Diff: DHT22.cpp

Revision:

0:a4f404326ee0

Child:

1:3385bd532812

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/DHT22.cpp	Wed Apr 15 18:45:36 2020 +0000
@@ -0,0 +1,121 @@
+#include "DHT22.h"
+
+DHT22::DHT22(PinName pin)
+{
+    pin_name = pin;
+
+    DigitalInOut dht(pin_name);
+    dht.output();
+    dht.write(1);
+}
+
+short DHT22::shortFromBits(bool bits[])
+{
+    short result = 0;
+    for (int i = 0; i<8; i++) {
+        result += bits[i]*(1 << (7-i));
+    }
+    return result;
+}
+
+// Implementation for reading a DHT22 according to the datasheet in
+// https://www.sparkfun.com/datasheets/Sensors/Temperature/DHT22.pdf
+int DHT22::read()
+{
+    Timer period;
+
+    DigitalInOut dht(pin_name);
+
+    // First, we send a low signal for 1ms to the sensor
+    dht.output();
+    dht.write(0);
+    thread_sleep_for(2);
+
+    // Now, we pull-up the voltage and wait for the MCU answer
+    dht.write(1);
+    dht.input();
+    // According to the documentation, the DHT will take between 20-40us to answer, so we just wait the minimum time
+    // Also, if the sensor doesn't change its status from 1, then it's broken.
+    period.start();
+    while (dht) {
+        if (period.read() > 1) {
+            return -1;
+        }
+    }
+    period.stop();
+
+    // Now the DHT has set to zero, it shall last 80us and then be another 80us in up
+    wait_us(80);
+    wait_us(80);
+
+    // Now it starts the data transmision, we expect first 50us before the first bit
+    // We expect 5 packets of 8 bits:
+    // - Integral RH Data
+    // - Decimal RH Data
+    // - Integral Temp Data
+    // - Decimal Temp Data
+    // - Checksum
+    bool packets[5][8];
+    for (int i = 0; i < 5; i++) {
+        bool result[8];
+        for (int j = 0; j < 8; j++) {
+            // We wait the start of the bit with wait just in case we have been carrying error
+            // wait_us(50);
+            while(!dht); // Low signal of 50us. Each bit has to start with it.
+            period.reset();
+            period.start();
+            while(dht);
+            period.stop();
+
+            // According to the documentation, it's 26us vs 70us
+            result[j] = period.read_us() > 60;
+        }
+        packets[i][0] = result[0];
+        packets[i][1] = result[1];
+        packets[i][2] = result[2];
+        packets[i][3] = result[3];
+        packets[i][4] = result[4];
+        packets[i][5] = result[5];
+        packets[i][6] = result[6];
+        packets[i][7] = result[7];
+    }
+
+    // Now we stop the sensor
+    dht.output();
+    dht.write(1);
+
+    // And we treat the received data
+
+    // First, we treat the MSB (bit 0) of the temp, if it's 1, then it's negative.
+    // The real value for the temperature is stored in the bits 1 ~ 16
+    int MSB = 1;
+    if (packets[2][0])
+        MSB = -1;
+    packets[2][0] = 0;
+
+    short high_humidity = shortFromBits(packets[0]);
+    short low_humidity = shortFromBits(packets[1]);
+    short high_temp = shortFromBits(packets[2]);
+    short low_temp = shortFromBits(packets[3]);
+
+    // If the values coincide with the checksum, then change the last values of temperature and humidity
+    // Otherwise, print the problem and exit the method
+    if ((high_humidity + low_humidity + high_temp + low_temp) % 256 == shortFromBits(packets[4])) {
+        m_humidity = (high_humidity * 256 + low_humidity) / 10;
+        m_temperature = MSB * (high_temp * 256 + low_temp) / 10;
+    } else {
+        return -1;
+    }
+
+    return 1;
+}
+
+int DHT22::getTemperature()
+{
+    return m_temperature;
+}
+
+int DHT22::getHumidity()
+{
+    return m_humidity;
+}
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