RISCV-ULP: Add DS18B20 1wire RISCV-ULP example

examples/system/ulp_riscv/gpio/main/ulp/main.c

@@ -0,0 +1,41 @@
+/* ULP-RISC-V example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+
+   This code runs on ULP-RISC-V  coprocessor
+*/
+
+#include <stdio.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include "ulp_riscv/ulp_riscv.h"
+#include "ulp_riscv/ulp_riscv_utils.h"
+#include "ulp_riscv/ulp_riscv_gpio.h"
+
+static bool gpio_level = false;
+
+/* this variable will be exported as a public symbol, visible from main CPU: */
+bool gpio_level_previous = false;
+
+int main (void)
+{
+    gpio_level = (bool)ulp_riscv_gpio_get_level(GPIO_NUM_0);
+    gpio_level_previous = gpio_level;
+
+    while(1) {
+        gpio_level = (bool)ulp_riscv_gpio_get_level(GPIO_NUM_0);
+
+        /* Wakes up the main CPU if pin changed its state */
+        if(gpio_level != gpio_level_previous) {
+            gpio_level_previous = gpio_level;
+            ulp_riscv_wakeup_main_processor();
+            break;
+        }
+    }
+    /* ulp_riscv_shutdown() is called automatically when main exits */
+    return 0;
+}