// USE LEDREG.BIT

#pragma code

#include <reg51.h>
#include <stdlib.h>

unsigned char xdata *ledaddress;

// Global values managed by the interrupt routines
unsigned int timeHigh,      // Copy of count for computing the frequency
  count;    // Counts the number of 250 usec chunks 
unsigned char timeLow;      // usec count

// Interrupts should always be short.
// This means no loops or other time consuming operations.
//
// We will use timer 1 as a counter that will interrupt after
// N input clocks cycles.  We then copy the time information
// and reset the time to count the next N input clock cycles.
// When this counter interrupts, we can compute the frequency
// based on the amount of time for the N input clock cycles
// We use timer 1 for this because the T input is a free pin
// not connected to the Xilinx chip on the XESS board

void counterInterrupt ( void ) interrupt 3 using 1 {
  timeLow = TL0;
  TL0 = 0;
  timeHigh = count;
  count = 0;
  if (timeHigh == 0 && timeLow < 16) *ledaddress = 0x6f;  // 6 (1us)
  else if (timeHigh == 0 && timeLow < 106) *ledaddress = 0x6b; // 5 (10us)
  else if (timeHigh < 4) *ledaddress = 0x3a;  // 4 (100us)
  else if (timeHigh < 40) *ledaddress = 0x5b;  // 3 (1ms)
  else if (timeHigh < 400) *ledaddress = 0x5d;  // 2 (10ms)
  else if (timeHigh < 4000) *ledaddress = 0x12;  // 1 (100ms)
  else if (timeHigh < 40000) *ledaddress = 0x77;  // 0 (1s)
  else *ledaddress = 0xff;  // default
}

// Timer 0 is used just measure time: all we do is increment the
// counter if 250 usec have passed.
void timerInterrupt ( void ) interrupt 1 using 1 {
  count++;  
}

void init ( void ) {
  // Timer1:  Counter, mode 2 (auto-reload), with reload value 256- 10 = 246
  // Timer0:  Timer, mode 2, with reload value 256- 250 = 6
  TMOD = 0x62; // 01100010;
  TCON = 0x50; // 01010000;
  TH0 = 6;
  TH1 = 246;
  

  // **************************************************************************** 
  // Enable Interruppts
  // The IE register sets the state of the interrupt vectors.
  // Every bit in IE is for a different interrupt (see documentation for definition)
  // 
  //(MSB)                               (LSB)
  // EAall  NA  ET2  ES  ET1  EX1  ET0  EX0
  //
  // ****************************************************************************     

  // Enable both timer interrupts
  IE = 0xCA; // 110001010;   msf 1 to enable all
}

void main() {
     
  init();

  ledaddress = 0x0f000;  // the FPGA is configured to memory map led to this address

  while (1) { // all programs run forever       
    //*ledaddress = 0x01;
  }
}