Ex-04: CMSIS Driver Introduction¶

In this exercise you will redo the Edge Detection Exercise by using the CMSIS driver. First read the theory concerning the CMSIS driver, then use it to reimplement the Ex-03.

CMSIS Driver¶

CMSIS description¶

Writing directly custom hardware addresses, as in the previous exercises, is error-prone, unreadable and impractical for reusing code. The MCU manufacturer ARM solve these problems with a driver library. The Common Microcontroller Software Interface Standard (CMSIS) enables device support and simple software interface to the processor and its peripherals. To use this driver with our MCU, two header files must be included:

#include "stm32f4xx.h"
#include "stm32f446xx.h"

The first include is a general include for stm32f4 controllers and the second is the specific include for stm32f446 controllers including all the addresses and register types.

CMSIS macros¶

The structure of these macros are always the same.

<PERIPHERAL>_<REGISTER>_<BITFIELD>_<ADDITIONALS>

GPIO_IDR_ID1

The description of the structure:

• PERIPHERAL

  The peripherals are the functional parts of the MCU such as GPIO, TIM, ADC, etc.

• REGISTER

  The name of the register in the peripheral device according to the reference manual RM0390 reference manual.

• BITFIELD

  The bit or bits to be changed. For example, the input register of a GPIO uses one bit for one input. The GPIO port mode register uses two bits for one pin.

• ADDITIONAL

  There are four possibilities:

  1. Nothing: Use directly the masked bitfield.

  2. Bit number: The specific bit in the bitfield. For example, in the GPIO port mode register, bit 0 and 1 are included in the bitfield of a pin.

  3. Pos: The bitfield’s position in the register.

  4. Msk: The masked bitfield as you have used in previous exercises.

CMSIS TypeDefs¶

In addition to the macros, CMSIS provides a struct of all the registers for each peripheral in the MCU. The struct is exactly the size of all the registers in a peripheral and the members are on the physical registers. This has the advantage that a pointer can be used to refer to the appropriate register with a readable name. CMSIS defines peripheral type macros at the correct address.

RCC->CR

CMSIS bit manipulation¶

In previous exercises you learned how to manipulate the registers of a peripheral with bit-wise operators. For a real-world implementation, we will probably use a set of macros to do this, rather than writing the plain bit operations. The code snippet below shows the most commonly used bit manipulation macros defined within the “stm32f4xx.h” header file.

#define SET_BIT(REG, BIT)     ((REG) |= (BIT))
#define CLEAR_BIT(REG, BIT)   ((REG) &= ~(BIT))
#define READ_BIT(REG, BIT)    ((REG) & (BIT))
#define CLEAR_REG(REG)        ((REG) = (0x0))
#define WRITE_REG(REG, VAL)   ((REG) = (VAL))
#define READ_REG(REG)         ((REG))
#define MODIFY_REG(REG, CLEARMASK, SETMASK)  WRITE_REG((REG), (((READ_REG(REG)) & (~(CLEARMASK))) | (SETMASK)))

SET_BIT(RCC->AHB1ENR, RCC_AHB1ENR_GPIODEN);

CLEAR_BIT(GPIOB->ODR, GPIO_ODR_OD4);

READ_BIT(GPIOC->IDR, GPIO_IDR_ID12)

CLEAR_REG(GPIOC->PUPDR);

WRITE_REG(SDIO->ARG, 0x2);

READ_REG(SPI1->DR)

MODIFY_REG(GPIOB->MODER, GPIO_MODER_MODE4_Msk, 0b01 << GPIO_MODER_MODE4_Pos);

Tasks¶

The aim is to reimplement the edge detection from ex-03, therefore refer to the task section in ex-03.