启动ucosii之七PC_SetTickRate(OS_TICKS_PER_SEC)

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/*
uCOS-II定义频率,以改变钟节拍的速率.
在DOS下,每秒产生18.20648次时钟节拍,或每隔54.925ms一次.这是因为82C54定时器芯片没有初始化,而使用默认值65535的结果.
如果初始化为58659,那么时钟节拍的速率就会精确地为20.000Hz
*/
void PC_SetTickRate (INT16U freq)
{
#if OS_CRITICAL_METHOD == 3                      /* Allocate storage for CPU status register           */
    OS_CPU_SR cpu_sr;
#endif
    INT16U     count;

//1.193Mhz/65536 ≈ 18.2HZ每秒18次中断
    if (freq == 18) {                            /* See if we need to restore the DOS frequency        */
        count = 0;
    } else if (freq > 0) {
                                                 /* Compute 8254 counts for desired frequency and ... */
                                                 /* ... round to nearest count                         */

//设置计数count=1.193Mhz/f + 0.5
        count = (INT16U)(((INT32U)2386360L / freq + 1) >> 1);
    } else {
        count = 0;
    }
    OS_ENTER_CRITICAL();
//控制字设置为通道0; 先读低字节,再读高字节; mode3-方波信号发生器; 计数器使用二进制.
    outp(TICK_T0_8254_CWR, TICK_T0_8254_CTR0_MODE3); /* Load the 8254 with desired frequency          */
    outp(TICK_T0_8254_CTR0, count & 0xFF);            /* Low byte                                     */
    outp(TICK_T0_8254_CTR0, (count >> 8) & 0xFF);     /* High byte                                     */
    OS_EXIT_CRITICAL();
}