A/D conversion time accuracy

[tmach]

Hello!
I am using the A/D functionality in 16F874A. I am trying to exactly determine the A/D conversion time by using timer 0 reading. Please advice me if my calculations are right. I am using 20MHz oscillator, which means timer increments at the rate of .2 us (1/(20000000*4)). My code follows,

***********************************************************
#include <16f874A.h>
#device ADC=10;
#fuses HS,NOWDT,NOPROTECT
#use delay(clock=20000000)
#use rs232(baud=115200, xmit=PIN_C6, rcv=PIN_C7)

#int_ad
toggle_incr()
{
toggle = 1;
}

#int_timer0
timer0_isr()
{
timer_overflow++;
}

int8 timer_overflow = 0; //number of times the counter has overflown in between A/D conversion

main()
{
int8 seconds = 0;

setup_adc_ports(ALL_ANALOG);
setup_adc( ADC_CLOCK_DIV_32 );
set_timer0(0);
setup_counters(RTCC_INTERNAL,RTCC_DIV_1); //count 256
enable_interrupts(INT_TIMER0);
enable_interrupts(GLOBAL);
enable_interrupts(INT_AD);

while(TRUE) // Main program loop
{
set_timer0(0);
timer_overflow = 0;

seconds = get_timer0(); //get initial count

A2D(); //calls A/D routine. Not included in code

seconds -=get_timer0(); //get final count

printf("%02u %02u \n",seconds, timer_overflow);

}

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seconds gives the counter reading and timer_overflow gives number of timer overflows. So the total A/D coversion is:
(timer_overflow * 256 + seconds) * 0.2 us + 50 * .2 us

The 50 is included to account for the interrupt routine time lapse.

[Ttelmah]