12.3. Structure of an API in Embedded Systems

These functions initialize the hardware or peripheral, setting it up for further use. For example, a function might initialize a GPIO pin to configure it as an output or configure a timer to generate periodic interrupts.

void gpio_init(uint8_t pin) {
    // Configure pin as output
    GPIO->MODER |= (1 << (pin * 2));
}

Initialization functions are essential in APIs as they set up the hardware components for operation. In embedded systems, components like General Purpose Input/Output (GPIO) pins must be configured before they can be used. For instance, when a GPIO pin is initialized, it is typically set as either an input or output. The provided code example shows how a GPIO pin is set to output by adjusting specific bits in a register. This process ensures that the system starts with the correct hardware configuration before performing any operations.

Think of initialization functions like preparing a kitchen before cooking. Just as you would organize your utensils, heat your oven, and arrange your ingredients before starting to cook, initialization functions prepare the hardware so the system can perform its tasks smoothly.

These functions allow the user to control the peripheral or hardware component, such as setting a value or triggering an action. For example, a function might write data to a UART data register or toggle a GPIO pin.

void gpio_write(uint8_t pin, uint8_t value) {
    if (value) {
        GPIO->ODR |= (1 << pin); // Set pin high
    } else {
        GPIO->ODR &= ~(1 << pin); // Set pin low
    }
}

Control functions are used to manipulate the state or behavior of hardware components. In the example provided, the gpio_write function determines whether to set a GPIO pin high (on) or low (off) based on the value passed to it. If the value is high, the function modifies the Output Data Register (ODR) to switch the pin on; if it's low, it turns the pin off. This flexibility allows developers to control the operation of peripherals easily.

Imagine control functions like a light switch in your home. Just as you can flip a switch to turn the light on or off, control functions allow software to change the state of hardware components, enabling or disabling their activities as needed.

These functions allow the user to query the status of the peripheral or hardware component, such as checking if data is available on a UART or if a timer has expired.

uint8_t uart_is_data_ready(void) {
    return (USART->SR & USART_SR_RXNE); // Return 1 if data is ready
}

Status functions provide a way to check the current state of hardware components. In our example, the uart_is_data_ready function checks if there is data available to read from the USART (Universal Synchronous/Asynchronous Receiver Transmitter) by assessing a specific status register. This enables the system to react accordingly, ensuring that it only attempts to read data when it is actually available.

You can think of status functions like checking the fuel gauge in your car. Just as you look at the gauge to see if you need to fill up on gas before a long drive, status functions allow software to check whether a device is ready to proceed with data transmission or any operation.

These functions manage the interrupt mechanism for the peripheral. APIs can provide functions to enable/disable interrupts, set interrupt priorities, and clear interrupt flags.

void uart_enable_interrupt(void) {
    USART->CR1 |= USART_CR1_RXNEIE; // Enable interrupt on receive
}

Interrupt handling functions are critical for managing how an embedded system responds to events. The example function uart_enable_interrupt configures the USART to generate an interrupt when new data is received, allowing the system to react immediately without continuous polling. These functions help streamline operations by allowing the microcontroller to perform other tasks while waiting for events to occur.

Think of interrupt handling functions as doorbells. Just as a doorbell alerts you to someone at the door, enabling interrupts allows the CPU to be notified of important events, so it doesn't have to constantly check if something is happening—it simply responds when it is alerted.

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