Monday, March 25, 2013

CNC controller case.

Though I don't think I'll have Apple banging down my door to design their next iHipTM device, I'm slightly proud of the fact that I turned this:

Into this:

Wednesday, March 20, 2013

Stainless steel is tough to work with.

Not only does stainless steel foil not cut well, stainless steel plates do not solder well with basic lead-tin solder. resin flux, and a cheapo 25 watt iron.

From the Internet, I have learned that you either need hydrochloric acid, or silver based solder and a much higher wattage iron or even a torch. I don't like using or keeping dangerous chemicals, and HCl is fairly high up in the danger scale.

I attempted to use a butane lighter to heat the stainless steel which I had sanded/roughed after my first failed attempt with an iron. The lead solder formed a ball and slid around on the surface of the plate, resembling mercury. It refused to bond, however.

This is for a probe attachment for my CNC. I've decided to skip the soldering, I will just bend at right angle in a new strip of stainless, and drill a screw hole and attach a lead wire to the screw.


Wednesday, March 13, 2013

Seeding the standard C random number generator on AVR chips.

The rand() function in C gives your a pseudo-random number generator. To purists, it's got a lot of flaws, but I'm glossing over that for this post. In many cases, it is "good enough" to get the job done. A lot of times, I don't care that the distribution is not strictly even when you do something like.

int foo = rand() % 10;

Many times, just a rough approximation like above is enough to make something "feel random".

The rand() function uses a formula that calculates new "random" numbers based on a formula that includes the previously generated value(s).

So where do you get the "starting point" for the first number in the formula?

The standard C library maintains an internal state for the random number generator, and you can "seed" this state with the "srand(unsigned int)" function.

So what do you pass to it?

Well for any given "seed", you will generate the same sequence of pseudo-random numbers. For instance:

int a = rand() % 10;
int b = rand() % 10;
int c = rand() % 10;

will yield the same sequence for a,b, and c every time it is run. What if that is undesirable? It's almost like you need a random number to seed the random number generator, a "catch 22".

On desktops, seed values are often taken from some system time register, on Linux systems, it's sometimes generated from a timer that measures the time between a user typing keys.

On microcontrollers, you often don't have inputs, and system up-time timers don't work because the time will likely have the same value in the power up initialization sequence.

It can be difficult to get these miracle computing machines to be non-deterministic when you want them to.

A trick you may be able to use, depending on your setup is to use the ADC (analog digital convert) built in to most AVR's (on other brand) micros to read a voltage level on a pin that is "floating" or otherwise not tied well to a particular voltage. Here's a short example of how that looks on an AtTiny85:

#include <avr/io.h>
#include <stdlib.h>

void setup_seed()
unsigned char oldADMUX = ADMUX;
ADMUX |=  _BV(MUX0); //choose ADC1 on PB2
ADCSRA |= _BV(ADPS2) |_BV(ADPS1) |_BV(ADPS0); //set prescaler to max value, 128

ADCSRA |= _BV(ADEN); //enable the ADC
ADCSRA |= _BV(ADSC);//start conversion

while (ADCSRA & _BV(ADSC)); //wait until the hardware clears the flag. Note semicolon!

unsigned char byte1 = ADCL;

ADCSRA |= _BV(ADSC);//start conversion

while (ADCSRA & _BV(ADSC)); //wait again note semicolon!

unsigned char byte2 = ADCL;

unsigned int seed = byte1 << 8 | byte2;


ADCSRA &= ~_BV(ADEN); //disable ADC


In my case, PB2 was connected to a resistor, which was then connected through an LED to ground. When the pin is in a "high z" state (eg, not driven by the CPU), this approximates "floating" close enough to give me nice erratic values. Notice that I use the "low bits" of the ADC. The low bits represent smaller voltage differences, and exhibit greater variance, so they're more likly to swing a lot on a floating pin.

I'm not sure if it was really needed to scale the IO clock down by 128x, I just added it for flourish, thinking more time would mean more variance. I have no scientific evidence that is true though.



Thursday, March 7, 2013

Updates, random bits.

Here are some things in the works for future posts.

  • My Zen Toolworks CNC is nearly complete for CNC functionality. I have all the parts and materials for 3D printing, I just haven't tired to do it yet, I'm still mastering CNC functionality, and tweaking my Marlin based firmware as I realize I want particular functionality.
  • I have a complete failed project started and scrapped in the space of two weeks, a record for me. I'll write it up later, but one of my goals was "to succeed, or else fail fast", mission accomplished. I did learn *a lot* though, so it was not wasted time at all.

Here are some things I learned recently: