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.

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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 the pads with a frequency meter or an oscilloscope and look for the clock signal around the nominal 270kHz frequency. By replacing this resistor with a smaller value one, the oscillator will run 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