add Chapter12

Chapter12/cpp_hal/hal/uart/inc/uart.hpp

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+#pragma once
+
+#include <cstdint>
+
+#include <span>
+
+namespace hal
+{
+class uart
+{
+  public:
+    virtual void init(std::uint32_t baudrate) = 0;
+    virtual void write_array(const char * ptr, std::size_t len) = 0;
+
+    virtual void write(std::span<const char> data)
+    {
+      write_array(data.data(), data.size());
+    }
+};
+}; // namespace hal

Chapter12/cpp_hal/hal/uart/inc/uart_stm32.hpp

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+#pragma once
+
+#include <span>
+
+#include <cstdint>
+
+#include <uart.hpp>
+
+#include <stm32f0xx_hal.h>
+#include <stm32f072xb.h>
+
+namespace hal
+{
+class uart_stm32 : public uart
+{
+  public:
+    uart_stm32(USART_TypeDef *inst);
+
+    void init(std::uint32_t baudrate = c_baudrate_default) override;
+
+    void write_array(const char * ptr, std::size_t len) override;
+
+  private:
+    UART_HandleTypeDef huart_;
+    USART_TypeDef *instance_;
+    std::uint32_t baudrate_;
+    static constexpr std::uint32_t c_baudrate_default = 115200;
+};
+}; // namespace hal

Chapter12/cpp_hal/hal/uart/src/uart_stm32.cpp

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+#include <uart_stm32.hpp>
+
+hal::uart_stm32::uart_stm32(USART_TypeDef *inst)
+    : instance_(inst)
+{
+}
+
+void hal::uart_stm32::init(std::uint32_t baudrate)
+{
+    huart_.Instance = instance_;
+    huart_.Init.BaudRate = baudrate;
+    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;
+    // TODO: add GPIO initialization for real hardware
+    huart_.MspInitCallback = nullptr;
+    HAL_UART_Init(&huart_);
+}
+
+void hal::uart_stm32::write_array(const char * ptr, std::size_t len)
+{
+    // we must cast away costness due to ST HAL's API
+    char * data_ptr = const_cast<char *>(ptr);
+    HAL_UART_Transmit(&huart_, reinterpret_cast<uint8_t *>(data_ptr), len,
+                      HAL_MAX_DELAY);
+}