They are the same, except for the LCD power supply, often called Vo or Ve.
As a reference, we remember the pinout of an alphanumeric LCD Module based on HD44780 or ST7066 is the following.
HD44780
VO is from 0 to Vcc , usually it can be tied to 0 or a trimmer can be used.
ST7066
V0 must be Vcc-10V to Vcc, so usually -5 to +5; however a good range is -2.5 to -0.5 V with respect to GND.
-2.0 to -1.1 is a good contrast and -1.5 can be considered a good average.
I have encountered (in an UniOp EK42) an interesting contrast control circuitry, fortunately designed to comprehend wide range for Vo supply. It was enough to replace a couple of resistors to change the mathematical function and adapt the circuit from HD44780 to ST7066.
With Vr that is a square wave from an MCU, with period about 10us, variable duty cycle; Vcc=5V, Vee=-5V (about).
component | original value | modified value |
---|---|---|
R1 | 100k | 100k |
R2 | 18k | 18k |
R3 | 100k | 68k |
R4 | 150k | 150k |
R5 | 1M | 560k |
R6 | 18k | 130k |
Maths:
V0 = R4/R3 * R2/R1 * Vr - R4/R3 * (R2+R1)/R1 * R6/(R5+R6) * Vcc
Note that Vee is not critical.
Original values meant a V0 from about -0.027 to +1.35 V (duty 50% gives about +0.65V)
Modified values mean a V0 from about -2.5 to -0.5 V (duty 50% gives about -1.1V)