STM32F103 can't config MCO（PA8） and USB CLK together

Hello All,

I use STM32F103VCT6 and VirtualComPort_loopback Project, it worked well. But now I want to output clk through PA6(MOC) ,but it did't work unless comment out Set_USBClock(); . Who knows why?

void MCO_Config(void) 
{ 
GPIO_InitTypeDef is; 
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA,ENABLE); 
is.GPIO_Pin = GPIO_Pin_8; 
is.GPIO_Speed = GPIO_Speed_50MHz; 
is.GPIO_Mode = GPIO_Mode_AF_PP; 
GPIO_Init(GPIOA,&is); 
RCC_MCOConfig(RCC_MCO_SYSCLK); 
return; 
}

***************************************************************************** 
* @file main.c 
* @author MCD Application Team 
* @version V4.0.0 
* @date 21-January-2013 
* @brief Virtual Com Port Demo main file 
***************************************************************************** 
int main(void) 
{ 
MCO_Config(); 
Set_System(); 
Set_USBClock(); // comment out this line MOC will work 
USB_Interrupts_Config(); 
USB_Init(); 
MainLoop(); 
}