NotVeryRandom.c

/* "Random" seed generator for ATmega168P.

Application: An instrument needs to turn on a light and sound a buzzer a random time
_after being powered on_. Since power may not be on for a reliable amount of time,
we do not want to write to EEPROM. Must use environmental inputs to construct a 
not-very-reproducible input that can be used as a random seed.


This is a fragment of code, and does not include all necessary
I/O setup or infrastructure

We use three environmental inputs: A floating input pin, a temperature sensor, and 
a resistor midpoint (the voltage in the middle of a voltage divider from Vcc to Gnd)
as inputs to our ADC. We sum these values and use them as our seed.


*/

    unsigned int ADoutTemp;
    unsigned int RandTemp;
 
    uint8_t i;
    
    // Initialize outputs: No pull ups, and output pins are all high.

    DDRD = _BV(5) | _BV(6) | _BV(7); // Output pins: D5, D6, D7.
    PORTD = DDRD;

    DDRB = 0;   // PORT B is all input
    DDRC = 0;   // PORT C is all input
    
    PORTB = DDRB;
    PORTC = DDRC;
   
    CLKPR = 0;                    // set clock to maximum
    WDTCSR = 0x00;                // disable watchdog timer
    
    // ADC Setup
    
    PRR &=  ~(_BV(PRADC));  //Allow ADC to be powered up
    
    ADMUX = 8;		// Channel 8: Temperature sensor
    
    // Enable ADC, prescale at 2.
    ADCSRA = _BV(ADEN);
    ADCSRA |= _BV(ADSC);	// Start initial ADC cycle
    
    
    
    while ((ADCSRA & _BV(ADSC)) != 0)		// while ADC conversion has not finished
    {
        asm("nop;");
    }
    ADoutTemp = ADCW;
    
    ADMUX = 3;		// Channel 3 (resistor midpoint)
    
    ADCSRA |= _BV(ADSC);	// Start next ADC cycle
    while ((ADCSRA & _BV(ADSC)) != 0)		// while ADC conversion has not finished
    {
        asm("nop;");
    }
    
    ADoutTemp += ADCW;

    ADMUX = 4;		// Channel 4 (unconnected)
 
    ADCSRA |= _BV(ADSC);	// Start next ADC cycle
    while ((ADCSRA & _BV(ADSC)) != 0)		// while ADC conversion has not finished
    {
        asm("nop;");
    }
    ADoutTemp += ADCW;
    
    srand(ADoutTemp);