Generating Random Numbers
- May 12th, 2011
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I was looking for example code for RF2500 last night and came across some TI code for using the ADC as a random number generator. The function was in assembly, so I rewrote it in C:
bool getRandomBit(){
ADC10CTL1 |= INCH_5;
ADC10CTL0 |= SREF_1 + ADC10SHT_1 + REFON + ADC10ON;
ADC10CTL0 |= ENC + ADC10SC;
while(ADC10CTL1 & ADC10BUSY);
return ADC10MEM & 0x01;
}Pin 1.5 is floating unconnected, and is measured by the adc. The LSB is used as the random bit. There’s a twist though. Pin 1.4 is also floating unconnected, and is used as Vref+, so the top end of the range is floating as well. I thought that was pretty clever. Nice, TI!
I wrote a few console programs to help me visualize the randomness. It turned out the generator was biased toward producing 0′s. This function used with the previous function seemed to remove the bias:
EDIT: it was just dumb luck that the following function removed the bias. I did the math afterward, and this 0′s the bias when getRandomBit() returns a 1 30% of the time. I guess that means the ’0′ to ’1′ ratio of getRandomBit() is close to 7 to 3. Not great. I’m mulling over the idea of using laser speckle as a cheap way to generate the randomness.
bool get0BiasRandomBit(){
if(getRandomBit()){
if(getRandomBit())
return 0;
else
return 1;
}
else{
if(getRandomBit())
return 1;
else
return 0;
}
}The following examples require the Full-duplex software UART for launchpad library, which was assembled by Rickta59.
To test for bias, I displayed a meandering line in a serial console. If I get more 0′s than 1′s, the line should slowly skew to the left or right:
#include "msp430g2231.h"
#include "config.h"
#include "softserial.h"
#include <stdbool.h>
#define CONSOLE_WIDTH 80
bool getRandomBit();
bool get0BiasRandomBit();
void main(){
int linePositon = CONSOLE_WIDTH / 2;
char cursorPosition;
DCOCTL = CALDCO_16MHZ;
BCSCTL1 = CALBC1_16MHZ;
WDTCTL = WDTPW + WDTHOLD; // Stop WDT
SoftSerial_init();
_enable_interrupts();
while(1){
for(cursorPosition = 0; cursorPosition < linePositon; cursorPosition++)
SoftSerial_xmit('8');
SoftSerial_xmit(' ');
cursorPosition++;
while(cursorPosition < CONSOLE_WIDTH){
SoftSerial_xmit('8');
cursorPosition++;
}
if(get0BiasRandomBit())
linePositon++;
else
linePositon--;
if(linePositon < 0)
linePositon = CONSOLE_WIDTH + linePositon;
else if(linePositon >= CONSOLE_WIDTH - 1)
linePositon = linePositon - CONSOLE_WIDTH;
}
}
bool get0BiasRandomBit(){
if(getRandomBit()){
if(getRandomBit())
return 0;
else
return 1;
}
else{
if(getRandomBit())
return 1;
else
return 0;
}
}
bool getRandomBit(){
ADC10CTL1 |= INCH_5;
ADC10CTL0 |= SREF_1 + ADC10SHT_1 + REFON + ADC10ON;
ADC10CTL0 |= ENC + ADC10SC;
while(ADC10CTL1 & ADC10BUSY);
return ADC10MEM & 0x01;
}and some example output from this code:
This sends a comma-separated list of random ints to the console:
#include "msp430g2231.h"
#include "config.h"
#include "softserial.h"
#include <string.h>
int adcGenRand16();
void reverse(char s[]);
void itoa(int n, char s[]);
void txString(char string[]);
void main(){
int random;
char string[7];
DCOCTL = CALDCO_16MHZ;
BCSCTL1 = CALBC1_16MHZ;
WDTCTL = WDTPW + WDTHOLD; // Stop WDT
SoftSerial_init();
_enable_interrupts();
while(1){
random = adcGenRand16();
itoa(random, string);
txString(string);
}
}
void txString(char string[]){
int iString = 0;
while(string[iString] != 0){
SoftSerial_xmit(string[iString]);
iString++;
}
SoftSerial_xmit(',');
SoftSerial_xmit(' ');
}
int adcGenRand16(){
char bit;
unsigned int random;
for(bit = 0; bit < 16; bit++){
ADC10CTL1 |= INCH_5;
ADC10CTL0 |= SREF_1 + ADC10SHT_1 + REFON + ADC10ON;
ADC10CTL0 |= ENC + ADC10SC;
while(ADC10CTL1 & ADC10BUSY);
random <<= 1;
random |= (ADC10MEM & 0x01);
}
return random;
}
/* itoa: convert n to characters in s */
void itoa(int n, char s[])
{
int i, sign;
if ((sign = n) < 0) /* record sign */
n = -n; /* make n positive */
i = 0;
do { /* generate digits in reverse order */
s[i++] = n % 10 + '0'; /* get next digit */
} while ((n /= 10) > 0); /* delete it */
if (sign < 0)
s[i++] = '-';
s[i] = '\0';
reverse(s);
}
/* reverse: reverse string s in place */
void reverse(char s[])
{
int i, j;
char c;
for (i = 0, j = strlen(s)-1; i<j; i++, j--) {
c = s[i];
s[i] = s[j];
s[j] = c;
}
}
I guess I got a little sidetracked from my original purpose of getting started with the RF2500, but I had fun.
