add Chapter12

Chapter12/cpp_hal/hal/adc/inc/adc.hpp

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <expected>
+
+#include <units.hpp>
+
+namespace hal
+{
+    class adc {
+    public:
+        enum class error {
+            timeout
+        };
+
+        virtual void init() = 0;
+        virtual std::expected<units::voltage, adc::error> get_reading() = 0;
+    };
+};