On the PCA8551B LCD driver, after writing segment registers, segments transitioning to off turn off over 230ms then segments transitioning to on turn on over 230ms. I would expect these both to occur at the same time. Is this expected behavior?

[dgardner]

See the attached video as a demonstration. As the digit on the far right transitions from 0 to 2, segments disable first and then the center segment enables afterwards.

This stuttered transition is aesthetically jarring. All segments change at the same time is preferable.

The LCD driver is configured to drive a static display, and the display has one backplane.

Device_ctrl = 0b00000100

Display_ctrl_1 = 0b00001101

Display_ctrl_2 = 0b00000000

Enabling a higher refresh rate or boost mode seems to have no effect.

The datasheet does not mention the timing of the segments transitioning when the segment registers are written, so it is unclear if this is expected behavior or not.

[reyes · Accepted solution]

Hi,

This is expected behavior and it is typically dependent on the LCD liquid used since suitable values for Von(RMS) and Voff(RMS) are dependent on the LCD liquid used. The RMS voltage, at which a pixel will be switched on or off, determine the transmissibility of the pixel.

For any given liquid, there are two threshold values defined. One point is at 10 % relative transmission (at Vth(off)) and the other at 90 % relative transmission (at Vth(on)), see Figure 11 on the datasheet.

The time between the 10% and the 90% is the time that you are seeing as “disabled” the pixel.

Regards,

Jose

NXP Semiconductor

[dgardner]

Hi Jose,

Thank you for the response. I was able to confirm that the driver is switching both segments simultaneously, so the cause must related to the LCD screen as you said.

I was surprised to find the RMS voltage across a segment to change almost instantaneously when a segment switches from off to on. Nothing electrically appears to be changing at the pins of the LCD screen during the time the segment is between clear and fully black.

Thanks again!

-David