Really cheap USB to TTL on Atmega : 1.70$

admin — Sat, 11 Sep 2010 16:58:41 +0000

One of the most common way to interface a microcontroler to a computer used to be serial port. But right now, serial port have been replaced with USB on most computers. A common way to fix this issue is to use a USB to TTL converter or a USB to RS232 converter + MAX232. That’s fine but :

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() ..

 1 #define UART_TX	D,1
 2 #define UART_RX	D,2
 3 #define UART_DELAY	52 // 1/9600 = 104uS : 1/19200 = 52uS
 4
 5
 6 void uart_putc(char c)
 7 {
 8   uchar i;
 9   uchar temp;
10
11   // start
12   set_output(UART_TX);
13   _delay_us(UART_DELAY);
14   clr_output(UART_TX);
15
16   for(i=0;i<8;i++)
17   {
18     temp = c&1;
19     if (temp==0)
20       set_output(UART_TX);
21     else
22       clr_output(UART_TX);
23     _delay_us(UART_DELAY);
24
25      c = c >>1;
26   }
27
28   // stop
29   set_output(UART_TX);
30   _delay_us(UART_DELAY);
31   clr_output(UART_TX);
32
33   _delay_us(UART_DELAY);
34 }
35
36 uchar uart_getc()
37 {
38   uchar i;
39   uchar ib = 0;
40   uchar currentChar=0;
41
42   while (ib != 1)
43     ib = get_input(UART_RX);
44
45   _delay_us(UART_DELAY/2); // middle of the start bit
46   for(i=0;i<8;i++)
47     {
48       _delay_us(UART_DELAY);
49       ib = get_input(UART_RX);
50
51       if (ib ==0)
52 	currentChar |= 1<<i; // this is a 1
53     }
54   return currentChar;
55 }

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 ? :)

Jkx@home » atmega

Really cheap USB to TTL on Atmega : 1.70$