Peter's electronic projects

Getting character LCDs to work at 3 volts

Generic character LCD modules contain an industry standard HD44780 compatible controller, which can operate down at 3 volts. But the modules are usually specified to work only at 5 volts, unless you choose a specific one designed for 3V operation.

This is the second page of the article.
< read the first page

Finding and adjusting the LCD oscillator

This modification is optional, you only need to do it if you have problems with the LCD being slow.

On both 5V LCD modules I checked, Rosc is a 91 kohm resistor. You can find this resistor by measuring it with a frequency meter or an oscilloscope and find the clock signal around the nominal 270kHz frequency. By replacing this resistor with a smaller value one, the oscillator will work faster.

This is a 2x16 character LCD module
Rosc on the LCD module

The signal on Rosc (R6 on the picture) is around 246 kHz
waveform on Rosc

This is a 4x20 character LCD module
Rosc on the LCD module

The signal on Rosc (R1 on the picture) is 206kHz
waveform on Rosc

The datasheet specification recommends a 91k 2% resistor at 5 volts, and a 75k 2% resistor at 3 volts. After replacing Rosc with a 75kohm resistor, you should recheck the frequency to be around the specified 270 kHz. Don't go above 350kHz, or the LCD may start to behave unreliably.

References