C8051F340延长喂狗间隔

C8051F340晶振12M,按照官方DataSheet中所讲使用PCA计时器，最大间隔为65.5ms，对于某些系统而言过于短暂。

但是没关系，PCA给我们提供了多个时钟源模式，只要配合定时器0就可以轻松的延长喂狗间隔。

首先设置PCA定时器

//-----------------------------------------------------------------------------
// PCA_Init
//-----------------------------------------------------------------------------
// This routine initializes the PCA to use the TR0 overflow
// as its clock source.  It also sets the offset value by writing to PCA0CPL2.
//-----------------------------------------------------------------------------
void PCA_Init (void)
{
   PCA0CN     =  0x40;                // PCA counter enable
   PCA0MD    &= ~0x40 ;               // Watchdog timer disabled-clearing bit 6
   // PCA0MD    &=  0xF1;                // timebase selected - System clock / 12
   PCA0MD    |=  0x04;                // use TR0 overflow as clock source
   PCA0MD    |=  0x80;
   PCA0CPL4   =  0xFF;                // Offset value
}

然后设置Timer0

//-----------------------------------------------------------------------------
// Timer0 Interrupt Init
// 定时器0,溢出作为PCA0时钟
// 使用系统时钟为定时器时钟
// 8位重载计数器,TH0 = 0x10,时间为240(0xF0)/12=20us
//-----------------------------------------------------------------------------
void Timer0_Init (void)
{
   EA  = 0;                // 关总中断
   ET0 = 0;                // 关定时器0中断
   CKCON &= 0XF8;          // 定时器0使用系统时钟为定时器时钟
   CKCON |= 0x04;
   TMOD  &= 0XF1;
   TMOD  |= 0X02;          // GATE0=0;C/T0=0(T0);02->8 bits reload
   TH0 = TIMER0_RELOAD;
   TL0 = TH0;
   // ET0 = 1;
   TR0 = 1;                // Timer0 ON
}

最后设置WDT

//-----------------------------------------------------------------------------
// 看门狗配置
//-----------------------------------------------------------------------------
void WatchDog_Config (void)
{
   PCA0MD  &= ~0x40;                   // WDTE = 0 (clear watchdog timer enable)
   PCA0L    = PCA0_L;                  // Set lower byte of PCA counter to 0  
   PCA0H    = 0x00;                    // Set higher byte of PCA counter to 0
   PCA0CPL4 = PCA0_RELOAD_L;           // Write offset for the WDT 
   PCA0CPH4 = PCA0_RELOAD_H;
   PCA0MD  |= 0x40;                    // Enable the WDT
}

其中宏是这么定义的

// Timer0
#define TIMER0_RELOAD 0x38       // Reload value for Timer0(200us)
#define TIMER0_RELOAD_100us 0x9C // 100us
// PCA(50,000)
// Offset = ( 256 x PCA0CPL4 ) + 256 - PCA0L
//        = ( 256 x 195(0xC3)) + 256 - 176(B0) = 50,000
// Time   = Offset * 20 us
//        = 1s ( PCA uses T0 overflow(20us) as its clock source)
#ifdef PCA_50000
#define PCA0_RELOAD_L   0xC3;
#define PCA0_RELOAD_H   0x01;
#define PCA0_L          0xB0;
#else
// PCA(10,000)
// Offset = ( 256 x PCA0CPL4 ) + 256 - PCA0L
//        = ( 256 x 39(0x27)) + 256 - 240 = 10,000
// Time   = Offset * 200 us
//        = 200ms ( PCA uses T0 overflow(20us) as its clock source)
#define PCA0_RELOAD_L   0x27;
#define PCA0_RELOAD_H   0x01;
#define PCA0_L          0xF0;
#endif