Variable being reset upon completion of function C - c

Question

I am trying to initiate a timer with CTC and external interrupts that will count to 60 seconds and use a function to display the output to LEDs on an ATmega128. The function, as follows, will count all the way to the first iteration of the tens place when debugging, but on the next run-through the tens variable is reset to zero and I cannot understand why it is resetting. Can anyone see where I might have made an error?
#include <avr/io.h>
#include <inttypes.h>
#include <avr/interrupt.h>
#include <util/delay_basic.h>
volatile uint8_t passval = 0;
void display(uint8_t value);
int main(void)
{
DDRC = 0xFF; //PORTC output for two seven segment displays(tenths and ones place)
DDRD = 0x0F; //PORTD output for one seven segment display(tens place)
EICRA = ((1 << ISC01) | (1 << ISC00) | (1 << ISC11) | (1 << ISC10)); //Interrupt set to trigger on rising edge for PINA1...0
TCCR1B |= (1 << WGM12); //Sets CTC Mode
OCR1A = 24999; //Prescaler as 64 with a count of 25000
TCNT1 = 0; //Clears clock
TIMSK |= (1 << OCIE1A); //Enables interrupt
sei(); //Enable Global Interrupt
while (1)
{
TCCR1B |= ((1 << CS11) | (1 << CS10));
display(passval);
}
return 0;
}
void display(uint8_t value)
{
static uint8_t tenths, ones, tens;
tenths = value;
if (tenths > 0x09) //Rolls Over tenths place at 10
{
passval = 0x00;
tenths = 0x00;
ones++; //Advances ones place
if (ones > 0x09) //Rolls over ones places at 10
{
ones = 0x00;
tens++; //Advances tens place
if (tens > 0x06) //Rolls over tens place at 6
{
tens = 0x00;
}
}
}
PORTC = ((ones << 4) + tenths); //Display Output
PORTD = tens;
}
ISR (TIMER1_COMPA_vect) //Timer ISR
{
passval++; //Advance tenths place
}