Author Topic: Understanding dual COM mode on NHD-2.7-12864WDW3  (Read 1609 times)


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Understanding dual COM mode on NHD-2.7-12864WDW3
« on: April 17, 2018, 02:32:07 PM »
I'm trying to get a NHD-2.7-12864WDW3 working over SPI.

What I dont get is how the bytes I send over to the display actually turn up on the pixels. I understand that this display uses the dual com mode. That means, two COM lines are connected to each pixel, right? How do these work together?

From the datasheet image (SSD1322 datasheet, Figure 8-10 : Segment and Common Driver Block Diagram Dual COM mode) it seems like the COM mosfets short themselves out when one COM is on, and the other is off (what the heck?). Does that mean, I must always make sure the two nibbles in each byte are equal? That would be a waste of memory, really. My displaybuffer would contain redundant information.

I was thinking: Maybe the two COMs on each pixel are switched on at separate times (as if they were actually connected to individual pixels) and don't short themselves out (like the datasheet drawing implies). That would mean that writing 0x05 and 0x50 to the display RAM would result in the same brightness - and sure enough, it does - from my experiments. However, I was assuming that this would allow me to actually gain 5 bits of resolution on each pixel (because I have two COM lines with each 4 bit of resolution) but I can't quite work out how to set the two nibbles for each pixel, because I don't really know when and how the two COM lines are actually switched on.

It would be great if someone could shed some light on this for me. I'm sorry to say that I find the datasheets are absolutely horrible and don't tell anything about this.

Thank you so much!


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