void usart_baudrate_set(uint32_t usart_periph, uint32_t baudval)
{
uint32_t uclk=0U, intdiv=0U, fradiv=0U, udiv=0U;
switch(usart_periph){
/* get clock frequency */
case USART0:
/* get USART0 clock */
uclk=rcu_clock_freq_get(CK_APB2);
break;
case USART1:
/* get USART1 clock */
uclk=rcu_clock_freq_get(CK_APB1);
break;
case USART2:
/* get USART2 clock */
uclk=rcu_clock_freq_get(CK_APB1);
break;
case UART3:
/* get UART3 clock */
uclk=rcu_clock_freq_get(CK_APB1);
break;
case UART4:
/* get UART4 clock */
uclk=rcu_clock_freq_get(CK_APB1);
break;
default:
break;
}
/* oversampling by 16, configure the value of USART_BAUD */
使用外部晶振初始化的时候，这里读到的 uclk 明显不对，
udiv = (uclk+baudval/2U)/baudval;
intdiv = udiv & 0xfff0U;
fradiv = udiv & 0xfU;
USART_BAUD(usart_periph) = ((USART_BAUD_FRADIV | USART_BAUD_INTDIV) & (intdiv | fradiv));
}
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