OK,,,,then you need just 16 port pins for the segment drivers and 1 more for the backplane control.

Typical software support for driving the direct LCD glass is to setup am interrupt for 600 -> 1200 Hz. In this interrupt you have software toggle the backplane signal level and you read 16 other bits and write them to the 16 port pins that drive the segments. When the BP is in one level you send all data from the 16 bits buffer to the ports in non-inverted manner. In the alternate state you invert the 16 bits before sending them to the ports. It can be convenient to connect the 7 digit selects of your two display digits to the low bits of two ports. Then maybe tie the upper 2 that make your 1 to the upper bits of the two ports. Arrange to keep the real time copy of these 16 bits in three bytes of DATA RAM - one byte for each if the three digits. If you assign the hardware connections so that the a-b-c-d-e-f-g segments selects are are connected to the same bits of the ports then the logcal layout of the segments in all 3 of these bytes can be the same. When an inernal subroutine wants to "write digits to the LCD" instead can write the digits to these 3 bytes I am talking about. The releatitive interrupt takes care of placing the display data out to LCD glass. You can construct a simple look-up-table that permits easy conversations from the internal digit numerical values to the segment patterns for the three bytes.

Michael Karas