Newhaven Display Forum

Newhaven Products => Character LCDs => Topic started by: ThomyLanty on November 17, 2017, 04:39:52 AM

Title: NHD-0420D3Z-FL-GBW-V3 backlight flicker
Post by: ThomyLanty on November 17, 2017, 04:39:52 AM
I'm using a NHD-0420D3Z-FL-GBW-V3 in SPI mode at 64kHz. All functions are working, but the backlight is flickering randomly.

I figured out that the PIC16 microcontroller that sits on the display PCB is dimming the backlight by PWM. I measured this PWM pin and I saw that it is skipping a cycle once in a while, which causes the flickering.
Then I measured the SPI lines simultaneously and I saw that the missing PWM cycle always occurs short after the end of a SPI write cycle, short after the CS pin getting high.
See the attached screenshots of the timing (Ch 4 is the SPI MOSI instead of CH3).

As far as I can tell I met all timing constraints. It looks like the PIC 16 controller can't keep up and first handles the SPI transfer and then misses the timing of the PWM. SPI will probably have a higher priority then PWM.

Does anyone have an idea to fix this issue?