• 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 only issue: I need to take it back to my computer when I want to upload a new film, that’s it .. When I first received this gift, I went back to the shop to exchange it against a networked one. But 1) they cost a lot more money, 2) not available at the shop. (even if it’s a really big one).

Last week, somebody sent me a mail. He found some cheap wireless access point with a USB port: Netgear WGT634U. (50Euro) Hum, ok let’s give it a try. First test, plug a Media Player to this USB port… ok It’s working fine. But you know, I don’t have a network cable near my bed, so I decided to transform the Access Point in a Wireless client. By this way, I can simply plug the Media Player on the WGT634U and remotely put some Divx via FTP, without any wire.

# hit Ctrl-C on the bootloader
CFE> ifconfig eth0 -auto
CFE> flash -noheader tftp_host:openwrt-wgt634u.bin flash0.os
CFE> reboot

config wifi-iface
option device   wifi0
option network  lan
option mode     sta
option ssid     MyDummySSID
option hidden   0
option encryption none

#### LAN configuration
config interface lan
option type     bridge
option ifname   "eth0.0"
option proto    'static'
option ipaddr   '192.168.3.2'
option netmask  '255.255.255.0'
option gateway  '192.168.3.1'
option dns      '192.168.1.254'

#!/bin/sh

# clear all firewall rules
for T in filter nat mangle ; do
iptables -t $T -F
iptables -t $T -X
done

# drop incomming packet
iptables -P FORWARD ACCEPT
iptables -P OUTPUT  ACCEPT
iptables -P INPUT   DROP

# accept traffic on localhost
iptables -A INPUT -p tcp -i lo -j ACCEPT
iptables -A INPUT -p udp -i lo -j ACCEPT

# accept ftp only from my home network
iptables -A INPUT -s 192.168.1.0/24 -p tcp -i br-lan --dport 21 -j ACCEPT

# accept incoming http / ssh
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# finaly accept already open Cnx
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

#!/bin/sh

while true
do
{
STATE=$(iwconfig ath0 |grep  00:13:13:53:DA:D1 | wc -l)
[ $STATE -eq 1 ] && echo 0 > /proc/diag/led/power
[ $STATE -eq 0 ] && echo 1 > /proc/diag/led/power
sleep 20
}
done

Step 6 / Finally

Here the result:

The finished product (grrr) works really fine and managed to achieve something like 850Ko/sec, I mean full speed ;) (11Mbs network) I need to remove the serial port right now, and close the box..

Important update

I read on Engadget that I use this to stream video to my TV, (like the Apple TV do), but this is absolutely wrong. This media player has a 320Go hard-drive. Why would stream film on this ? I just put it on that’s it :)

This is really amazing how people doesn’t read the article to see what I’ve done. It’s not the first time this happen in fact.

Here in France, most advanded users have some network TV from a long time, and this little boxes can play network stream (like the Apple TV) for a long time now. Beside you don’t pay for this service (rent for the boxes are included in DSL bill), the only issue is that you must have another computer to stream.

I guess nobody here (in France) would by a Apple TV, since we already have this kind of products for free for a long time. Look at a Freebox for example.

It’s time to watch a film :)