Tuesday, September 30, 2014

One wire debug display

Lately I haven't had much time to work on hobby projects but I did manage to do a few things. Sometimes for debugging I use an LED if the UART pins are not free to output to a serial terminal. This works alright for simple things but gets complicated when I start outputting different patterns of flashes. Instead I decided to wire a small screen to a microcontroller so that I can bitbang out debug information with just one pin.

At the beginning, I considered using a WM-1611-62C LCD from Russian telephones that I have had for the last two years. It runs on 1.5v, so I haven't been in a hurry to do anything with it. The documentation for it shows Vdd supplied by 5v through a 4.7k resistor with two standard diodes between Vdd and Vss. Running from the 3.6v of the launchpad, the voltage between Vdd and Vss was well under 1.5v and the display was very dim. Adding another diode brought the voltage up and resulted in the clear display you see on the right. Poking at the pins revealed what some of them do and I was able to bitbang some characters to the screen. This was possible even at 3.6v through diodes since the inputs are open drain. Unfortunately, other than 0-9, only a few other specialized characters are possible. Displaying anything in hexidecimal would not be possible, so I scratched the idea of using it.

The next idea I had was to use a cell phone screen. In Kyrgyzstan I found some 1202 cell phone screens for about $2 and I tried soldering 0.10mm wires to one of them. At 700°F the iron was way too hot and even at 550°F I managed to destroy the tiny ribbon cable of the LCD. The next LCD I tried was from a Nokia 3510i but I managed to lift the pads off of that LCD. It seems that the temperature displayed by the soldering iron might be wrong. Even at 350°F the iron melts solder very quickly.

Another idea I had was to use some small green 7 segment displays I have lying around. With one MSP430 I could multiplex all four of them. Even at the 6mA that a single pin can supply, these displays were very dim. Maybe it has something to do with running green LEDs at 3.6v. Next I tried a red 7 segment display from an answering machine that was very bright even at 1mA, so I soldered everything to perfboard like you see on the left. The MSP430 on there waits for data to come in on the purple line and flashes a number out digit by digit. It is probably not practical for displaying 32 bit numbers in decimal but seems to work well enough for 8 and 16 bit numbers in hex. The headers on the right are for relaying incoming data to a PC over UART, even if the UART pins on the main microcontroller are in use.

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