Tutorial: Creating Application API

Introduction

In this tutorial, you will learn how to create user application API for a simple, user-friendly programming of MCU.

We will create our own application API (EC API) , in similar format as mbed API.

EC API is based on the EC_HAL API that is based on CMSIS-CORE.

Structure of mbed-os

Comparison mbed API vs EC API

mbed API example code

Example code for Digital In and Out using mbed

#include "mbed.h"

DigitalIn  button(USER_BUTTON);
DigitalOut led(LED1);

int main() {
    while(1) {
        if(!button)  led = 1;  //if(!button.read())
        else         led = 0;
    }
}

EC API example code

We are going to create EC API is similar form, such as

EC_API

#include "EC_GPIO.h"

#define LED_PIN 	5
#define BUTTON_PIN 13

EC_DigitalIn button(GPIOC,BUTTON_PIN);
EC_DigitalOut led(GPIOA,LED_PIN);

	
int main(void) { 
	while(1){
		if(!button)	led=1;			//if(!button.read())
		else 				led=0;
	}
}

Case study: mbed API

Lets analyze how user API is structured in mbed. The application API is defined with C++ class and its methods. Each methods are based on HAL API, which is defined based on CMSIS-CORE.

For example, GPIO Digital In.

* mbed API: Class Digital In (DigitalIn.h)

* mbed HAL API: gpio_api.h

mbed API - DigitalIn.h

* mbed Microcontroller Library
 * Copyright (c) 2006-2020 ARM Limited

#ifndef MBED_DIGITALIN_H
#define MBED_DIGITALIN_H

#include "platform/platform.h"
#include "interfaces/InterfaceDigitalIn.h"
#include "hal/gpio_api.h"

namespace mbed {

class DigitalIn
{

public:
    
    DigitalIn(PinName pin) : gpio()
    {
        gpio_init_in(&gpio, pin);
    }

    DigitalIn(PinName pin, PinMode mode) : gpio()
    {
        gpio_init_in_ex(&gpio, pin, mode);
    }

    ~DigitalIn()
    {
        gpio_free(&gpio);
    }

    int read()
    {
        return gpio_read(&gpio);
    }

    void mode(PinMode pull);

    int is_connected()
    {
        return gpio_is_connected(&gpio);
    }

    operator int()
    {
        return read();
    }

protected:
#if !defined(DOXYGEN_ONLY)
    gpio_t gpio;
#endif //!defined(DOXYGEN_ONLY)
};


} 

#endif

mbed HAL - gpio_api.h

mbed API: Class Digital In (DigitalIn.h)

DigitalIn header defines the application API designed in C++ class structure. After class construction/initiation, the methods are easy to be used by the user. Here, you don't need to specifically define and refer to the register pointer for specific digital in pins.

In Each methods, it calls the functions defined in mbed HAL_API.

mbed HAL API: gpio_api.h

Underneath the simple application API, it calls more complex, more lower level HAL API. For example, in class construction (initialization), it finds which GPIO to be applied from the Pinname, using the call back function. gpio_init_in(&gpio, pin);

Tutorial: Create EC_API - for Digital In

Lets borrow the DigitalIn class from mbed API. To eliminate any redundancy defintion of variables, we will use prefix 'EC_ ' for Class, Variable names.

Create Application API source file

Application API: EC_GPIO_API.h, EC_GPIO_API.cpp

First, create header and source file as EC_ GPIO ___API. h and EC_ GPIO ___API. cpp

we will use *.cpp, which is C++ source file

Define Application API

Use the following source code to start. ecGPIO.h is the file you have created in LAB:GPIO Dgital InOut.

Unlike mbed API, we are going to input the GPIO and the pin number for initialization.

In " EC_GPIO_API.cpp ", you can define each methods. For this tutorial, we will use only *.h header file

EC_API - EC_GPIO_API.h

#include "stm32f411xe.h"
#include "ecGPIO.h"
#include "ecRCC.h"
#include <stdint.h>

#ifndef __EC_GPIO_API_H
#define __EC_GPIO_API_H

#define EC_DIN 		0

#define EC_PU 1
#define EC_PD 0
#define EC_NONE 0

#define EC_LOW 		0
#define EC_MEDIUM 1
#define EC_FAST 	2
#define EC_HIGH 	3

/* System CLOCK is HSI by default */

class EC_DigitalIn // declare class set the Application API name and declare class
{
public:
    EC_DigitalIn(GPIO_TypeDef *Port, int pin)
    // API initial seting
    {
			uint8_t mode=EC_DIN; // mode=0
			GPIO_init(Port, pin, mode);
			Port_t=Port;
			pin_t=pin;
			mode_t=mode;	
    }

    ~EC_DigitalIn()
    {
			 delete[] Port_t;
    }

    int read()
    {
				val_t = GPIO_read(Port_t, pin_t);
				return val_t;
    }
		
		void pupdr(int _pupd){
			GPIO_pudr(Port_t, pin_t, _pupd);
		}
    
    operator int()
    {
        return read();
    }

	private:
			GPIO_TypeDef *Port_t;
			int	pin_t;
			int mode_t;	
			int val_t;	
};

EC_HAL - ecGPIO.h

Use Application API

Lets compare the simple code based on 'EC_HAL' vs EC API'.

EC-HAL

/**
******************************************************************************
* @author  SSSLAB
* @Mod		 2021-8-12 by YKKIM  	
* @brief   Embedded Controller:  LAB Digital In/Out
*					 - Toggle LED LD2 by Button B1  pressing
* 
******************************************************************************
*/

#include "stm32f4xx.h"
#include "ecGPIO.h"
#include "ecRCC.h"

#define LED_PIN 	5
#define BUTTON_PIN 13

void setup(void);
	
int main(void) { 
	// Initialiization --------------------------------------------------------
	setup();
	
	// Inifinite Loop ----------------------------------------------------------
	while(1){
		if(GPIO_read(GPIOC, BUTTON_PIN) == 0)	GPIO_write(GPIOA, LED_PIN, HIGH);
		else 																	GPIO_write(GPIOA, LED_PIN, LOW);
	}
}


// Initialiization 
void setup(void)
{
	RCC_HSI_init();	
	GPIO_init(GPIOC, BUTTON_PIN, INPUT);  // calls RCC_GPIOC_enable()
	GPIO_init(GPIOA, LED_PIN, OUTPUT);    // calls RCC_GPIOA_enable()
}

EC-API

Exercise: Create EC_API - for Digital Out

Lets borrow the Digital Out class from mbed API. To eliminate any redundancy defintion of variables, we will use prefix 'EC_ ' for Class, Variable names.

Download source file

Download the source files: EC_GPIO_API_student.h, EC_GPIO_API_student.cpp

Rename the files as 'EC_GPIO_API.cpp ' and 'EC_GPIO_API.h'

Define Application API

Complete the definition of the class EC_DigitiOut.

You need to define the function in "EC_GPIO_API.cpp"

Then, run the following code.

#include "EC_GPIO_API.h"

#define LED_PIN 	5
#define BUTTON_PIN 13

EC_DigitalIn button(GPIOC,BUTTON_PIN);
EC_DigitalOut led(GPIOA,LED_PIN);

	
int main(void) { 
	// Initialiization --------------------------------------------------------

	// Inifinite Loop ----------------------------------------------------------
	while(1){
		if(button.read() == 0)	led.write(HIGH);
		else 										led.write(LOW);
		//if(!button)	led=1;
		//else 				led=0;
		

	}
}