What exactly is happening when a graphing calculator LCD malfunctions when batteries are weak or overloaded?

Question

I'm a beginner in electrical engineering, and I have limited knowledge of how electronics work internally. I'm trying to understand some odd LCD behavior when the batteries are weak or subjected to a heavy load.

I'm running a TI-Nspire with Clickpad graphing calculator in diagnostic mode. When I run the USB OTG test and connect a USB device that accepts power over USB using the mini-A-to-mini-B cable (supplied with the calculator), the batteries are subjected to a heavy drain. Because the batteries are not monitored in this mode, if the batteries become weak, the handheld malfunctions. Specifically, the calculator no longer responds to keystrokes and most notably, the LCD goes haywire, displaying a fast-moving pattern of horizontal and vertical lines. I've read a voltage of as low as 3.24 volts from the 4 AAA batteries on my DMM while a USB device is connected. No hardware damage actually occurs, only the AAA batteries get drained. The handheld seems to eventually regain its sanity and reboot normally. The LCD appears to be that of the STN type.

The question I'm trying to ask is how this fast-moving pattern of lines is generated on the display and why this occurs instead of normal operation. In other words, why does the LCD do this with insufficient power and an active heavy load?

Edit: The display is a grayscale 320x240 dot-matrix LCD. When it starts to act up, slight disruptions appear first: horizontal lines appear on top of the normal content and the content may shift slightly or shrink vertically by up to 12% as more and more lines flash on the screen. Eventually, the content is lost and the screen is dominated by randomly flashing horizontal lines. At this point, the background is simply a series of squiggly vertical lines, which can be hard to see behind all of the horizontal lines. Vertical lines occasionally flash on the screen momentarily in place of horizontal lines.

If the load is removed (by disconnecting the USB cable), one of three things can happen:

1. If the batteries are OK, the calculator will function normally and accept input.
2. If the screen was starting to flash horizontal lines, but the normal content was still present beneath these lines, the horizontal lines disappear and normal content will be restored but the calculator is frozen and does not accept input. The display will malfunction again when the cable is reconnected and the batteries are placed under load again.
3. If the normal content was lost, the horizontal lines will disappear, but all that is left is either squiggly vertical lines or a blank screen. The calculator is actually powered on, even if the screen is totally blank. If the cable is reconnected, either the horizontal lines reappear, or the calculator shuts down altogether, momentarily flashing a single horizontal line on the screen.

Answer 1

All ICs (Integrated circuits or "chips") have a working voltage range outside which their behaviour is undefined. This means the manufacturers of the chips don't specify what will happen, just that it (probably) won't work as it should.

So the microcontroller in your Nspire may have a operating range of say 4V - 5.5V and outside this it might do just about anything. Typical stuff would be resetting, running code it's not meant to, etc.

This goes for all the other chips in there, including the LCD controller chip (the chip that drives the display) so odd behaviour is to be expected.
As long as the voltage is lower than it should be, damage will be very unlikely, it just won't work properly.

EDIT - some speculation:

My guess at what is actually happening is RAM that holds the pixel information for the LCD is not being read correctly at the lower supply voltages. Either that or the pixel matrix drivers are not transferring the data correctly.
It's hard to say without more info on the display type (e.g. TN, STN, TFT, OLED) as the method of driving them is a bit different. Also seeing the "fast moving lines" would help - for example if they are horizontal then that might fit with the scanning row by row from top to bottom that is usually the way the pixels are driven in LCDs.

Of course it's also possible the LCD read port is not reading correctly, the microcontroller is acting up, etc, etc. Normally it should have a low voltage sense and brownout active, but if it's in diagnostic mode these features are probably turned off.

All the effects described will be mainly due to the transistors no longer acting enough like switches as vicatcu discusses in his answer.

EDIT 2 - more info provided

Thanks for updating with more info. However, there's not much more that can be added which isn't vague speculation. To find the exact cause of things you would need to check things like: The display and microcontroller datasheets, microcontroller source code, scope the uC -> LCD connections, possibly even scope the LCD controller chip. Even then you may not be guaranteed to find the exact (original) cause as it may be on an unreachable part of the silicon (so all you see is secondary effects) for example an internal RC oscillator changing frequency so timing is not met on the chip, etc.

Answer 2

I know an answer has already been accepted for this, but I wanted to throw this one in for elaboration on the point of "exactly" what is happening. Oli's answer is good in practical terms.

Under the covers, as we all know, IC chips are implemented with transistors. One of the fundamentall bases of design in virtually all digital logic circuits is that the transistors are all operating in a region of their performance characteristic (relative voltage levels between their gate, source, and drain terminals) where they act like "switches" which is to say they output one of two stable states with a very rapid transition region between them.

The reality is that's not the only "operating mode" for our friend the transistor, though. In fact the transistor is really an analog device at the end of the day and it only "acts" like a digital device under the contract that the voltages in the system are within various limits with respect to one another. When your battery voltage drops below some level, you have effectively broken the contract with your transistors throughout the system and they no longer feel obliged to behave as digital devices, and from there things get "interesting" in terms of how they might behave at any particular moment.