
{66	,1.9970 },
{67	,1.9283 },
{68	,1.8623 },
{69	,1.7989 },
{70	,1.7380 },
{71	,1.6794 },
{72	,1.6231 },
{73	,1.5689 },
{74	,1.5168 },
{75	,1.4667 },
{76	,1.4185 },
{77	,1.3722 },
{78	,1.3275 },
{79	,1.2845 },
{80	,1.2431 },
{81	,1.2033 },
{82	,1.1649 },
{83	,1.1279 },
{84	,1.0923 },
{85	,1.0580 },
{86	,1.0249 },
{87	,0.9930 },
{88	,0.9623 },
{89	,0.9326 },
{90	,0.9040 },
{91	,0.8764 },
{92	,0.8498 },
{93	,0.8241 },
{94	,0.7994 },
{95	,0.7754 },
{96	,0.7523 },
{97	,0.7300 },
{98	,0.7085 },
{99	,0.6877 },
{100,0.6676 },
{101,0.6482 },
{102,0.6295 },
{103,0.6113 },
{104,0.5938 },
{105,0.5769 },
{106,0.5605 },
{107,0.5447 },
{108,0.5293 },
{109,0.5145 },
{110,0.5002 },
{111,0.4863 },
{112,0.4729 },
{113,0.4599 },
{114,0.4474 },
{115,0.4352 },
{116,0.4234 },
{117,0.4120 },
{118,0.4009 },
{119,0.3902 },
{120,0.3799 },
{121,0.3698 },
{122,0.3601 },
{123,0.3506 },
{124,0.3415 },
{125,0.3326 },
{126,0.3240 },
{127,0.3157 },
{128,0.3076 },
{129,0.2998 },
{130,0.2922 },
{131,0.2848 },
{132,0.2776 },
{133,0.2707 },
{134,0.2640 },
{135,0.2574 },
{136,0.2511 },
{137,0.2449 },
{138,0.2389 },
{139,0.2331 },
{140,0.2274 },
{141,0.2220 },
{142,0.2166 },
{143,0.2114 },
{144,0.2064 },
{145,0.2015 },
{146,0.1968 },
{147,0.1922 },
{148,0.1877 },
{149,0.1833 },
{150,0.1791 },
{151,0.1749 },
{152,0.1709 },
{153,0.1670 },
{154,0.1632 },
{155,0.1595 },
{156,0.1559 },
{157,0.1524 },
{158,0.1490 },
{159,0.1457 },
{160,0.1425 },
{161,0.1394 },
{162,0.1363 },
{163,0.1333 },
{164,0.1304 },
{165,0.1276 },
{166,0.1249 },
{167,0.1222 },
{168,0.1196 },
{169,0.1170 },
{170,0.1146 },
{171,0.1121 },
{172,0.1098 },
{173,0.1075 },
{174,0.1053 },
{175,0.1031 },
{176,0.1010 },
{177,0.0989 },
{178,0.0969 },
{179,0.0949 },
{180,0.0930 },
{181,0.0911 },
{182,0.0893 },
{183,0.0875 },
{184,0.0858 },
{185,0.0841 },
{186,0.0824 },
{187,0.0808 },
{188,0.0792 },
{189,0.0777 },
{190,0.0762 },
{191,0.0747 },
{192,0.0733 },
{193,0.0719 },
{194,0.0705 },
{195,0.0692 },
{196,0.0679 },
{197,0.0666 },
{198,0.0654 },
{199,0.0642 },
};
static volatile float rMid,tempR,tempDeltaL,tempDeltaR;
static volatile	u8 left,right,mid;
static volatile	int16 t;
int16 FindTempByRes(float res)
{	

	
	
	t = 0;
	rMid = 0;
	left = mid = 0;
	right = 219;
	
	while(left<right)
	{
		mid = (left+right)/2;
		rMid = pgm_read_float(&(tr_array[mid].r));//tr_array[mid].r;//pgm_read_float(&(tr_array[mid].r));
		

		if(rMid>res){//The larger the array index, the smaller the resistance value.
			//Check maximum
			tempR = pgm_read_float(&(tr_array[mid + 1].r));
			if(res>tempR)//tr_array[mid+1].r < res < tr_array[mid].r
			{
				tempDeltaL = rMid - res;
				tempDeltaR = res - tempR;
				if(tempDeltaL < tempDeltaR)
				{
					t = pgm_read_word(&(tr_array[mid].t));
					break;
				}else
				{
					t = pgm_read_word(&(tr_array[mid+1].t));
					break;
				}
			}else//Not between mid and mid+1
			{	
				left = mid+1;
			}
		}else if(rMid<res){
			tempR = pgm_read_float(&(tr_array[mid - 1].r));
			if(tempR>res)//tr_array[mid].r < res < tr_array[mid-1].r
			{
				tempDeltaL = tempR - res;
				tempDeltaR = res - rMid;
				if(tempDeltaL < tempDeltaR)
				{
					t = pgm_read_word(&(tr_array[mid-1].t));
					break;
				}else
				{
					t = pgm_read_word(&(tr_array[mid].t));
					break;
				}				
			}else//Not between mid and mid-1
			{
				right = mid-1;
			}
		}else{
			t = pgm_read_word(&(tr_array[mid].t));//tr_array[mid].t;//pgm_read_word(&(tr_array[mid].t));
			break;
		}

		if(left == right)//
		{
			t = pgm_read_word(&(tr_array[left].t));
			break;
		}
	}
	

	return t;
}

expectation

Changing the platform-related codes (pgm_read_float, pgm_read_word) can be transplanted to other platforms. For similar resistors, the corresponding tables are compiled to support them, and the algorithm remains unchanged.