add Chapter12

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

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+#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;
+    };
+};

Chapter12/cpp_hal/hal/adc/inc/adc_stm32.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <adc.hpp>
+#include <units.hpp>
+
+#include <stm32f0xx_hal.h>
+#include <stm32f072xb.h>
+
+namespace hal 
+{
+
+    class adc_stm32 : public adc{
+    public:
+        adc_stm32(units::voltage ref_voltage) : ref_voltage_(ref_voltage) {}
+
+        void init() override;
+        std::expected<units::voltage, adc::error> get_reading() override;
+    private:
+        ADC_HandleTypeDef adc_handle_;
+        units::voltage ref_voltage_;
+    };
+};