GD32F450 200M时USB不稳定

使用GD32F450的demo修改usb驱动，发现120M和168M时正常，200M时很不稳定。怀疑USB时钟分频有问题，一查果然是，记录如下：

200M是库函数主时钟分频代码如下

static void system_clock_200m_25m_hxtal(void)
{
    uint32_t timeout = 0U;
    uint32_t stab_flag = 0U;

    /* enable HXTAL */
    RCU_CTL |= RCU_CTL_HXTALEN;

    /* wait until HXTAL is stable or the startup time is longer than HXTAL_STARTUP_TIMEOUT */
    do{
        timeout++;
        stab_flag = (RCU_CTL & RCU_CTL_HXTALSTB);
    }while((0U == stab_flag) && (HXTAL_STARTUP_TIMEOUT != timeout));

    /* if fail */
    if(0U == (RCU_CTL & RCU_CTL_HXTALSTB)){
        while(){
        }
    }

    RCU_APB1EN |= RCU_APB1EN_PMUEN;
    PMU_CTL |= PMU_CTL_LDOVS;

    /* HXTAL is stable */
    /* AHB = SYSCLK */
    RCU_CFG0 |= RCU_AHB_CKSYS_DIV1;
    /* APB2 = AHB/2 */
    RCU_CFG0 |= RCU_APB2_CKAHB_DIV2;
    /* APB1 = AHB/4 */
    RCU_CFG0 |= RCU_APB1_CKAHB_DIV4;

    /* Configure the main PLL, PLL_M = 25, PLL_N = 400, PLL_P = 2, PLL_Q = 9 */
    RCU_PLL = (25U | (400U << 6U) | (((2U >> 1U) - 1U) << 16U) |
                   (RCU_PLLSRC_HXTAL) | (9U << 24U));

    /* enable PLL */
    RCU_CTL |= RCU_CTL_PLLEN;

    /* wait until PLL is stable */
    while(0U == (RCU_CTL & RCU_CTL_PLLSTB)){
    }

    /* Enable the high-drive to extend the clock frequency to 200 Mhz */
    PMU_CTL |= PMU_CTL_HDEN;
    while(0U == (PMU_CS & PMU_CS_HDRF)){
    }

    /* select the high-drive mode */
    PMU_CTL |= PMU_CTL_HDS;
    while(0U == (PMU_CS & PMU_CS_HDSRF)){
    } 

    /* select PLL as system clock */
    RCU_CFG0 &= ~RCU_CFG0_SCS;
    RCU_CFG0 |= RCU_CKSYSSRC_PLLP;

    /* wait until PLL is selected as system clock */
    while(0U == (RCU_CFG0 & RCU_SCSS_PLLP)){
    }
}

这里注意几个重要信息，PLL_M=25,PLL_N=400,PLL_Q=9

再查看demo里的usb时钟配置代码，从代码里我们可以了解,USB时钟来源是PLLM_48M，而PLLM_48M是HXTAL倍频后经由PLLQ分频得到

    rcu_pll48m_clock_config(RCU_PLL48MSRC_PLLQ);

    rcu_ck48m_clock_config(RCU_CK48MSRC_PLL48M);

    rcu_periph_clock_enable(RCU_USBFS);

对比数据手册里USB这块的时钟树我们计算下USB时钟

输入时钟25Mhz，USB时钟=25/PLL_M*PLL_N/PLL_Q=25/25*400/9=44.44444MHz，而我们知道USB需要48MHz的时钟才能稳定工作，所以这导致了USB工作的不稳定。

为什么120MHz和168MHz时不会有这个稳定呢？那是因为不同时钟主时钟分频函数不一样，在120MHz和168MHz时能分出48M的时钟给USB。

知道问题哪后，就好解决了，方法如下：

1、直接修改库函数里PLL的分频系数，这个方法新人不推荐

2、使用内部RCK_48M作为USB的时钟源，

rcu_ck48m_clock_config(RCU_CK48MSRC_IRC48M)
rcu_osci_on(RCU_IRC48M)
/* wait till RCU_IRC48M is ready */
while(SUCCESS != rcu_osci_stab_wait(RCU_IRC48M)){
}

3、修改PLLM_48M由PLLAI分频来

    //使用PLLAI_Q作为PLL_SEL的输入
    rcu_pll48m_clock_config(RCU_PLL48MSRC_PLLSAIP);
    //使用PLL_SEL作为usb主频
    rcu_ck48m_clock_config(RCU_CK48MSRC_PLL48M);

    /* configure PLLSAI
     　PLLAI_N=192;PLL_P=192/4=48MHZ,PLL_Q=192/2=96MHZ,PLL_R=192/3=64MHZ
       注意，此处的PLL_P用于后面的USB主时钟，要确保为48MHZ
    */
    if(ERROR == rcu_pllsai_config(, , , ))
    {
        while();
    }

    rcu_osci_on(RCU_PLLSAI_CK);

    if(ERROR == rcu_osci_stab_wait(RCU_PLLSAI_CK))
    {
        while();
    }