2x16 LCD display in battery powered application?

You may be confusing a number of different things here -- the LCD itself (*without* drivers) is a very low power device. Add the usual controller chip, and you've got a different story entirely.

Powering down the display when it's not in use will save power, at the added complexity on the software side of having to reinitialize the display when you power it up (that includes the fancy song-and-dance needed to put 8-bit data path controllers into 4-bit mode if you're trying to save on pins as well as on power), and the additional milliseconds that setup and initialization requires.

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Namaste--

I'm using one of those 2x16 alphanumeric LCD displays in a battery powered device but I'm unsure about the power consumption. I've set the goal that it should work for about a year on two AA alkaline batteries (assume 2500mAh per AA battery). As the MCU is mostly in sleep mode and all processing is done on an interrupt basis, the LCD display is the primary source of power consumption. Now looking at several datasheets the powerconsumption of 2x16 character LCD's varies between 0.4mA to 10mA. Sometimes current consumption for both the logic and the LCD driving is specificied but I'm not sure if the logic is always consuming current or just when the LCD display contents changes/refreshes.

Has anyone have any experience with using 2x16 character displays in a battery powered product? Could it be made to last for 1 year when the display is on continuously assuming the use of two 2500mAH AA batteries?

TIA

I think you'll need to do the latter. It's better anyway, there is no real reset line on those things.

Best regards, Spehro Pefhany

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One 2500mAh AA battery is 2500mAh * 1.25v = 3125mWh So with two cell your power budget is 6250mWh

Let's say your power supply has a 90% conversion efficiency (to convert from the 1.25v or 2.5v to your final application voltage).

That means you only have 5625mWh of usable power for 1 year (=8760h). So your mean instantaneous power consumption must be lower than 0.64 mW. Assuming a 3.3v vcc that's about 200µA.

So in short ... you'd better find better batteries and very low power LCD. 200µA is not much of current to play with.

Sylvain

You may have thought about this, but couldn't you use a PMOS to turn off the LCD most of the time? If the user wants to see the information, s/he can just push a button momentarily. Then it would stay on until a timer expires.

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It's a 5V system so that would translate into 128uA (0.128mA). I read a datasheet from Rohm which claims their LCD module will use typically 0.5mA so that will translate into only about 2months of operation and possibly less since I didn't take into account power usage of the microcontroller.

So how would one go about this? Is it just about segments? Segmented LCD displays (with custom symbols and all that) ussually only consume microamps. Are there no character or graphic LCD displays out there with very low power consumption?

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Yes, a good idea. That seems to be the only sensible solution at this point. I believe the display driving circuitry can be turned off in software so that will cut consumption at least in half but most likely it will not disable the logic part. By disabling the Chips Select line I may be able to completely turn off the module (including logic) if I'm lucky (anyone know this?). Otherwise I would need to redesign it to power the LCD module from a MCU I/O line.

I think a big part of the power if for the logic. On custom display, you often have 1 dataline per 'symbol', the controller is not included. Here the display needs to be refreshed constantly and there is the char ram etc ...

As suggested in another post, the panel might not need to be powered all the time. Cutting the whole power would be better.

Depending on space constraint, you can find better batteries. One D sized cell might give you > 8500mAh.

Or some camcorder battery pack are like 7.2V with >2000mAh.

Add a solar panel ? ;)

Sylvain

It is about segments. Getting rid of them (or more accurately, the number of clock lines)!... Basically, all graphic LCD's are multiplexed. If you take the 16*2 alphanumeric example, there are potentially 5*7*16*2 segments on such a display, that need to be accessed. 1120 segments. If you go graphic, it gets even worse. I had a custom display made for an application, with 256 segments, and with two row direct drive, the consumption was in the order you require. However it meant having in my case 128 segment lines, and two clocks. Using fourteen segment characters, allowed me to have twelve characters, six decimal points, and then still have a large numner of custom message/symbols. Custom LCD's like this are not expensive now, but involve a huge number of interconnections. Even using zebra strip mounting, you will run out of possible connections for a display with 16*2 full alpha characters... The more clock lines you have, the more power is used. With a 16*2 module, there will normally be 80 segment drives, and fourteen clocks. The best chips driving this sort of number of segments, will still draw perhaps

1uA/segment line, and typically 20uA/clock line, depending on the speed used for the multiplexing. Even ignoring the consumption for other parts of the chip, this still takes you past your continuous consumption requirement. Could you use a low number of digits for continuous display (like a 4 digit LCD, with a couple of enunciators), and a smaller text LCD for 'messages', only enabled when required?. This is achievable, and relatively simple to do.

Best Wishes

FWIW, I have a little credit-card-sized calculator, and its only power source is a photovoltaic cell on the front. It forgets everything in the dark, and the display goes off, but while it's in use, it seems to work fine.

Good Luck! Rich

De-energize the display and driver circuitry until the operator pushes a 'display' button.

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I was recently playing around with some LCD character modules (16x2 large, and 40x2 standard), and even though the data sheet specified +5V for VDD-VSS, I was able to run them to 2.3V without problems, and if not communicating, then they worked down to 1.8V before the display went stupid. Of course the LCD voltage (VDD-VL) bust be maintained at around

5V. I didn't measure current in the VL pin.