SHT3x driver minor changes

- new high level function sht3x_measure added, it comprises all three
  steps to perform one measurement in only one function
- example with two sensors removed

examples/sht3x/sht3x.c

@@ -0,0 +1,153 @@
+/**
+ * Simple example with SHT3x sensor. 
+ *
+ * It shows different user task implementations in *single shot mode* and
+ * *periodic mode*. In *single shot* mode either low level or high level
+ * functions are used.
+ * 
+ * Constants SINGLE_SHOT_LOW_LEVEL and SINGLE_SHOT_HIGH_LEVEL controls which
+ * task implementation is used.
+ *
+ * Harware configuration:
+ *
+ *   +------------------------+     +----------+
+ *   | ESP8266  Bus 0         |     | SHT3x    |
+ *   |          GPIO 5 (SCL)  ------> SCL      |
+ *   |          GPIO 4 (SDA)  ------- SDA      |
+ *   +------------------------+     +----------+
+ */
+
+// #define SINGLE_SHOT_LOW_LEVEL
+// #define SINGLE_SHOT_HIGH_LEVEL
+ 
+#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
+
+#if defined(SINGLE_SHOT_HIGH_LEVEL)
+/*
+ * User task that triggers a measurement every 5 seconds. Due to power
+ * efficiency reasons it uses *single shot* mode. In this example it uses the
+ * high level function *sht3x_measure* to perform one measurement in each cycle.
+ */
+void user_task (void *pvParameters)
+{
+    float temperature;
+    float humidity;
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+
+    while (1) 
+    {
+        // perform one measurement and do something with the results
+        if (sht3x_measure (sensor, &temperature, &humidity))
+            printf("%.3f SHT3x Sensor: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3, temperature, humidity);
+
+        // wait until 5 seconds are over
+        vTaskDelayUntil(&last_wakeup, 5000 / portTICK_PERIOD_MS);
+    }
+}
+
+#elif defined(SINGLE_SHOT_LOW_LEVEL)
+/*
+ * User task that triggers a measurement every 5 seconds. Due to power
+ * efficiency reasons it uses *single shot* mode. In this example it starts the
+ * measurement, waits for the results and fetches the results using separate
+ * functions
+ */
+void user_task (void *pvParameters)
+{
+    float temperature;
+    float humidity;
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+
+    // get the measurement duration for high repeatability;
+    uint8_t duration = sht3x_get_measurement_duration(sht3x_high);
+    
+    while (1) 
+    {
+        // Trigger one measurement in single shot mode with high repeatability.
+        sht3x_start_measurement (sensor, sht3x_single_shot, sht3x_high);
+        
+        // Wait until measurement is ready (constant time of at least 30 ms
+        // or the duration returned from *sht3x_get_measurement_duration*).
+        vTaskDelay (duration);
+        
+        // retrieve the values and do something with them
+        if (sht3x_get_results (sensor, &temperature, &humidity))
+            printf("%.3f SHT3x Sensor: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3, temperature, 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 (*sht3x_periodic_1mps*).
+ */
+void user_task (void *pvParameters)
+{
+    float temperature;
+    float humidity;
+
+    // Start periodic measurements with 1 measurement per second.
+    sht3x_start_measurement (sensor, sht3x_periodic_1mps, sht3x_high);
+
+    // 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(sht3x_high));
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+    
+    while (1) 
+    {
+        // Get the values and do something with them.
+        if (sht3x_get_results (sensor, &temperature, &humidity))
+            printf("%.3f SHT3x Sensor: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3, temperature, 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, multiple sensors are possible.
+    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.
+}