GD32VF103怎么输出系统时钟SYS_CLK，时钟是通过哪个引脚输出的？

GD32VF103芯片的时钟输出通过PA8引脚输出。以下是配置输出系统时钟的代码示例：

```c
/* Enable AFIO and GPIOA clock */
rcu_periph_clock_enable(RCU_AF);
rcu_periph_clock_enable(RCU_GPIOA);

/* Configure PA8 as alternate function */
gpio_init(GPIOA, GPIO_MODE_AF_PP, GPIO_OSPEED_50MHZ, GPIO_PIN_8);

/* Select timer15 PWM output as alternate function */
gpio_pin_remap_config(GPIO_SWJ_DISABLE_REMAP, ENABLE);

/* Enable TIMER15 clock */
rcu_periph_clock_enable(RCU_TIMER15);

/* Configure TIMER15 channel 0 as PWM output */
timer_oc_parameter_struct timer_ocinitpara;
timer_ocinitpara.oc_output_state = TIMER_OC_OUTPUT_IDLE_STATE;
timer_ocinitpara.oc_mode = TIMER_OC_MODE_PWM2;
timer_ocinitpara.oc_polarity = TIMER_OC_POLARITY_HIGH;
timer_ocinitpara.oc_idle_state = TIMER_OC_IDLE_STATE_LOW;
timer_output_oc_config(TIMER15, TIMER_CH_0, &timer_ocinitpara);

/* Enable TIMER15 */
timer_enable(TIMER15);

/* Enable system clock output */
rcu_cfg0 &= ~RCU_CFG0_I2SCKSEL;
rcu_cfg0 |= RCU_CFG0_I2SCKEN;
RCU_CFG0 = rcu_cfg0;
```

请注意，该代码配置了PA8引脚为TIM15_CH0的PWM输出功能，并将系统时钟输出到该引脚。你可以通过测量PA8引脚的频率来测试芯片时钟的精确性。