Props Working ammo counter based on atmega8 (schematics and source code)

#include <avr/io.h>
#include <avr/interrupt.h>

#ifndef F_CPU
#define F_CPU	1000000
#endif

// fire key port
#define KEY_FIRE	(1 << PC2)

// reload key port
#define KEY_RELOAD	(1 << PC3)

#define KEYS		KEY_FIRE | KEY_RELOAD

// LED muzzle flash port
#define LED_MUZZLE	(1 << PB2)

// define PORTD bit mask for digits...
const uint8_t digits[] = {
	(0 << PD0) | (1 << PD1) | (1 << PD2) | (1 << PD3) | (1 << PD4) | (1 << PD5) | (1 << PD6) | (0 << PD7),
	(0 << PD0) | (0 << PD1) | (0 << PD2) | (1 << PD3) | (0 << PD4) | (0 << PD5) | (1 << PD6) | (0 << PD7),
	(1 << PD0) | (0 << PD1) | (1 << PD2) | (1 << PD3) | (1 << PD4) | (1 << PD5) | (0 << PD6) | (0 << PD7),
	(1 << PD0) | (0 << PD1) | (1 << PD2) | (1 << PD3) | (0 << PD4) | (1 << PD5) | (1 << PD6) | (0 << PD7),
	(1 << PD0) | (1 << PD1) | (0 << PD2) | (1 << PD3) | (0 << PD4) | (0 << PD5) | (1 << PD6) | (0 << PD7),
	(1 << PD0) | (1 << PD1) | (1 << PD2) | (0 << PD3) | (0 << PD4) | (1 << PD5) | (1 << PD6) | (0 << PD7),
	(1 << PD0) | (1 << PD1) | (1 << PD2) | (0 << PD3) | (1 << PD4) | (1 << PD5) | (1 << PD6) | (0 << PD7),
	(0 << PD0) | (0 << PD1) | (1 << PD2) | (1 << PD3) | (0 << PD4) | (0 << PD5) | (1 << PD6) | (0 << PD7),
	(1 << PD0) | (1 << PD1) | (1 << PD2) | (1 << PD3) | (1 << PD4) | (1 << PD5) | (1 << PD6) | (0 << PD7),
	(1 << PD0) | (1 << PD1) | (1 << PD2) | (1 << PD3) | (0 << PD4) | (1 << PD5) | (1 << PD6) | (0 << PD7)
};

// MA5B: 32, MA5C: 48, BR55: 48
#define DELAY 48

// MA5B: 60, MA5C: 32, BR55: 36
#define MAX_AMMO 32

// number of segments max
#define NUM_SEG 2


// current ammo in mag
volatile uint8_t ammo;

// current segment for multiplexing
volatile uint8_t segment;

// this is the multiplier for the different rifle types,
// the timer is set up the same for all, and the multiplier
// controls how fast the rifle fires
volatile uint8_t multi;


int main(void)
{
	DDRC	&= ~KEYS;
	DDRB	|= (1 << PB0) | (1 << PB2) | (1 << PB6) | (1 << PB7);	
	DDRD	|= 0xFF;

	PORTC	= KEYS;
	PORTB	= 0xFF;

	PORTB	&= ~LED_MUZZLE;

	ammo			= MAX_AMMO;
	segment			= 0;

	// multiplexer, debouncer
	TCCR0	= (1 << CS01); // prescaler 8
	TIMSK	= (1 << TOIE0);

	// interrupts are go
	sei();

	while (1)
	{
		asm volatile("nop" ::);
	}
}

ISR(TIMER0_OVF_vect)
{
	// multiplex off
	PORTB &= ~(1 << (PB3 + segment));

	// leds dark (~0x00)
	PORTD = 0xFF;
	
	// reload pressed?
	if (!(PINC & KEY_RELOAD))
	{
		ammo = MAX_AMMO;
	}

	// increment the multiplier
	multi++;

	// fire pressed?
	if (!(PINC & KEY_FIRE))
	{
		// we could fire again
		if (multi > DELAY)
		{
			// ammo left?
			if (ammo > 0)
			{
				// one less round
				ammo--;

				 // turn on flash LED
				PORTB |= LED_MUZZLE;
			}

			multi = 0; // reset
		}
		else
		{
			// turn of LED after 15 timer cycles
			// too fast and you can't see it
			// too slow and it's not a flash
			if (multi == 15)
			{
				PORTB &= ~LED_MUZZLE;
			}
		}
	}
	else
	{
		// fire not pressed, DODGY DEBOUNCE LOGIC (!!!)

		// if button released, but not yet indicated in the multiplier count
		if (multi < 100)
		{
			// add 100 to signalize release
			multi += 100;
		}

		// button was released more than 20 cycles ago
		if (multi > 120)
		{
			// set to 20 again, wait for fire trigger
			multi--;
		}
		else
		{
			// turn of LED after 15 timer cycles
			// too fast and you can't see it
			// too slow and it's not a flash
			if (multi == 115)
			{
				PORTB &= ~LED_MUZZLE;
			}
		}
	}

	// multiplex on
	segment++;

	// end of segment?
	if (segment >= NUM_SEG)
	{
		segment = 0;
	}

	uint8_t temp = ammo;

	// this is basically shifting the digit of the segment
	// to the first digit so we can read it
	for (int i = 0; i < segment; i++)
	{
		temp = temp / 10;
	}

	// get the bitmask for the segment digit
	PORTD = ~(digits[temp % 10]);

	// go for multiplexing
	PORTB |= (1 << (PB3 + segment));
}