Sht31.cpp

00001 /***************************************************
00002   This is a library for the SHT31 Digital Humidity & Temp Sht31
00003 
00004   Designed specifically to work with the SHT31 Digital Sht31 from Adafruit
00005   ----> https://www.adafruit.com/products/2857
00006 
00007   These displays use I2C to communicate, 2 pins are required to
00008   interface
00009   Adafruit invests time and resources providing this open source code,
00010   please support Adafruit and open-source hardware by purchasing
00011   products from Adafruit!
00012 
00013   Written by Limor Fried/Ladyada for Adafruit Industries.
00014   BSD license, all text above must be included in any redistribution
00015  ****************************************************/
00016 
00017 #include "Sht31.h"
00018 #include "mbed.h"
00019 
00020 Sht31::Sht31(PinName sda, PinName scl) : _i2c(sda, scl) {
00021     _i2caddr = 0x44;
00022     printf("\r\n");
00023     reset();
00024     readStatus();
00025 }
00026 
00027 float Sht31::readTemperature(void) {
00028     if (! readTempHum()) return NAN;
00029     return temp;
00030 }
00031 
00032 float Sht31::readHumidity(void) {
00033     if (! readTempHum()) return NAN;
00034     return humidity;
00035 }
00036 
00037 void Sht31::reset(void) {
00038     writeCommand(SHT31_SOFTRESET);
00039     wait_ms(10);
00040 }
00041 
00042 uint16_t Sht31::readStatus(void) {
00043     writeCommand(SHT31_READSTATUS);
00044     char val[1];
00045     _i2c.start();
00046     _i2c.read(_i2caddr, val, 1);
00047     _i2c.stop();
00048     uint16_t stat = val[0];
00049     stat <<= 8;
00050     _i2c.start();
00051     _i2c.read(_i2caddr, val, 1);
00052     _i2c.stop();
00053     stat |= val[0];
00054     printf("0x%X\r\n", stat);
00055     return stat;
00056 }
00057 
00058 void Sht31::writeCommand(uint16_t cmd) {
00059     char buf[2];
00060     _i2c.start();
00061     buf[0] = (cmd >> 8);
00062     buf[1] = (cmd & 0xFF);
00063     _i2c.write(_i2caddr, buf, 2);
00064     _i2c.stop();
00065 }
00066 
00067 bool Sht31::readTempHum(void) {
00068     char readbuffer[6];
00069 
00070     writeCommand(SHT31_MEAS_HIGHREP);
00071 
00072     wait_ms(500);
00073     _i2c.start();
00074     _i2c.read(_i2caddr, readbuffer, 6);
00075     _i2c.stop();
00076     for (uint8_t i = 0; i < 6; i++) {
00077         printf("0x%Xd\r\n", readbuffer[i]);
00078     }
00079     uint16_t ST, SRH;
00080     ST = readbuffer[0];
00081     ST <<= 8;
00082     ST |= readbuffer[1];
00083 
00084     if (readbuffer[2] != crc8((uint8_t *) readbuffer, 2)) {
00085         return false;
00086     }
00087 
00088     SRH = readbuffer[3];
00089     SRH <<= 8;
00090     SRH |= readbuffer[4];
00091 
00092     if (readbuffer[5] != crc8((uint8_t *) readbuffer+3, 2)) {
00093         return false;
00094     }
00095 
00096     printf("ST = %d\r\n", ST);
00097     double stemp = ST;
00098     stemp *= 175;
00099     stemp /= 0xffff;
00100     stemp = -45 + stemp;
00101     temp = stemp;
00102 
00103     printf("SRH = %d\r\n", SRH);
00104     double shum = SRH;
00105     shum *= 100;
00106     shum /= 0xFFFF;
00107 
00108     humidity = shum;
00109 
00110     return true;
00111 }
00112 
00113 uint8_t Sht31::crc8(const uint8_t *data, int len) {
00114     const uint8_t POLYNOMIAL(0x31);
00115     uint8_t crc(0xFF);
00116 
00117     for ( int j = len; j; --j ) {
00118         crc ^= *data++;
00119 
00120         for ( int i = 8; i; --i ) {
00121             crc = (crc & 0x80) ? (crc << 1) ^ POLYNOMIAL : (crc << 1);
00122         }
00123     }
00124     return crc;
00125 }