Random Number Generation with PIC

[ABSF]

Just my sudden thought, kind of agree with generating the number from key press.

Though is not really random, but if the microcontroller is running at 20MHz, and each instruction is executed at 200ns. If changing the number at 7-segment take a few instructions, it will take around 1us to 2us. Basically no one cannot see and also predict the number that will appear on 7seg when it release the keypress.

Therefore it become random

Myke Predko has a smarter way to generate random number. Page 128 of my "Evil Genius" book has an example on how to generate random number using TMR0. The result is an integer between 0 and 255:

OPTION = 0b01001111; // Run TMR0 From PIC MCU Clock
;
;
; when random number is needed:
;
i = TMR0; // Save TMR0 Value
if (i > 100) // Convert TMR0 Value to Decimal
LCDWrite((i / 100) + '0', 1);
else // No Hundreds to Display
LCDWrite(' ', 1);
if (i > 10)
{
i = i % 100; // Not Greater than 10
LCDWrite((i / 10) + '0', 1);
}
else
LCDWrite(' ', 1);
LCDWrite((i % 10) + '0', 1);

As the program is copyrighted. I can only extract a portion out of the whole program.

Allen

[ABSF]

The random number part from the "Flickering Candle" experiment in his book is also very useful. He is using the Linear Shift Feedback Register (LFSR). The idea isn't all that new, but it's a pretty neat trick.

Yes I just found it in Exp 50 - The Pumpkin LED Display. But I dont quite understand how LFSR works.

int fourBitLFSR, sixBitLFSR;
int fourBitCount, sixBitCount;

fourBitLFSR = ((fourBitLFSR << 1) & 0x0F) +
((fourBitLFSR >> 3) ^
((fourBitLFSR >> 2) & 1));
fourBitCount = fourBitCount+ 1; // Keep Track of the Number of States

sixBitLFSR = ((sixBitLFSR << 1) & 0x3F) +
((sixBitLFSR >> 5) ^
((sixBitLFSR >> 4) & 1));
sixBitCount = sixBitCount+ 1; // Keep Track of the Number of States

PORTC = sixBitLFSR | fourBitLFSR; //Maximum number of LED on

Does "^" mean "to the power of" or "exponent of" ?
What does "|" mean ? Logical OR or AND?

Allen

[Brian Griffin]

Hello,

The LFSR is explained in a picture form. Since I do not have a picture, you need to imagine it:

Take an example, 8-bit number. Fill it with a number, any number: 0-255. Picture a strip with 8 boxes inside. Each bit correspond to the each box inside.

Then, take a bit, and another bit, example: the 5th and 6th bit, and XOR it. The result of XOR-ing the 5th and the 6th bit is then "fed" (hence the "feedback") back to the very first bit of the number, which is the bit 0th.

Afterwards, shift the 8-bit number to the right. The 5th and the 6th bit is XOR-ed and then bit 0th is overwritten by the result.

Repeat the process. Every shift gives a different number. The number is pseudo-random, which means the same number will appear again after a sequence, but it's not noticable if you don't really check on it.

A more different apporach is extending the number of gates, or chain-link it with more gates in between.

The ^ means bitwise XOR, and the | means bitwise OR.

[pic16]

i hope someday, i could understand what you guys talking about PIC here...