Custom 7 Segment Display | Design Guide

This custom 7 segment display design guide provides you with helpful online resources to reduce your design time and development cost.

The question we are asked to address is this: I am designing a hand-held glucose screening product that will incorporate a custom segmented LCD module. The display will contain a total of 164 segments, how many pins will I need?

Calculate the # of pins on segmented LCDs

The equation to calculate the number of pins necessary for segmented custom LCD modules is shown below.

Number of pins = (Number of Segments) X (Mux Ratio) + (Number of Backplanes.)

What is a pin and what is a segment

Below is a photo showing a custom LCD module that contains seven (7) segment digits and the pins, or connections, necessary to connect the glass to the customer’s printed circuit board.

There are two types of drives, or interfaces used on a custom segment display glass: static (or direct drive) and a multiplex (or Mux).

Note: There are rare exceptions where an additional pin or backplane

will need to be added to avoid ghosting of your LCD.

This will be explained in more detail at the end of this article.

Static Drive | Custom 7 Segment Display

Static Drive LCDs are very simple, because there is one pin for every segment. Each segment is either ON 100% of the time or OFF 100% of the time. The advantage of the static configuration is the segments will look sharp and dark, but the disadvantage is custom “static drive” segment displays with multiple segments will need more pins than can fit on the display.

If you have a large number of segments and do not have room for the necessary amount of pins, the next option is to a use multiplex interface.

Multiplex Custom 7 Segment Display

The multiplex custom 7 Segment display allows one pin to control more than one segment. The number of segments addressed by each pin is called the mux ratio. An example of this would be a 4:1 multiplex ratio; one pin will control four different segments. Other ratios, such as ½, 1/3 and others are possible.

A 4:1 mux display contains four separate backplanes and unlike the static display, which is ON or OFF 100% of the time, the 4:1 display is only ON ¼ or 25% (.25) of the time.

The higher the mux rate the more segments that can be controlled by one pin. So what is the catch? The catch is that the segments will not appear as sharp as those of a static display. The higher the mux rate, the lower the sharpness of the segments.

What is a backplane?

A backplane is a layer of glass that is placed between the front and rear polarizers. A Custom 7 segment display module with a 4:1 mux ratio contains four (4) back planes.

Formula to calculate the number of pins

Step by Step Instructions

  1. Determine the total number of segments you will need. The glucose screening device requires 164 segments
  2. Decide on the level of multiplex necessary. Since the display is not large enough to hold 164 pins, the design engineer choose a mux ratio of 4:1.
  3. The total number of pins =
    1. Number of pins = (Number of Segments) X (Mux Ratio) + (Number of Backplanes.)
    2. There are (164 segments) X (1/4 duty cycle) + (4 backplanes).
    3. Total pins will be (164)X(1/4) + 4 = 46
  4. If you arrive at an odd number of pins, add an additional pin to bring the total number up to an even number. This last pin will be a ‘no connect’

Why is my 7 Segment LCD module ghosting?

Sometimes the segment on one backplane is too close to the segment on a different backplane. When this occurs, the result is called ghosting. Ghosting is the phenomena of one segment being nice and dark, while another segment is gray. The photo below is an example of ghosting.

The solution is to add more pins and may require an additional backplane.

Ready to design your custom 7 Segment display? Let Focus Displays be your partner in your Custom LCD module development.