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:
- If the batteries are OK, the calculator will function normally and accept input.
- 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.
- 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.