Requirements for RGB/TTL - 16 vs 18 bit bus
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"EOMA-68's RGB/TTL interface is 18-bit-wide. If a particular SoC only has e.g. 16-bit or 15-bit RGB/TTL then the LSB (lower) bits MUST be set to logic output level 0 when the LCD interface is enabled: they must NOT be left floating or tri-state. This ensures that devices which are expecting the full 18-bits do not receive noise on the lower bits of each of the R,G and B 8-bit inputs."
While I agree floating is bad, I'm not certain requiring those pins to be grounded is the best solution as it will prevent the display from ever showing full Red, Blue, Green, or White. Instead, if enough drive-strength is available, any extra LSBs can be tied to the corresponding MSBs for each channel. Or, it could be tied to other pins driven by actual data.