An embedded system is a specialized computer system that is integrated into a device or machine to perform specific functions. These systems are typically based on an embedded processor, with the control program stored in ROM. In reality, almost every modern device with a digital interface—such as watches, microwave ovens, video recorders, and even cars—uses embedded systems. While some embedded systems include an operating system, most operate using a single, dedicated program that controls all functions. An embedded system generally consists of two main parts: the embedded computer system and the execution device. The embedded computer system serves as the core of the entire system and is composed of multiple layers, including hardware, middleware, system software, and application software. The execution device, also known as the actuator, receives commands from the embedded system and performs the required actions. These actuators can range from simple components like a small motor in a phone that vibrates, to complex systems such as a SONY smart robot dog, which integrates multiple microcontrollers and sensors to perform intricate movements and gather real-time data. From an application perspective, an embedded system is a combination of both hardware and software, and may also include auxiliary mechanical components. In China, the commonly accepted definition of an embedded system emphasizes its application-centered design, where it is built around a computer system tailored to meet strict requirements for functionality, reliability, cost, size, and power consumption.  1. **Small external buses**, such as those found in single-chip microcontrollers, allow connections to devices like true-color LCDs. Most mobile phones use synchronized screen interfaces, where addresses are first sent, followed by continuous data updates. 2. **ARM processor applications** vary depending on the model: - **ARM7**: Used in industrial control and mobile devices. - **ARM9**: Ideal for image display tasks. - **Cortex-A**: High-performance applications, such as smartphones and tablets. - **Cortex-M**: Low-end applications with minimal processing needs. - **Cortex-R**: Mid-range applications, less commonly used. 3. **ARM exceptions** are partially predefined by ARM itself, while others are defined by chip manufacturers. 4. **System Timer (Systick)** can serve as a general-purpose timer when no OS is present, or as the system clock when an OS is running. 5. **Interrupt handling** involves different registers for disabling and enabling interrupts. Disabling an interrupt is done by writing 1 to the mask register, while enabling it is done via the enable register. This separation prevents unnecessary read-modify-write operations and avoids affecting other interrupts. 6. **OD gate output** is useful when there's a mismatch between the processor’s voltage level and the peripheral’s. 7. **Reducing power consumption** by setting unused IO pins to either high or low output, or enabling internal pull-up resistors if configured as inputs. 8. **ADC and DAC** should be disabled during sleep mode to save power. 9. **Watchdog usage** is important for system stability. Avoid feeding the watchdog in interrupt service routines, and ensure regular resets for better reliability. 10. **I2C bus** requires pull-up resistors on both SDA and SCL lines. The communication process includes sending a start signal, addressing the slave, transferring data, and ending with a stop signal. Software-emulated I2C lacks bus arbitration and makes implementing slave devices more challenging. 11. **Logic 0 page mapping** allows for flexible boot configurations, enabling multiple systems to run from flash memory by redefining the boot address.

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