LPC1758的pwm输出脉冲

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设置频率20HZ，实际输出5.429Hz，需要修改哪里才能输出准确20Hz呢，不知道为什么没有找到lpc1758的用户手册，不知道怎么配置
static uint32_t pwm_period = 0;
void PWM_CH6_Init(uint32_t frequency)
{
    LPC_PINCON->PINSEL4 &= ~(3 << 10);
    LPC_PINCON->PINSEL4 |= (1 << 10);   
    LPC_PINCON->PINMODE4 &= ~(3 << 10);
    LPC_PINCON->PINMODE4 |= (2 << 10);       
    LPC_SC->PCONP |= (1 << 6);  
    LPC_PWM1->TCR = (1 << 1);
    LPC_PWM1->PR = 0;   
    PWM_CH6_SetFrequency(frequency);   
    LPC_PWM1->MCR |= (1 << 1);   
    LPC_PWM1->MR6 = 0;   
    LPC_PWM1->PCR |= (1 << 14);
    LPC_PWM1->PCR |= (1 << 6);  
    LPC_PWM1->LER |= (1 << 0) | (1 << 6);
    LPC_PWM1->TCR = 0;                  
    LPC_PWM1->TCR = (1 << 0) | (1 << 3);  
}
void PWM_CH6_SetFrequency(uint32_t frequency)
{
    pwm_period = SystemCoreClock / frequency - 1;

    if (pwm_period < 1)
                        {
        pwm_period = 1;
      }
    LPC_PWM1->MR0 = pwm_period;
    LPC_PWM1->LER |= (1 << 0);
}
void PWM_CH6_SetDutyCycle(float duty_cycle)
{
    uint32_t match_value;
    if (duty_cycle > 100) {
        duty_cycle = 100;
    }   
    match_value = (uint32_t)((pwm_period * duty_cycle) / 100.0f+0.5f);   
    LPC_PWM1->MR6 = match_value;   
    LPC_PWM1->LER |= (1 << 6);
}
void PWM_CH6_Enable(void)
{
    LPC_PWM1->PCR |= (1 << 6);  // 使能PWM6输出
}
void PWM_CH6_Disable(void)
{
    LPC_PWM1->PCR &= ~(1 << 6); // 禁用PWM6输出
}

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用户手册这里找：https://www.nxp.com/products/LPC1758FBD80#documentation