帮我写一个stm32f103定时器TIM3四通道pwm输出的代码

好的，我可以回答这个问题。以下是一个基本的stm32f103定时器TIM3四通道pwm输出的代码：

#include "stm32f10x.h"

void TIM3_PWM_Init(void)
{
    GPIO_InitTypeDef GPIO_InitStructure;
    TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
    TIM_OCInitTypeDef TIM_OCInitStructure;

    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB | RCC_APB2Periph_AFIO, ENABLE);
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE);

    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0 | GPIO_Pin_1 | GPIO_Pin_2 | GPIO_Pin_3;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOB, &GPIO_InitStructure);

    TIM_TimeBaseStructure.TIM_Period = 999;
    TIM_TimeBaseStructure.TIM_Prescaler = 71;
    TIM_TimeBaseStructure.TIM_ClockDivision = 0;
    TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
    TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);

    TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;
    TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
    TIM_OCInitStructure.TIM_Pulse = 0;
    TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;

    TIM_OC1Init(TIM3, &TIM_OCInitStructure);
    TIM_OC1PreloadConfig(TIM3, TIM_OCPreload_Enable);

    TIM_OC2Init(TIM3, &TIM_OCInitStructure);
    TIM_OC2PreloadConfig(TIM3, TIM_OCPreload_Enable);

    TIM_OC3Init(TIM3, &TIM_OCInitStructure);
    TIM_OC3PreloadConfig(TIM3, TIM_OCPreload_Enable);

    TIM_OC4Init(TIM3, &TIM_OCInitStructure);
    TIM_OC4PreloadConfig(TIM3, TIM_OCPreload_Enable);

    TIM_Cmd(TIM3, ENABLE);
}

int main(void)
{
    TIM3_PWM_Init();

    while (1)
    {
        // do something
    }
}

这个代码使用了stm32f103的定时器TIM3来实现四通道PWM输出。具体实现细节可以参考代码注释。