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
The signal on Rosc (R6 on the picture) is around 246 kHz
This is a 4x20 character LCD module
The signal on Rosc (R1 on the picture) is 206kHz
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