TTL USB dongles: Hacker's duct tape

For micro-controller projects, a TTL serial UART has a multitude of uses.  At a cost that is often under $1, it's not hard to justify having a few of them on hand.  I happen to have several

The first and probably simplest use is a breadboard power supply.  Most USB ports will provide at least 0.5A of 5V power, and the 3.3V regulator built into the UART chip can supply around 250mA.  With a couple of my dongles, I used a pair of pliers to straighten the header pins in order to plug them easily into a breadboard.

I've previously written about how to make an AVR programmer, although now that USBasp clones are widely available for under $2, there is little reason to go through the trouble.  Speaking of the USBasp, they can also be used along with TTL USB dongle to do 2.4Msps 2-channel digital capture.

Since TTL dongles usually have Rx and Tx LEDs, they can be used as simple indicator lights.  To show that a shell script has finished running, just add:
$ cat < /dev/zero > /dev/ttyUSB0
to the end of the script.  The continuous stream of zeros will mean the Tx LED stays brightly illuminated.  To cause the Tx LED to light up or flash for a specific period of time, set the baud rate, then send a specific number of characters:
$ stty -F /dev/ttyUSB0 300
$ echo -n '@@@@@@@@' > /dev/ttyUSB0
Adding a start and a stop bit to 8-bit characters means a total of 10 bits transmitted per characters, so sending 8 characters to the port will take 80 bit-times, which at 300 baud will take 267ms.

It's also possible to generate a clock signal using a UART.  The ASCII code for the letter 'U' is 0x55, which is 01010101 in 8-bit binary.  A UART transmits the least significant bit first, so after adding the start bit (0) and stop bit (1), the output is a continuous stream of alternating ones and zeros.  Simply setting the port to a high baud rate and echoing a stream of Us will generate a clock signal of half the baud rate.  Depending on OS and driver overhead, it may not be possible to pump out a continuous stream of data sending one character at a time from the shell.  Therefore I created a small program in C that will send data in 1KB blocks.  Using this program with a 3mbps baud rate I was able to create a 1.5Mhz clock signal.

If you have server that you remote monitor, you can use TTL dongles to reset a server.  This works best with paired servers, where one server can reset the other.  The RESET pin on a standard PC motherboard works by being pulled to ground, so the wiring is very basic:
RESET <---> TxD
  GND <---> GND
When one server hangs, log into the other, and send a break (extended logic level 0) to the serial port.  That will pull RESET on the hung server low, causing it to reboot.