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Home > Products > Liquid Crystal Display Products > Notes > Troubleshooting
Notes

Troubleshooting

The most common causes of a dark STN LCD are driving / contrast voltage and the DISP signal. Improper input timing will not damage a display if applied for a short time. Misapplied power can quickly damage a display, so double check all supply and contrast voltages.

Signal cable lengths should be as short as feasible. Generally lengths up to 0.5 meter are acceptable. The new KCG-series is less tolerant of long cables than the classic KCS-series. Generally the signal cables do not need to be shielded.

LCDs do not experience image "burn-in" like older monochrome CRTs. At most there could be a slight temporary image retention due to segregation of liquid crystal impurities, but any such retained image quickly disappears when the displayed image is changed.

Quick STN LCD Signal Summary
Signal Name Common Video
Signal Name		 Description
CP Dot clock Data shift clock
LOAD Horizontal Sync Start of the horizontal line
FRM Vertical Sync Start of the frame
DISP Display Enable Enables display operation

STN LCD Troubleshooting Matrix
Symptom Problem / Corrective Action
Display is completely black Backlight is not operating
  • Verify that inverter input and output connectors are properly seated.
  • Verify that the inverter can supply correct voltages and current.
  • Verify that the on/off control is on.
  • Verify that the inverter has the correct input voltage.
Backlight is on, but no image is visible Contrast/driving voltage or logic signals are incorrect
  • Verify that signal connector(s) is seated properly.
  • Verify that the signal cables(s) are wired correctly.
  • With the display hoooked up, verify that contrast/driving voltage is within spec.
  • Verify that DISP is high and up to VDD level.
  • Verify signals on CP, LOAD, FRM.
  • Check power sequencing. Improper sequencing may cause latch-up.
Visible image is scrambled Input signals are not correct
  • Verify that CP, LOAD, FRM are within specification limits.
  • Verify that input voltages do not have excessive noise.
  • Verify at the input connector that each pin has the correct signal.
Image is correct but it jitters or blurs CP to DATA setup and hold time is incorrect or there is excessive noise on DATA
  • Reduce cable length.
  • Reduce loading on CP.
  • Add termination to CP.
  • Use higher quality cable.
  • Buffer the output signals.
Image jumps vertically Noise on FRM is causing false triggers to the LCD
  • Reduce cable length.
  • Add in-line resistance to FRM.
  • Add a 1 pf cap to FRM to filter noise.
Image tears jumps horizontally Noise on LOAD is causing false triggers to the LCD
  • Reduce cable length.
  • Add in-line resistance to LOAD.
  • Add a 1 pf cap to LOAD to filter noise.
Image appears washed out Contrast voltage is incorrect
  • Verify that contrast is adjusted to optimum.
  • Verify that the adjustment has correct input voltage.
  • Verify that FRM is correct.
Crosstalk or shadowing If FRM frequency is too high, these effects will become more pronounced
  • Reduce FRM freq by reducing CP freq.
  • Reduce FRM freq by extending LOAD time.
Display loses uniformity over time Contrast changes with temperature
  • Move hot items away from LCD.
  • Shield LCD from hot components.

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