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Support Forum » LCD and lower voltages

October 14, 2012
by mblythe86
mblythe86's Avatar

Hi,

I'm trying to interface the LCD module from my NerdKit with an STM32F0 Discover board. The Discovery board runs at 3V, so I'm trying to use the same power source for the LCD as well. The spec sheet for the LCD controller (HD44780) suggests that it should work down to 2.7V (it just requires mor time for self-init); however, when I give it only 3V, nothing seems to happen. Does the LCD module itself (instead of the controller chip) have a requirement that it's run at 5V? Is there something else I'm overlooking?

Thanks, Matt

October 15, 2012
by mongo
mongo's Avatar

When the specs are written, the writers look at ideal and perfect conditions. In the real worls, things like that don't really happen. So, if they say it may work at 2.7 volts to 7.5 volts, it's best to stick with the recommended voltage, typically 5 volts.

October 16, 2012
by pcbolt
pcbolt's Avatar

Matt -

Did you try changing the "contrast" resistor value? I'd think you'd still see something with the resistor used for 5v, but then again you are operating near the lower end of the operating values, so who knows... might be worth a try.

October 16, 2012
by mblythe86
mblythe86's Avatar

mongo,
Yes, I agree that datasheets are often written with ideal conditions in mind; however, when there is a section with the heading "DC Characteristics (VCC = 2.7 to 4.5 V)", it should be safe to assume that the part will work under the specified conditions. Also, I don't see anything in the LCD documentation itself that states that 5V is recommended.

pcbolt,
I've tried running the "contrast" line to 3V or ground (without any resistor), and still don't see anything displayed.

Unfortunately, I can't find much documentation about what the contrast pin is or how the circuit behind it works. The closest thing I've found is this which suggests that connecting contrast to ground is usually sufficient, but a connection to a negative voltage may be necessary. (Probably won't bother trying this)

It looks like the LCD does work with the Discovery board as long as the LCD itself is powered by 5V. The 3V from the GPIOs of the discovery board seems to be high enough to allow communication with the LCD module.

I guess I'll settle with this as a solution unless there are any other ideas.

Thanks all!

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