Understanding code for LED light for stm32f334R8? - c

Question

#include "stm32f30x_conf.h"
uint16_t read_pos(void);
void PC_Conf(void);
uint8_t get_bit(uint8_t, uint8_t);
// PROCESSORTACT = 64 MHz
// AHB Prescaler = 1
// APB1 Prescaler = 2
// APB2 Prescaler = 1
long pin = 1;
long dir = 1;
char recvd;
int main(void){
/*
*/
//Definitions
GPIO_InitTypeDef GPIO_Initstructure;
TIM_TimeBaseInitTypeDef TIM_TimeBaseInitStructure;
NVIC_InitTypeDef NVIC_NVICInitStructure;
//ENABLE CLOCK
RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOB, ENABLE);
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);
//GPIO
GPIO_Initstructure.GPIO_Mode=GPIO_Mode_OUT;
GPIO_Initstructure.GPIO_OType=GPIO_OType_PP;
GPIO_Initstructure.GPIO_Pin=GPIO_Pin_All;
GPIO_Initstructure.GPIO_PuPd=GPIO_PuPd_NOPULL;
GPIO_Initstructure.GPIO_Speed=GPIO_Speed_50MHz;
GPIO_Init(GPIOB, &GPIO_Initstructure);
//TIMER
TIM_TimeBaseInitStructure.TIM_Prescaler = (64000-1); //TIM2 cycle to 1kHz
TIM_TimeBaseInitStructure.TIM_Period = 500; //Every 500ms
TIM_TimeBaseInit(TIM2,&TIM_TimeBaseInitStructure);
//NVIC
NVIC_NVICInitStructure.NVIC_IRQChannel=TIM2_IRQn;
NVIC_NVICInitStructure.NVIC_IRQChannelCmd=ENABLE;
NVIC_NVICInitStructure.NVIC_IRQChannelPreemptionPriority=0;
NVIC_NVICInitStructure.NVIC_IRQChannelSubPriority=0;
NVIC_Init(&NVIC_NVICInitStructure);
//Enable everything
TIM_Cmd(TIM2, ENABLE);
TIM_ITConfig(TIM2, TIM_IT_Update, ENABLE);
/* Infinite loop */
while (1)
{
}
}
void TIM2_IRQHandler(void)
{
GPIOB->ODR &= 0;
GPIOB->ODR ^=1<<pin;
//Pin to new value
if(dir == 1)
{
if(pin == 5)
pin=11;
else if(pin == 15)
{ pin-=1;
dir = 0;
}
else
pin+=1;
}
else
{
if(pin == 11)
pin = 5;
else if(pin == 1)
{
pin+=1;
dir = 1;
}
else
pin-=1;
}
TIM_ClearITPendingBit(TIM2,TIM_IT_Update);
}
I have trouble understanding this code. First part is okay, but I don't understand the last function.
Can someone explain me simply what is going on in the last part of this code in TIM2_IRQHandler(void)?
I am new in this, any help would be welcome.

Answer 1

In many interrupts you need to clear the interrupt pending flag. If you don't when you exit from the handler, the interrupt will be called again. Every interrupt is described in the Reference manual of your micro. For example SPI interrupt pending flag is cleared by writing or reading from the DR register. Another one requires that bit to be cleared by the programmer.
The rule of thumb: clear the flag as soon as you enter the interrupt.
If the interrupt can be triggered by more than one event, the programmer should check which one was the source of the exception.