Driver for Sensirion SHT3x sensor added

This is a driver for Sensirion SHT3x temperature and humidity sensors
connected via I2C.

This commit is a rebasing and contains some interface changes based on
the review from @ourairquality.

SHT3x driver changes

- additional include to satisfy the Travis CI test build
- README.md has been shortened
- special handling for the timer overflow in sht3x_is_measuring removed
- some whitespaces removed
- crc8 computation changed to a non table lookup version
- measurement duration is now given in ticks and can be used directly
  for vTaskDelay (documentation and examples changed accordingly)
- number of ticks for measurement duration takes now into account
  portTICK_PERIOD_MS
- clock stretching disabled on sensor to avoid blocking when data are
  not ready to read
- calculation of maesurement duration adds now one and a half ticks to
  be sure that measurement duration is not too short
- function sht3x_is_measuring is now private and only for internal use,
  user task has always to use function vTaskDelay to wait for
  measurement results
- function sht3x_is_measuring was simplified and returns now just a
  boolean value
- private function sht3x_reset added which is used to reset the sensor
  during initialization
- active flag in sensor data structure not needed anymore
- function sht3_get_raw_data simplified
- function sht3x_start_measurement returns now only a boolean
- function sht3x_start_measurement does not check anymore whether there is
  already a measurment running
- new function sht3x_get_measurement_duration which returns the measurement
  duration in ticks for configured repeatability

examples/sht3x/sht3x_one_sensor/Makefile

@@ -0,0 +1,3 @@
+PROGRAM=SHT3x_One_Sensor
+EXTRA_COMPONENTS = extras/i2c extras/sht3x
+include ../../../common.mk

examples/sht3x/sht3x_one_sensor/sht3x_one_sensor.c

@@ -0,0 +1,119 @@
+/**
+ * Simple example with one SHT3x sensor. 
+ *
+ * It shows two different user task implementations, one in *single shot mode*
+ * and one in *periodic mode*.
+ * 
+ * Which implementation is used is defined by constant SINGLE_SHOT_MODE.
+ *
+ * Harware configuration:
+ *
+ *   +------------------------+     +----------+
+ *   | ESP8266  Bus 0         |     | SHT3x    |
+ *   |          GPIO 5 (SCL)  ------> SCL      |
+ *   |          GPIO 4 (SDA)  ------- SDA      |
+ *   +------------------------+     +----------+
+ */
+
+// #define SINGLE_SHOT_MODE
+ 
+#include "espressif/esp_common.h"
+#include "esp/uart.h"
+
+#include "FreeRTOS.h"
+#include "task.h"
+
+// include SHT3x driver
+#include "sht3x/sht3x.h"
+
+// define I2C interfaces at which SHTx3 sensors are connected
+#define I2C_BUS       0
+#define I2C_SCL_PIN   GPIO_ID_PIN((5))
+#define I2C_SDA_PIN   GPIO_ID_PIN((4))
+
+static sht3x_sensor_t* sensor;    // sensor device data structure
+
+#ifdef SINGLE_SHOT_MODE
+/*
+ * User task that triggers a measurement every 5 seconds. Due to
+ * power efficiency reasons, it uses the SHT3x *single_shot*.
+ */
+void user_task (void *pvParameters)
+{
+    sht3x_values_t values;
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+
+    while (1) 
+    {
+        // Trigger one measurement in single shot mode.
+        sht3x_start_measurement (sensor, single_shot);
+        
+        // Wait until measurement is ready (constant time of at least 30 ms
+        // or the duration returned from *sht3x_get_measurement_duration*).
+        vTaskDelay (sht3x_get_measurement_duration(sensor));
+        
+        // retrieve the values and do something with them
+        if (sht3x_get_results (sensor, &values))
+            printf("%.3f SHT3x Sensor: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3,  
+                   values.temperature, values.humidity);
+
+        // wait until 5 seconds are over
+        vTaskDelayUntil(&last_wakeup, 5000 / portTICK_PERIOD_MS);
+    }
+}
+#else  // PERIODIC MODE
+/*
+ * User task that fetches latest measurement results of sensor every 2
+ * seconds. It starts the SHT3x in periodic mode with 1 measurements per
+ * second (*periodic_1mps*).
+ */
+void user_task (void *pvParameters)
+{
+    sht3x_values_t values;
+
+    // Start periodic measurements with 1 measurement per second.
+    sht3x_start_measurement (sensor, periodic_1mps);
+
+    // Wait until first measurement is ready (constant time of at least 30 ms
+    // or the duration returned from *sht3x_get_measurement_duration*).
+    vTaskDelay (sht3x_get_measurement_duration(sensor));
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+    
+    while (1) 
+    {
+        // Get the values and do something with them.
+        if (sht3x_get_results (sensor, &values))
+            printf("%.3f SHT3x Sensor: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3,  
+                   values.temperature, values.humidity);
+                   
+        // Wait until 2 seconds (cycle time) are over.
+        vTaskDelayUntil(&last_wakeup, 2000 / portTICK_PERIOD_MS);
+    }
+}
+#endif
+
+void user_init(void)
+{
+    // Set UART Parameter.
+    uart_set_baud(0, 115200);
+
+    // Give the UART some time to settle.
+    sdk_os_delay_us(500);
+    
+    // Init I2C bus interfaces at which SHT3x sensors are connected
+    // (different busses are possible).
+    i2c_init(I2C_BUS, I2C_SCL_PIN, I2C_SDA_PIN, I2C_FREQ_100K);
+    
+    // Create the sensors.
+    if ((sensor = sht3x_init_sensor (I2C_BUS, SHT3x_ADDR_2)))
+    {
+        // Create a user task that uses the sensors.
+        xTaskCreate(user_task, "user_task", 256, NULL, 2, 0);
+    }
+
+    // That's it.
+}