Arduino I2C + 温湿度传感器Si7021

  1 /*
  2 Measurement of relative humidity and temperature using Si7021
  3 */
  4 
  5 #include <Wire.h>
  6 
  7 #define ADDRESS_SI7021 0x40
  8 #define MEASURE_RH_HOLD 0xE5
  9 #define READ_T_FROM_PRE_RH_MEASUREMENT 0xE0
 10 
 11 byte buffer[] = {0, 0, 0};
 12 byte crcHumi;
 13 
 14 word outHumi = 0;
 15 word outTemp = 0;
 16 float valueHumi = 0;
 17 float valueTemp = 0;
 18 
 19 void setup()
 20 {
 21     Wire.begin();
 22     Serial.begin(115200);
 23 }
 24 
 25 void loop()
 26 {
 27     //perform a RH measurement and read back the RH value
 28     Wire.beginTransmission(ADDRESS_SI7021);
 29     Wire.write(MEASURE_RH_HOLD);
 30     Wire.endTransmission();
 31 
 32     Wire.requestFrom(ADDRESS_SI7021, 3);
 33     if(Wire.available() >= 3)
 34     {
 35         buffer[0] = Wire.read(); //high byte
 36         buffer[1] = Wire.read(); //low byte
 37         buffer[2] = Wire.read(); //crc
 38     }
 39     outHumi = (buffer[0]<<8) | buffer[1];
 40     crcHumi = CRC8(buffer, 3);
 41     
 42     //read temperature from previous RH measurement  
 43     Wire.beginTransmission(ADDRESS_SI7021);
 44     Wire.write(READ_T_FROM_PRE_RH_MEASUREMENT);
 45     Wire.endTransmission();
 46 
 47     Wire.requestFrom(ADDRESS_SI7021, 2);
 48     if(Wire.available() >= 2)
 49     {
 50         buffer[0] = Wire.read(); //high byte
 51         buffer[1] = Wire.read(); //low byte; no crc
 52     }
 53     outTemp = (buffer[0]<<8) | buffer[1];
 54 
 55     valueHumi = 125.0*outHumi/65536 - 6;
 56     valueTemp = 175.72*outTemp/65536 - 46.85;
 57 
 58     Serial.print(valueHumi);    Serial.print(" \%RH\t");
 59     Serial.print(valueTemp);    Serial.println(" `C");
 60 
 61     if (crcHumi == 0)
 62     {
 63         Serial.println("CRC checked.");
 64     }
 65     else
 66     {
 67         Serial.println("CRC Error!");
 68     }
 69 
 70     delay(4000);
 71 }
 72 
 73 byte CRC8(byte *data, byte len)
 74 {
 75     byte crc[8];
 76     byte result = 0;
 77     byte i;
 78     byte doInvert;
 79     String bitString = MakeString(data, len);
 80 
 81     for (i = 0; i < 8; i++)
 82     {
 83         crc[i] = 0;
 84     }
 85 
 86     for (i = 0; i < bitString.length(); i ++)
 87     {
 88         doInvert = (byte(bitString[i] == ‘1‘)) ^ crc[7];
 89         crc[7] = crc[6];
 90         crc[6] = crc[5];
 91         crc[5] = crc[4] ^ doInvert;
 92         crc[4] = crc[3] ^ doInvert;
 93         crc[3] = crc[2];
 94         crc[2] = crc[1];
 95         crc[1] = crc[0];
 96         crc[0] = doInvert;
 97     }
 98 
 99     for (i = 0; i < 8; i++)
100     {
101         bitWrite(result, i, crc[i]);
102     }
103 
104     return result;
105 }
106 
107 String MakeString(byte *data, byte len)
108 {
109     int i, j;
110     String tempString[len];
111     String bitString="";
112 
113     for (i = 0; i < len; i++)
114     {
115         tempString[i] = String(data[i], BIN);
116         int zeros = 8 - tempString[i].length();
117         for (j = 0; j < zeros; j++)
118         {
119             tempString[i] = ‘0‘ + tempString[i];
120         }
121     }
122 
123     for (i = 0; i < len; i++)
124     {
125         bitString = bitString + tempString[i];
126     }
127 
128     return bitString;
129 }