Add ds18b20 driver and example.

examples/ds18b20_onewire/Makefile

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+PROGRAM=ds18b20_onewire
+EXTRA_COMPONENTS = extras/onewire extras/ds18b20
+include ../../common.mk

examples/ds18b20_onewire/ds18b20_onewire.c

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+/* ds18b20 - Retrieves temperature from ds18b20 sensors and print it out.
+ *
+ * This sample code is in the public domain.,
+ */
+#include "espressif/esp_common.h"
+#include "esp/uart.h"
+
+#include "FreeRTOS.h"
+#include "task.h"
+#include "timers.h"
+#include "queue.h"
+
+// DS18B20 driver
+#include "ds18b20/ds18b20.h"
+
+void print_temperature(void *pvParameters)
+{
+    int delay = 500;
+    uint8_t amount = 0;
+    uint8_t sensors = 2;
+    DSENSOR t[sensors];
+    
+    // Use GPIO 13 as one wire pin. 
+    uint8_t GPIO_FOR_ONE_WIRE = 13;
+
+    while(1) {
+        // Search all DS18B20, return its amount and feed 't' structure with result data.
+        amount = readDS18B20(GPIO_FOR_ONE_WIRE, t);
+
+        if (amount < sensors){
+            printf("Something is wrong, I expect to see %d sensors \nbut just %d was detected!\n", sensors, amount);
+        }
+
+        for (int i = 0; i < amount; ++i)
+        {
+            printf("Sensor %d report: %d.%d C\n",t[i].id, t[i].major, t[i].minor);
+        }
+        printf("\n");
+        vTaskDelay(delay / portTICK_RATE_MS);
+    }
+}
+
+void user_init(void)
+{
+    uart_set_baud(0, 115200);
+
+    printf("SDK version:%s\n", sdk_system_get_sdk_version());
+
+    xTaskCreate(&print_temperature, (signed char *)"get_task", 256, NULL, 2, NULL);
+}
+