delay_t0.c

Embedded C 这本书的范例光碟程式

/*
   Delay_T0.C (v1.00)
   Simple hardware delays based on T0.
*/

#include "Main.H"

// ------ Private constants ----------------------------------------
 
// Timer preload values for use in simple (hardware) delays
// - Timers are 16-bit, manual reload ('one shot').
//
// NOTE: These values are portable but timings are *approximate*
//       and *must* be checked by hand if accurate timing is required.
//
// Define Timer 0 / Timer 1 reload values for ~1 msec delay
// NOTE: Adjustment made to allow for function call overheard etc.
#define PRELOAD01  (65536 - (tWord)(OSC_FREQ / (OSC_PER_INST * 1020)))    
#define PRELOAD01H (PRELOAD01 / 256)
#define PRELOAD01L (PRELOAD01 % 256)

/*
  DELAY_T0()
  Function to generate N millisecond delay (approx). 
  Uses Timer 0 (easily adapted to Timer 1).
*/
void DELAY_T0_Wait(const tWord N)   
{
   tWord ms;
   
   // Configure Timer 0 as a 16-bit timer 
   TMOD &= 0xF0; // Clear all T0 bits (T1 left unchanged)
   TMOD |= 0x01; // Set required T0 bits (T1 left unchanged) 

   ET0 = 0;  // No interupts

   // Delay value is *approximately* 1 ms per loop   
   for (ms = 0; ms < N; ms++)
   {                 
      TH0 = PRELOAD01H;  
      TL0 = PRELOAD01L;   
      TF0 = 0;          // clear overflow flag
      TR0 = 1;          // start timer 0
      while (TF0 == 0); // Loop until Timer 0 overflows (TF0 == 1)
      TR0 = 0;          // Stop Timer 0
   }
}        

/*------------------------------------------------------------------*-
  ---- END OF FILE -------------------------------------------------
-*------------------------------------------------------------------*/