• 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 ? :)

I decided to open this little key, to look closely. After a quick look, I didn’t find anything. So this little PCB stuff was laying on me desk, and people play with it. After a couple of weeks, I discover the memory chip is simply de-solder. My friend at work play so much with it, the failure is now apparent. Nice no ?

Hum, this little TQFN chip will be hard to solder, that’s right. But this is do-able no ? I tried this with a small soldering iron and here the result:

And yes, the key works fine right now ;) That’s fine no ? I definitively won’t trust this drive enought to store important document, but fine for a couple of mp3, or other..

This stuff looks great and you can even plug a hard-drive. The main issue is that the firmware only support 999 files on a single USB device, so that’s enougth for ~4Go of MP3 but not more. So I decided to buy some USB sticks, it’s far more simple .. no external power supply, and just the room to place a quite large MP3 collection (simply buy 2/3 sticks and that rules)

That’s ok, but hum, as you can see on this picture the USB plug is on the front panel. And that’s a mess, cause you have to unplugged it when you want to remove the front panel (some infamous guys already try to open my car 4 times..), or to put a CD in. Beside this is a gift, I decided to open it, and try add a external USB in the back. I think my friends will understand this: “If you can’t open it, you don’t own it” ..

First, I decided to open the front panel, to look how the USB plug is connected. As you can see on the aboves pictures, the USB power-supply come form the main board.

That’s fine.. We simply have to add take the power from the source ..

When I look closely, I discover that the PSU provide 5.5v instead of 5.0v. The main reason, is that the front panel contain a small chips do the final regulation.. Ok, we can do the same ? :) I used a low drop 5.0v regulator: LP38691 with 2 small caps..

The final step, is to connect the data bus (DATA- / DATA+), this was really hard, because the front panel USB bus is connected to the main decoder board throught a small wire. As I’m not able to find the right plug, I decided to solder it on the board ..

So we have something like this:

The small PCB:

As you can see on this picture, on the left, there is the wires from the main board, and the 5.0v regulator. On the right the wire that goes to the decoder board, and the external new USB connector.

Here the finished product:

Ok, that’s fine, now, you can place this small board in system (there is a lot of room), and enjoy the external USB cable ;)

With time, Python really became a powerfull tool for hardware hackers :) That’s perhaps why I like it so much ?

• Download latest eagle-usb
• Untar
• make
• make install
• eagleconfig (to enter your config..)
• rm /etc/init.d/eagle-usb (this doesn’t work on debian ..)
• eaglectl -w (to load kernel modules and so on..)
• startadsl