A smaller rand()?

[SolidSilver]

I have a little project on the ATtiny13 that needs some random numbers. I got one rand() call in and the code went from ~150 bytes to almost 800. A second call pushed it over the limit. Sure, I could move it to a tiny85, but a) I don't have any, and b) I'm curious about alternatives to the stdlib rand() code. Part of the bloat might be the avr/delay.h stuff, as well. I'm looking into cutting that down.

Meanwhile, does anyone have a smaller solution to get some random?

[mtbf0]

if you're not using all of the a/d pins, you could try just reading an unconnected one. or, if you've got a nice noisy analog signal hooked up you sample it a few times and build a random number by shifting in the low order bit(s) of whatever you get.

[westfw]

How random do you need? I used the something like the following to drive some LEDs in a "flicker" brightness that was supposed to look like flames.

Code: Select all

/*
* 127 state pseudo-random numer
* Shift register with feedback at bits 0,6
* (The macro is tested, the function is not.)
*/

#define nextrandom(last)  \
    newbit = last & 0b01000000;  /* bit 6 */  \
    if (last & 1)            /* bit 0 */ \
      newbit ^=  0b01000000; \
    last = (last>>1) + newbit;

unsigned char rand(void)
{
    static unsigned char rand_seed;
    unsigned char newbit;

    nextrandom(rand_seed);
    return(rand_seed);
}


And here's a 16bit version...

Code: Select all

/*
* pseudorandom
* return the next pseudo-random number (PRN) using a standard maximum
* length xor-feedback 16-bit shift register.
* This returns the number from 1 to 65535 in a fixed but apparently
* random sequence.  No one number repeats.
* (This is the "standard" linear shift register with xor feedback PRN;
* other PRN generators could be used.)
*/
static unsigned short pseudorandom16 (void)
{
    unsigned short newbit = 0;

    if (randreg == 0) {
        randreg = 1;
    }
    if (randreg & 0x8000) newbit = 1;
    if (randreg & 0x4000) newbit ^= 1;
    if (randreg & 0x1000) newbit ^= 1;
    if (randreg & 0x0008) newbit ^= 1;
    randreg = (randreg << 1) + newbit;
    return randreg;
}


PRNGs make for pretty ugly C code; it's one of those bit-intensive things that CAN work more elegantly in assembler.

[SolidSilver]

Perfect! Swapping in the 16-bit rand routine cut the object code down to 342 bytes. Interestingly enough, this project is a flickering LED. Looks like I'll have enough room to get a more realistic dither in the pattern.

Thanks for the tip!

[SolidSilver]

Thanks for the suggestions, everyone. The end result is now an Instructable project.

[SolidSilver]

Woo hoo! My Instructables project got linked from Hack-a-day! (no, the link is in the story, not the headliner)