Random number generate in assembly.

[Peet19]

Hi!
I would like to generate a random number in assmebly from 0 to 50. I would like some help with this.
The PIC a 18F24K20.
Thank you in advance for your help!

 

[MaxHeadRoom78]

Try the Piclist site, IIRC a search brings up a result.

 

[upand_at_them]

Indeed, PICList has tons of stuff.

 

[Nigel Goodwin]

Bear in mind all these type of things only create pseudo random numbers, you need to somehow generate a random seed to start with - if you start with the same seed, you get the exact same pseudo-random sequence.

A simple solution, depending on your application, is to use a timer to generate a 'random' seed number - as the program starts up start a fast timer counting, and display "press button to start" - when the button is pressed, read the counter value and use that as the random seed.

 

[Peet19]

I looking, thanks.

Nigel: I understand, but there is no outside intervention. The program should automatically generate a random number at certain intervals.

 

[MaxHeadRoom78]

IIRC there is one on the Piclist site that uses a timer as a random generator.

 

[Nigel Goodwin]

IIRC there is one on the Piclist site that uses a timer as a random generator.

Which presumably still requires some kind of random input?, as I suggested - but presumably the OP is asking as it's a homework question?, and they are probably perfectly happy with pseudo random?.

 

[danadak]

Maybe this -

How to generate random numbers in C++ without using time as a seed

My program is supposed to generate random number with a fixed maximum of 9999. These numbers are used to create objects. Now the problem is when using time as a seed, if I create two objects one af...

stackoverflow.com

Thius is not ASM but you could use a compile to produce the ASM to incorporate
into your code.


Regards, Dana.

 

[Beau Schwabe]

If you have a spare I/O, an external RC timer with 'R' being a photo sensor works well

 

[Nigel Goodwin]

If you have a spare I/O, an external RC timer with 'R' being a photo sensor works well

That sounds a good idea - I was thinking along the lines of a resistor and electrolytic capacitor, on the grounds that the leakage and charging time of an electrolytic probably isn't all that accurate and repeatable, and by using a fast timer/counter to generate the seed a minor change of capacitor timing would produce a decent range of counter variation.

Presumably not a good idea to have the LDR inside a light proof box

 

[Ian Rogers]

I would try the ADC route... A spare input from a floating adc pin can produce random noise..

 

[Nigel Goodwin]

I would try the ADC route... A spare input from a floating adc pin can produce random noise..

Good point, they certainly do - having inadvertently left an ADC pin floating

 

[danadak]

I would try the ADC route... A spare input from a floating adc pin can produce random noise..

Leakage can drive the pin to one rail or the other.

But use A/D to look at its Vdd pin, typically thats 100+ mV of noise on it,
somewhat correlated but usable as a seed.


Regards, Dana.

 

[Peet19]

Thanks everyone for your help. It's slowly coming together.
One more question.
I would like to copy the lower four bits of a byte to another register.
How do I do this?

 

[MaxHeadRoom78]

Check out the Math Logic instructions,AND, XOR, IOR etc, in this case ANDLW or ANDWF If the lower four bits are a 1 when and'ed with the data to be copied, the lower four bits are copied over the high four are cleared, if this will do it for you.?

 

[Peet19]

Thanks, but that's not much help to me. An example would be nice.
For now, I solved it by erasing the upper four bits one by one and copying the value of the register to WREG.
But that's not the real thing.

 

[MaxHeadRoom78]

............XXXX XXXX
AND 0000 1111
Result 0000 XXXX

Andlw or andwf.

 

[Pommie]

Long time since I did any pic assembly but it's something like,

    movfw    var            ;get variable
    andlw    0x0f           ;keep only bottom 4 bits
    movwf    lowerbits      ;store in new variable

Mike.

 

[Triode]

Hey, if it's on a microcontroller you could read a floating analog pin, cut off the first part that isn't varying, multiply the part that is varying by some appropriately sized number, modulo it, and use it as a seed.

You can't just use that AS the random number because it's likely a normal distribution about some value, not random.

 

[Pommie]

If using a floating ADC pin then I'd just use the LSB and shift it into a variable until you have the number of bits you need. To get a 32 bit random seed would be very quick.

Mike.