Twinkling LED algorithm for http://kennethfinnegan.blogspot.com/2009/12/twinkling-led-snowflakes.html

twinkle.c

#include <avr/io.h>      // this contains all the IO port definitions
#include <util/delay.h>

// Global used to store the generated randomness
unsigned int reg=1;

// This function basically wastes time
void delay_ms(long int ms) {
	for( ; ms > 0; ms--) {
		_delay_ms( 1);
	}
}

// Generate one random bit using a feedback shift register
// pseudorandom number generator algorithm
// The coefficients for this came from the TTL Cookbook, Lancaster
void gentick(void) {
	int newbit = (((reg>>3)^(reg>>12))^((reg>>14)^(reg>>15)))&1;
	reg = (reg<<1) + newbit;
}

int main(void) {
	int i;

	DDRB = 0xFF;        // set port B to output only

	PORTB = 0; // turn off all LEDs
	while (1) {
		// Generate this cycles randomness
		for (i=3+8; i; i--) {
			gentick();
		}
		// Flip a random LED
		PORTB = PORTB ^ (1<<(reg&7));
		// wait a random 0-0.5 seconds
		delay_ms(((reg>>3)&0xFF) * 2);
	}
}