How do I generate random numbers in a microcontroller efficiently?

Question

How do I generate random numbers in a microcontroller efficiently? Are there any general guidelines or a particular fast method?

Answer 1

If you can leave a pin floating, it could help to generate random numbers with linear feedback shift register. I'm not sure this is the way to go, but please have a look at my code:

// This code was written for 8051 (AT89S52)
unsigned char lfsr = 231; //8 bit shift register, with the seed of 231 or '11100111b'
unsigned char input_bit, i;

void main (void) 
{
    //Setup
    P0_0 = 0; // Leave Pin 0 Port 0 floating
    uart_init(); //Initializing uart/serial communication with pc


    while (1) 
    {
        for (i = 0; i < 255; i++) 
        {
            if (P0_0 == 1) // If Pin 0.0 is HIGH
            {
                input_bit = ((lfsr >> 0) ^ (lfsr >> 2) ^ (lfsr >> 3) ^ (lfsr >> 4)) & 1;
                lfsr = (lfsr >> 1) | (input_bit << 7);
            }
        }
        printf_tiny("%u\n", lfsr); //Send the random number to PC
        soft_delay(65535); //Simple delay function
    } //end while (1) loop

}

EDIT: I found out my answer may be a bad one. More details on why should not use a floating digital pin: https://electronics.stackexchange.com/questions/50476/random-number-generators-using-a-gpio-pin