我用nisoii里的定时器定时1s，使能中断，中断服务程序里控制IO口，关闭我的硬件计数器计数功能，同时读出计数。所得计数即为我要测的频率。

#include "alt_types.h"
#include <stdio.h>
#include "system.h"
#include "sys/alt_irq.h"
#include "altera_avalon_timer_regs.h"
#define alt_cpu_freq 50000000
volatile alt_u32 counter;
volatile int ito_capture;
void init_timer0(void)
{
//设定1s定时周期
void *ito_capture_ptr=(void *)& ito_capture;//重置ito捕获事件，使其符合alt_irq_register()函数的要求
IOWR_ALTERA_AVALON_TIMER_PERIODL(TIMER0_BASE, (short)(alt_cpu_freq&0x0000ffff));
IOWR_ALTERA_AVALON_TIMER_PERIODH(TIMER0_BASE, (short)((alt_cpu_freq>>16)&0x0000ffff));

//开启定时器，并设定为循环工作,允许中断
IOWR_ALTERA_AVALON_TIMER_CONTROL(TIMER0_BASE,7);

//注册定时器0中断服务函数
int alt_irq_register(TIMER0_IRQ,ito_capture_ptr,timer0_interrupts);
}

void timer0_interrupts (void *context,alt_u32 id)
{
volatile int *ito_capture_ptr=(volatile int *) context;//调整context指针为整数型
// *ito_capture_ptr=IORD_ALTERA_AVALON_TIMER_CONTROL_ITO_MSK(TIMER0_BASE); //读计数器值
//向状态寄存器执行写操作，清除定时器溢出状态,即清timer0标志
IOWR_ALTERA_AVALON_TIMER_STATUS(TIMER0_BASE, 0x0);
IOWR_ALTERA_AVALON_PIO_DATA(PB0_BASE, *ito_capture_ptr); //写到pb0输出口

}

int main(void)
{
// volatile alt_u32 counter;//暂存计数器计数（PB引脚状态）
init_timer0();
while (1);
return 0;
}
编译出现这样的错误，好像是中断注册不对，不知道咋改了.
**** Build of configuration Debug for project pinlvji ****

make -s all includes
Compiling pinlvji.c...
../pinlvji.c: In function `init_timer0':
../pinlvji.c:21: error: parse error before numeric constant
../pinlvji.c:13: warning: unused variable `ito_capture_ptr'
../pinlvji.c: In function `timer0_interrupts':
../pinlvji.c:30: warning: implicit declaration of function `IOWR_ALTERA_AVALON_PIO_DATA'
make: *** [obj/pinlvji.o] Error 1
Build completed in 3.313 seconds