• USB to TTL PCB cost a bit of money : you can find some on Ebay around 7€ (shipped) and 15$ on Sparfun !!!  That’s about 2 or 5 times the cost of the microcontoler !
• USB to RS232 cost 1.70$ (shipped) but need some extra level shifting and doesn’t really feet on a PCB (need a DB9 connector …)

In fact, USB to RS232 is a mass product, and the cost is really low. I decided to order a couple of this, just to look if I can use this stuff on a PCB. So I bought a 1.70$ USB to RS232 on Ebay.

I decided to rip the plastic off the DB9 and discovered a really tiny PCB. I removed the DB9, and decided to pass this little PCB to a scope session. How the hell do they manage to do a USB to RS232 with only a couple of external components ? They is no big capacitor for level shifter  (remember  RS232 is a +12/-12v ) ? The answer is simple, they don’t !!

This device isn’t RS232 compliant at all, the signals on the DB9 are TTL compliant, but not RS232. The ouput is between 0/5V and the input can handle -12/+12V but works great with a 0/5V too. I simply removed used pads on one side and added a couple on pins.

Please note that RX pin is missing on this pix but needed of course. The next step : How can I use this with an AVR Atmega (I used a Atmega8 but any will do the trick). Serial connection on a micro is TTL like this board, but the TTL signal is just inverted. A “1″ on the RS232 side is a -12V and +5V on a TTL, and a 0 on the RS232 side is a + 12V and a 0v on the TTL. You can find all the information here.

In fact MAX232 do both level shitting and inverting, but as I’m to lazy to wire a MAX232 (and will destroy the cheap aspect of this hack), I decided to handle this by software. This mean, I won’t be able to use the Atmega serial builtin port but need to write some additional code, to do the RS232 encoding/decoding by hand. Let’s give it a try :

I simply put this on a verroboard, connect VCC to USB Vcc, GND, RX and TX  to random pins on the AVR and let’s go to RS232 software serial. This can be done easily in fact, and I managed to handle 19200bauds with the internal 8Mhz clock of the Atmega. Above you will find the popular uart_putc() and uart_getc() ..

 #define UART_TX	D,1
 #define UART_RX	D,2
 #define UART_DELAY	52 // 1/9600 = 104uS : 1/19200 = 52uS


 void uart_putc(char c)
 {
   uchar i;
   uchar temp;

   // start
   set_output(UART_TX);
   _delay_us(UART_DELAY);
   clr_output(UART_TX);

   for(i=0;i<8;i++)
   {
     temp = c&1;
     if (temp==0)
       set_output(UART_TX);
     else
       clr_output(UART_TX);
     _delay_us(UART_DELAY);

      c = c >>1;
   }

   // stop
   set_output(UART_TX);
   _delay_us(UART_DELAY);
   clr_output(UART_TX);

   _delay_us(UART_DELAY);
 }

 uchar uart_getc()
 {
   uchar i;
   uchar ib = 0;
   uchar currentChar=0;

   while (ib != 1)
     ib = get_input(UART_RX);

   _delay_us(UART_DELAY/2); // middle of the start bit
   for(i=0;i<8;i++)
     {
       _delay_us(UART_DELAY);
       ib = get_input(UART_RX);

       if (ib ==0)
 	currentChar |= 1<<i; // this is a 1
     }
   return currentChar;
 }

Nothing more to say, this hack works really great, and I can now build a bunch of USB board without paying so much. The only drawback of this approach is that you can’t use an interrupt for the uart_getc() so you have deal with that in your code. Another approach would use a single transistor for the RX pin to make the RX compliant w/ the AVR serial builtin routine.

You can find the whole project C files + Makefile in a zip here. I think this little hack is really useful, so please send it to all to your DIYer friends, this can save them money, time …

// Enjoy cheap USB ? :)

The AVR Dragon is nice because you can use it as a small developpement device without any other requirement: Simply drop the needed ATMega on the board, some little wrapping for : Jtag + power supply.

As you can see, this is compact and nothing else is needed. The power supply come from the USB port, and I soldered a DIP on the board.. and that’s it.

I use the Jtag connector, so now I can use a real debugger instead of playing with the UART. Simply put a breakpoint, and enjoy :) By this way, I figure out that most of the time I simply push some stuff in arrays, and inspect them with debugger. This is really efficient. For example, last week I need to fix a timing issue with a IR sensor, simply wrap the little board, and push all interrupts in a array with the related timing. Of course, this can be done with a serial connection too, but it will take more time, and even worst if you encounter a bug, you will have to find where is it (the UART printf, or the code itself) ..

So, how to use this with a Linux OS ?

First you need to use AVaRICE to program the ATMega with a command like this :

avarice -g -j usb --erase --program --file main.hex :4242

Here the result:

AVaRICE flash the hex file to the ATMega, and wait for a GDB connection on port 4242. GDB is fine, but not really visual ;)

Let’s take a look at DDD

To use DDD with avr-gdb (the gdb for AVR), you need to edit a config file, for example gdb.conf and put this in :

file main.out
target remote localhost:4242

And the final command, just launch DDD like this :

ddd --debugger "avr-gdb -x gdb.conf"

Next step: Simply place some breakpoint, and the press “Cont” inue button in DDD. Et voilà :

I hope this little tuto will help people looking for a nice AVR debuger for the AVR on Linux (or any OSS system). The AVR Dragon is definitively a must have for low budget user in AVR scene.

Enjoy bug ? :)

My first thought was to use the avrlib, so I wired the LCD and started to use avrlib, but I quickly run into issues. In fact, I want to use some icons on, and the avrlib seems to be buggy.

After a little compile/test/run/compile, I remember an old post on a website about KS108 chips. I find it back, it’s on the science prog

This is really great, you can build some custom fonts easily (that’s exactly what i’m looking for)..

Here a couple of pics…

As you can see, I use a ATMega32 (on a futurelec card), a simple pot for the contrast, and the lumex.

I can use a mix of different fonts on the same screen.

And I can even display some pics too :)

Here the famous tux.

I still have to deal with menu and other stuff, this should be a little harder than expected, but will be nice. If somebody know a good library to use on a T6963C, please post a comment..

For users who wants high res pics, you can find them in my gallery (as usual).

After a little googling, I found there is 3 ways to fixe this kind of issue:

• Use a Jtag (as it doesn’t need a clocked micro)
• Use a high voltage programmer (you can erase the fuse, with a programmer that do a parallel programming with RESET plug to a 12v PSU). This kind of programmer aren’t really easy to find. The only one I know right now is the STK500
• Use an external clock (This is what I used, and describe here)

As, I don’t have any Jtag, or STK500, the only way to fix that is to use an external clock. In fact, I decided to ask on the #avr channel on freenode. And a really kool guy (“rue_mohr”) give me this tips. On every AVR micro, you can use a external clock if you messed the fuses settings. Simply plug a external clock on the clock pin (XTAL1 on the Mega32), and changes the fuses by this way.

Of course, you need a external clock at something like 1Mhz. Of course I don’t have this kind generator at home, but this can be easily done with another AVR micro this something like this:

void main()
{
  DDRA = 0xFF;
  while (1)
  {
     PORTA = ~PINA;
     nop();  // added some nop, to slow the clock a bit
     nop();
   }
}

This one use quite the same stuff than my previous I IV-18 clock, the main difference is the VFD power supply, but if you read this weblog, you will understand easily. I will publish the whole design and source code for free soon.

Anyway, here some pictures:

The PCB made with toner transfert

Soldering SOIC isn’t that hard ;)

The beast

The VFD tube is really big, and nice no ?

As you can see on the latest picture, there is a little gradian due to the filament supply. I haven’t find a simple to use (as the MAX6921) VFD driver that support AC filament. (MAX6931 is to small for hands soldering)

Enjoy :)

Ok .. so I decided to use a Maxim VFD driver: the MAX6921:

• 20 outputs
• 4 wire serial-interface
• supply voltage: 8v / 76v
• 40mA output

The main issue is that it only exist in a SO package, so I need to use a little adapter to use MAX6921 on a veroboard .. (I bought this at my local store, but they cost twice the price of the MAX)

The next step, is the clock itself. For my nixie clock, I tested several stuff like 50hz (mainlines) sync, or external oscillator but this isn’t accurate enought. This time, I decided to use a plain old DS32kHz crystal oscillator. I’m pretty sure this won’t loose a second in a week ;)

To finish the clock, I need a micro-controller, as I need a really little amout of IO (2 for buttons, 4 for the VFD driver, and 1 for the oscillator), I decided a ATtiny2313 could do the job perfectly and give me the enought room for other stuff (like a LDR for automatic dimming)

So, here the final schematic:

The full schematic

It’s the first time I used the ATtiny2313, but AVR-GCC is a great tools so.. To program this little stuff, I used the in-circuit programming, check out tuxgraphics doc for more infos about the whole stuff.

That’s fine, but does it look great ? ;)

Hum, yeah ! ;)

Here the (quite) finished product

And if you look closer at the VFD:

And the finished product :

Download

You will find a complete archive file here, it contains:

• All the sources need to program the ATtiny
• All the schematics to build you own ;)

/Enjoy

Here a little pic of this big guy:

As you can see this is quite large, and really nice looking. I spent a little time to figure out, how to make it working (reading russian isn’t really easy). After sending a mail to the NeonNixie group, Chris help me to fix my issue.

So here the IV-18 pinouts:

And the values:

• power supply for a digit : 20v-30v (if not muxed)
• power supply in muxed (for the 9 digit): 50v-70v
• the filament: 4.3v-5v at 100mA

The first step was to build the PSU

Power Supply

I want to build a small clock which feet in a small box without the use of a wallmart (external) transformer. So I went to my junk box, and found a small 2x6v – 350mA transformer. Hum, after a little test, I found that using this with a voltage doubler, I can get around 40v. Beside I should use 50v in muxed, I discover that 40v is far enought, and It will save the VFD life. (even if this kind of VFD should live at least 10 years without issue)

That fine.. but I need some 5v too. My first thought was to simply plug a 7805 in the middle of the voltage doubler… That works fine, except the 7805 is really hot, and need a large heatsink. So I decided to use a switching PSU based on a LM2575.

Here the final power supply. (as you can see, I added a diode the filament PSU to avoid it to glow too much)

Here’s a pic:

You can see

• the transformer
• the two caps and diodes from the doubler
• the LM2575 and its parts
• the ATtiny2313 (micro-controller used to do the logic)
• the DS32kHz (32.768kHz Temperature-Compensated Crystal Oscillator)
• and the little plug I used to do the in-circuit programming.

Some people asked what I’m doing .. that it .. here a teaser

This is some New Old Stock Russian VFD vaccum tubes.. ready for my next RSS reader !

I’m looking for some good electronics blogs, or planet.. If you know one please give me a comment

Update video is here (RSS reader block the embeded video)

Enjoy

Now, this is a little reminder for people (like me) which doesn’t read binary op fluently.

# This will off the bits according to value
x &= ~(value) 

# Here a little reverse: This will set bits on according to value
x |= value

Have fun w/ bits :)