Segment LCD displays available in static and multiplex drives
We receive many calls regarding direct drives and multiplex drives for custom segment LCD displays. The Segment LCD is a common type of custom display module; in fact, it is the lowest cost custom LCD glass option available. One reason for the low cost is that it does not contain a controller/driver chip. Therefore, instead of having a parallel / IC2 / SPI interface options, you now have two options: Direct (or static) drives and multiplex drives.
Drive options for a segment LCD modules
There are two drive types available for a custom segmented LCD module. Direct drives or static and multiplex drives. There is no difference in tooling cost between the two. Before we explain the difference between direct and multiplex drives, let’s talk about segments.
Figure one below is a custom segmented LCD module. The number ‘8’ is made up of seven (7) independent segments and a decimal point. Each one of these segments can be turned on and off individually. This allows the user to create any number and many letters such as ‘E’, ‘F’, ‘L’ and others.
Figure two shows a custom segment LCD with leads or pins that are coming off the side of the LCD glass. These pins are connected to the segments.
Direct Drives | Segment LCD
Direct drive custom segmented LCDs contain one pin for every segment plus supporting pins such as power, ground and COM’s. Figure one shows eight segments, this would require a total of eight pins for the segments, plus supporting pins.
Figure two shows an LCD with eight number ‘8’s. This equates to 8 numbers multiplied by 7 segments per number, which equals 56 segments. If you were to use direct drive on this display, you would need 56 pins plus supporting pins. The higher quantity of pins not only increases the size of the display , since you will need a larger display to hold all the leads, but it also increases the unit cost and the amount of time to install the LCD on a printed circuit board (PCB)
One advantage of direct drive custom segmented LCD is that it offers the sharpest contrast, because the segment is on 100% of the time.
Multiplex Drives | Segment LCD
The most common type of multiplex LCD display used on a custom segment LCD is called a 4:1 mux. The means that there is one pin for every four segments. The advantage of this configuration is that it reduces the number of pins by 75%. Add up all the segments and divide by four and you arrive at the number of pins necessary, plus supporting pins.
The display in figure two would require an estimated 56 segments divided by 4 for a total of 14 pins. With the additional supporting pins the number of pins would be closer to 20.
One down side to a multiplex custom segmented LCD is the reduced LCD contrast. On a direct drive each segment is on 100% of the time, in a 4:1 mux configuration, each segment is on 25% of the time. The reduced sharpness is not very noticeable and difficult for the human eye to notice.