[PIC] How to achieve 2ms sampling rate by using PIC16F877a

Code C - [expand]
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#include<htc.h>
#include<pic16f877a.h>
 
#define LCD_DATA  PORTB
#define LCD_RS RB2
#define LCD_EN RB3
 
#define AN0  0
#define AN1  1
#define AN2  2
#define AN3  3
#define AN4  4
#define AN5  5
#define AN6  6
#define AN7  7
 
 
#define _XTAL_FREQ   20000000 
 
void init();
void lcddata(unsigned char);
void lcddisp(unsigned char);
void lcdcmd(unsigned char);
void delay(unsigned int);
void display(const char*);
unsigned int msCounter = 0;
unsigned int secCounter = 0;
unsigned int minCounter = 0;
unsigned int ADC_value = 0;
 
 
void interrupt ISR();
void InitTimer0(void);
void Init1msecTimerInterrupt(void);
void UpdateTimeCounters(void);
 
 
void InitADC(void);
unsigned int GetADCValue(unsigned char);
 
void main(void)
{
    unsigned int ADC_value = 0;
    unsigned int digit1, digit2, digit3, digit4;
 
    InitADC();          // Initialize ADC
    //PORTA=0xff;
    PORTB=0x00;
    TRISB=0x00;
    init();         // Initialize LCD
 
 
Init1msecTimerInterrupt();      // Start 2 msec timer
 
    
    while(1)
{
    if(msCounter == 0)       // msCounter becomes zero after exact 2 sec
            {
        
 
        ADC_value = GetADCValue(AN3);       // Read ADC value from RE2(AN7) pin
 
        // ADC_value can have a value from 0 (0v) to 1023(5v) only.
        // SO display 4 digits of ADC_value
        digit1 = (unsigned int)(ADC_value/1000);                                     // Calculate digit1 of ADC_value
        digit2 = (unsigned int)((ADC_value - digit1*1000)/100);                      // Calculate digit2 of ADC_value
        digit3 = (unsigned int)((ADC_value - (digit1*1000+digit2*100))/10);          // Calculate digit3 of ADC_value
        digit4 = (unsigned int)(ADC_value - (digit1*1000+digit2*100+digit3*10));     // Calculate digit4 of ADC_value
 
        lcdcmd(0xc0);lcdcmd(0x00);
        lcddisp(digit1+0x30);       // Display digit1 of ADC_value on LCD
        lcddisp(digit2+0x30);       // Display digit2 of ADC_value on LCD
        lcddisp(digit3+0x30);       // Display digit3 of ADC_value on LCD
        lcddisp(digit4+0x30);       // Display digit4 of ADC_value on LCD
 
        __delay_ms(500);                    // Half second delay before next reading
}
        UpdateTimeCounters();       // Update sec, min, hours counters*/
    }
}
 
void InitADC(void)
{
    ADCON1  = 0x80;      // Make PORTA and PORTE analog pins
                         // Also, Vref+ = 5v and Vref- = GND
    TRISA   = 0x2f;      // Make RA5, RA3, RA2, RA1, RA0 input
    TRISE   = 0x07;      // Make RE0, RE1 and RE2 input
    ADCON0  = 0x81;      // Turn on the A/D Converter
}
 
unsigned int GetADCValue(unsigned char Channel)
{
    ADCON0 &= 0xc7;         // Clear Channel selection bits
    ADCON0 |= (Channel<<3); // Select channel pin as ADC input
    
    __delay_ms(10);         // Time for Acqusition capacitor 
                            // to charge up and show correct value
    GO_nDONE  = 1;          // Enable Go/Done
 
    while(GO_nDONE);        // Wait for conversion completion
 
    return ((ADRESH<<8)+ADRESL);   // Return 10 bit ADC value
}
 
 
void delay(unsigned int v)
  {
  unsigned int i,j;
  for(i=0;i<v;i++)
  for(j=0;j<=1275;j++);
  return;
  }
  
 void init()
    {
    lcdcmd(0x00);lcdcmd(0x20);      // return home
    delay(5);
 
    lcdcmd(0x20);lcdcmd(0x80);      // 4 bit interfacing
    delay(5);
 
    lcdcmd(0x00);lcdcmd(0xc0);      // display on cursor off
    delay(5);
 
    lcdcmd(0x00);lcdcmd(0x60);      // to increment cursor 
    delay(5);
 
    return; 
    }
 
void lcdcmd(unsigned char v)  
    {
    LCD_DATA=v;  // send the data into the port
    LCD_RS=0;        // command register is selected 
    LCD_EN=1;        // enable the command register
    delay(5);
    LCD_EN=0;        // disable the command register 
    return; 
    }
 
void lcddata(unsigned char v)
    {
    LCD_DATA=v;   // send the data into the port
    LCD_RS=1;         // data register is selected 
    LCD_EN=1;         // enable the command register
    LCD_EN=0;         // disable the command register 
    return;
    } 
    
 void lcddisp(unsigned char v)
    {                                                     
    unsigned int disp,msb,lsb;
    disp=v;
    msb=disp&0xf0; 
    disp=disp<<4;
    lsb=disp&0xf0;
    lcddata(msb);
    lcddata(lsb);
    return;
    }
    
void display(const char *s)
{
    while(*s)
    lcddisp(*s++);
}
 
void InitTimer0(void)
{
    // Timer0 is 8bit timer, select T0CS and PSA to be zero
 
    OPTION_REG &= 0xC1;     // Make Prescalar 1:4
    INTE    = 1;
    GIE     = 1;        // Enable global interrupts
    T0IE = 1;               // Enable Timer0 interrupt
}
 
void Init1msecTimerInterrupt(void)
{
    InitTimer0();       // Intialize timer0 to genrate 1msec interrupts
}
 
void UpdateTimeCounters(void)
{
 
    if (msCounter==2000)
    {
        secCounter++;
        msCounter=0;
     }
    
    }
 
void interrupt ISR(void)
{
    if(T0IF)  //If Timer0 Interrupt
    {
        TMR0 = 0x08; // Timer0 should overflow after 250 instructions (250x4 = 1msec)
        T0IF = 0;    // Clear the interrupt 
        //INTF=0;
        msCounter++;
    }
 
}