add Chapter04 bare example

Chapter04/bare/hal/inc/hal.hpp

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+#pragma once
+
+#include <cstdint>
+
+#include <stm32f0xx_hal.h>
+
+namespace hal
+{
+inline void init()
+{
+    HAL_Init();
+}
+
+struct time
+{
+    inline static std::uint32_t get_ms()
+    {
+        return HAL_GetTick();
+    }
+};
+}; // namespace hal

Chapter04/bare/hal/uart/inc/uart.hpp

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+#pragma once
+
+#include <cstdint>
+
+namespace hal
+{
+class uart
+{
+  public:
+    virtual void init() = 0;
+
+    virtual void putchar(char c) = 0;
+
+    void puts(const char *str)
+    {
+        std::size_t ind = 0;
+
+        if(str != nullptr)
+        {
+            while(str[ind] != '\0')
+            {
+                putchar(str[ind]);
+                ind++;
+            }
+        }
+    }
+};
+}; // namespace hal

Chapter04/bare/hal/uart/inc/uart_stm32.hpp

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+#pragma once
+
+#include <cstdint>
+
+#include <uart.hpp>
+
+#include <stm32f0xx_hal.h>
+
+namespace hal
+{
+class uart_stm32 : public uart
+{
+  public:
+    uart_stm32(USART_TypeDef *inst, std::uint32_t baud = 115200);
+
+    void init() override;
+
+    void putchar(char c) override;
+
+  private:
+    UART_HandleTypeDef huart;
+    USART_TypeDef *instance;
+    std::uint32_t baudrate;
+};
+}; // namespace hal

Chapter04/bare/hal/uart/src/uart_stm32.cpp

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+#include <uart_stm32.hpp>
+
+hal::uart_stm32::uart_stm32(USART_TypeDef *inst, std::uint32_t baud)
+    : instance(inst), baudrate(baud)
+{
+}
+
+void hal::uart_stm32::init()
+{
+    huart.Instance = USART2;
+    huart.Init.BaudRate = 115200;
+    huart.Init.WordLength = UART_WORDLENGTH_8B;
+    huart.Init.StopBits = UART_STOPBITS_1;
+    huart.Init.Parity = UART_PARITY_NONE;
+    huart.Init.Mode = UART_MODE_TX_RX;
+    huart.Init.HwFlowCtl = UART_HWCONTROL_NONE;
+    huart.Init.OverSampling = UART_OVERSAMPLING_16;
+    huart.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
+    huart.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
+
+    // huart.MspInitCallback = mspInitCallback;
+
+    if(HAL_UART_Init(&huart) != HAL_OK)
+    {
+        // Error_Handler();
+    }
+}
+
+void hal::uart_stm32::putchar(char c)
+{
+    HAL_UART_Transmit(&huart, reinterpret_cast<uint8_t *>(&c), 1,
+                      HAL_MAX_DELAY);
+}