Diff: main.cpp

Revision:

0:062188c0bb61

Child:

1:f4a1942b4486

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp	Thu Aug 10 13:25:37 2017 +0000
@@ -0,0 +1,153 @@
+#include "mbed.h"
+
+/**
+ * A simple library for SHT21 humidity and temperature sensor.
+ * Based on the Arduino library.
+ */
+
+class SHT21 {
+public:
+    SHT21(PinName sda, PinName scl, char addr = (0x40 << 1));
+    SHT21(I2C &i2c,  char addr = (0x40 << 1));
+    
+    float temperature();
+    float humidity();
+    
+    void reset();
+    void serialNumber(uint8_t *serialNumber);
+    
+    ~SHT21();
+private:
+    enum commands {
+        TRIGGER_T_MEASUREMENT_HM    = 0xE3,
+        TRIGGER_RH_MEASUREMENT_HM   = 0xE5,
+        TRIGGER_T_MEASUREMENT_NHM   = 0xF3,
+        TRIGGER_RH_MEASUREMENT_NHM  = 0xF5,
+        USER_REGISTER_W             = 0xE6,
+        USER_REGISTER_R             = 0xE7,
+        SOFT_RESET                  = 0xFE
+    };
+
+    static const uint16_t POLYNOMIAL;
+
+    char addr_;
+    I2C *pi2c_;
+    I2C &i2c_;
+    
+    uint16_t readRaw(uint8_t cmd);
+    static bool crcChecksum(const uint8_t *data, uint8_t sz, uint8_t checksum);
+};
+
+const uint16_t SHT21::POLYNOMIAL = 0x131;
+
+SHT21::SHT21(PinName sda, PinName scl, char addr) :
+    addr_(addr),
+    pi2c_(new I2C(sda, scl)),
+    i2c_(*pi2c_) {
+}
+
+SHT21::SHT21(I2C &i2c, char addr) :
+    addr_(addr),
+    pi2c_(NULL),
+    i2c_(i2c) {
+}
+
+float SHT21::temperature() {
+    uint16_t r = readRaw(TRIGGER_T_MEASUREMENT_HM);
+    r &= ~0x0003;
+    return (-46.85 + 175.72/65536 * (float) r);
+}
+
+float SHT21::humidity() {
+    uint16_t r = readRaw(TRIGGER_RH_MEASUREMENT_HM);
+    r &= ~0x0003;
+    return (-6.0 + 125.0/65536 * (float) r);
+}
+
+void SHT21::reset() {
+    uint8_t pcmd [] = { SOFT_RESET };
+    i2c_.write(addr_, (const char *) pcmd, 1);
+    wait_ms(15);
+}
+
+void SHT21::serialNumber(uint8_t *serialNumber) {
+    uint8_t data[8];
+    uint8_t pcmd0[2] = { 0xFA, 0x0F };
+    uint8_t pcmd1[2] = { 0xFC, 0xC9 };
+    uint8_t i = 0;
+    i2c_.write(addr_, (const char *) pcmd0, 2);
+    i2c_.read(addr_, (char *) data, 8);
+    
+    serialNumber[5] = data[i++];
+    i++;
+    serialNumber[4] = data[i++];
+    i++;
+    serialNumber[3] = data[i++];
+    i++;
+    serialNumber[2] = data[i++];
+    i = 0;
+    
+    i2c_.write(addr_, (const char *) pcmd1, 2);
+    i2c_.read(addr_, (char *) data, 6);
+    
+    serialNumber[1] = data[i++];
+    serialNumber[0] = data[i++];
+    i++;
+    serialNumber[7] = data[i++];
+    serialNumber[8] = data[i++];
+    i = 0;
+}
+
+SHT21::~SHT21() {
+    if (pi2c_) delete pi2c_;
+}
+
+uint16_t SHT21::readRaw(uint8_t cmd) {
+    uint8_t checksum;
+    uint8_t data[2];
+    uint16_t result;
+
+    uint8_t pcmd [] = { cmd };
+
+    i2c_.write(addr_, (const char *) pcmd, 1);
+    i2c_.read(addr_, (char *) data, 2);
+    i2c_.read(addr_, (char *) &checksum, 1);
+
+    result =    data[0] << 8;
+    result +=   data[1];
+
+    if (crcChecksum(data, 2, checksum)) {
+        reset();
+        return 1;
+    }
+    return result;
+}
+
+bool SHT21::crcChecksum(const uint8_t *data, uint8_t sz, uint8_t checksum) {
+    uint8_t crc = 0;
+    
+    for (uint8_t i = 0; i < sz; ++i) {
+        crc ^= data[i];
+        for (uint8_t j /* bit index */ = 8; j > 0; --j) {
+            if (crc & 0x80)
+                crc = (crc << 1) ^ POLYNOMIAL;
+            else
+                crc = (crc << 1);
+        }
+    }
+    
+    return crc == checksum;
+}
+
+SHT21 sht21(PB_9 /* I2C_SDA */, PB_8 /* I2C_SCL */);
+Serial pc(USBTX, USBRX);
+
+int main() {
+    while(1) {
+        pc.printf(
+            "%2.2f degC, %2.2f %%\n",
+            sht21.temperature(),
+            sht21.humidity());
+        wait(1);
+    }
+}