pwm输出50hz的周期的pwm来驱动舵机但pwm时钟不分频是什么原因？

调了一晚上了，想让pwm输出50hz的周期的pwm来驱动舵机，然而pwm时钟就是不分频。我设置的是16分频，但用pmwclkget函数，返回的是“不分频”，示波器测出来的是800hz，求大家解答。
附上pwm代码
#include"main.h"
#include"duoji.h"
#include"pwm.h"
#include "inc/hw_memmap.h"
#include "driverlib/gpio.h"
#include "driverlib/pin_map.h"
#include "driverlib/pwm.h"
#include "driverlib/sysctl.h"
#include "driverlib/uart.h"
void pwminit(void)
[
SysCtlPeripheralEnable(SYSCTL_PERIPH_PWM0);

PWMClockSet(PWM0_BASE,SYSCTL_PWMDIV_16);

SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOF);

GPIOPinConfigure(GPIO_PF0_M0PWM0);
GPIOPinConfigure(GPIO_PF1_M0PWM1);

GPIOPinTypePWM(GPIO_PORTF_BASE, GPIO_PIN_0|GPIO_PIN_1);

PWMGenConfigure(PWM0_BASE, PWM_GEN_0, PWM_GEN_MODE_UP_DOWN |
PWM_GEN_MODE_NO_SYNC);

PWMGenPeriodSet(PWM0_BASE, PWM_GEN_0, 20000);
PWMPulseWidthSet(PWM0_BASE, PWM_OUT_0|PWM_OUT_1,
1000);

long aw=PWMGenPeriodGet(PWM0_BASE, PWM_GEN_0);

PWMOutputState(PWM0_BASE, PWM_OUT_0_BIT|PWM_OUT_1_BIT, true);
PWMGenEnable(PWM0_BASE, PWM_GEN_0);
]
//然后main是这样的
int main(void)
[

g_ui32SysClock = MAP_SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ |
SYSCTL_OSC_MAIN |
SYSCTL_USE_PLL |
SYSCTL_CFG_VCO_480), 16000000);

pwminit();
uartinit();

adcinit();
IntEnable(INT_PWM0_0|INT_PWM0_1|INT_UART3);
IntMasterEnable();
while(1)
[


pmwclk2333=PWMClockGet(PWM0_BASE);

]
]