I'm trying to make a fairly simple alarm clock for a school project. I am trying to use the 8051 timer 0 in 16 bit auto-reload mode to keep time. I figured out a timer seed for 10 millis and put it into the auto reload vars (I'm using Keil C51). The problem is that when I try to have the timer0_isr handle the time incrementing it takes way longer than if I have another function handle it...not good. I need to have the interrupt handle the time incrementing so I can have the user do other functions and still have the device keep time. Here's some code:

--------------
Works:

int count = 0;
int seconds = 0;
void foo(void) {
count++;
if (count == 100) {
seconds++;
count = 0;
}
}

void main(void) {
unsigned short seed = 55536;
while(1) {
TMOD = 1;
TR0 = 0;
TH0 = seed >> 8;
TL0 = seed & 0xff;
TF0 = 0;
TR0 = 1;
while(!TF0);
foo();
}
}

----------------
Doesn't work (seconds take about 2.5 seconds to increment):

int count = 0, seconds = 0;
void timer0_ISR (void) interrupt 1 {
if (count == 100) {
seconds++;
count = 0;
}
count++;
}

void main(void) {
unsigned short seed = 55536;
TMOD = 1;
TR0 = 0;
TH0 = seed >> 8;
TL0 = seed & 0xff;
TF0 = 0;
TR0 = 1;
EA = 1;
ET0 = 1;
while(1);
}