Dear All,

I have a problem interfacing the LCD HD44780.

When I power my Embed with an external source (No USB) and powering the Display either on the 40th pin of embed or by external power supply at 12V. it lost its contrast.

What I have observed is:

Every time I switch on the device I need to adjust the Potentiometer placed between the (pin 3) LCD and GND, and more after calibrating it lost its calibration during the time

I have already tested the MBED with 3 different displays which are from different manufacturer and all of three does the same

I look forward to hearing from you soon

Regards

odd problem, i have worked with the hitachi 44780 chipset for over 20 years. it was my first peripheral / controller handshaking experience. did you try pin 39 on mbed at 5 V instead of 3.3V?

vin range is 4.5V to 9V DC, that allows for internal saturation and dissipation of about 1.5V. so 12V input is overkill. if you are running into a different voltage each time, you should be using something like a 10K trim POT or a simple diode. if the pot is dirty, then that is why you have to keep readjusting the output.

cheers

Ron

Please note that the 5V output (pin 39) is only available when mbed (lpc1768) is powered from USB. The pin is not powered when you use an external powersupply. HD44780 LCDs wont work properly at voltages below 5V. Some of the more recent HD44780 clones will work, but since 5V supply is missing there may be problems. The display could still work because of spurious power supply through some of the datapins. The voltage level for the contrast may be effected and that means you have to readjust it. Check the displays actual powersupply voltage at pin2 and the contrast voltage level at pin 3.

Another option you might try is making a voltage inverter using a ICL7661 or similar and using it to turn the +3.3v into -3.3v. Now power the LCD from the MBED VOUT (+3.3V) not VUSB and take the contrast pot to -3.3V. With a negative contrast voltage you should be able to get acceptable contrast. The downside is the time delays are slightly longer.

Edit: The datasheet I checked was for HD44780U, and its supply voltage range includes 3.3v. I don't know that all the equivalents support 3.3v but I would be suprised if they didn't. It is the actual LCD "glass" that needs more volts.