Segment LCD Displays with a 4:1 MUX Interface
This article is a continuation in the Focus Display’s LCD Design Guide for Engineers series and will cover segment LCD Displays with the 4:1 multiplex bus.
Segmented display technology is the most popular custom LCD technology that Focus Display Solutions designs; customized solutions allow the design engineer to choose the size of glass, placement of the icons, and type of interface or bus.
This article covers all segment types of LCDs including 7-segment, 14-segment structures and all fluid types including UWVD (Ultra-Wide Viewing Display Technology) FSTN, STN and TN.
Custom Segmented displays interface (bus) options
The function of the LCD interface (or bus) is to communicate between the segments of the LCD and the customer’s pic controller or microprocessor.
The four most popular interface options include:
- 4:1 mux interface
- Direct drive interface
- SPI interface
- I2C interface
There are other LCD interface options including 4bit and 8 bit parallel, but these are rarely requested and will not be covered in this article.
Segment LCDs with a 4:1 mux interface
The 4:1 mux interface (or multiplex bus) is the most popular option for custom segment LCDs and is preferred for a display containing multiple segments and when low cost is important.
As a general rule, a 4:1 mux interface means that there are four segments tied to one pin (or connection) plus an additional four pins for each of the commons.
The formula to calculate the exact number of pins necessary for a 4:1 mux is listed below and can be located at the Focus Displays custom LCD design guide.
The display does not contain a segment LCD driver instead there are four commons (aka backplanes or coms). The absence of the chip lowers the unit cost and the tooling cost.
Below is a pin out of a 4:1 mux segment LCD. (Photo caption: Segmented LCD pin-out with a 4:1 mux interface).
The segments are energized (or on) 25% of the time, this is also referred to as a 25% duty cycle. 25% may seem like a short time for the segment to be on, but the human eye cannot see a flicker and the segment looks to be on all the time. A similar example of this is a florescent light bulb. The light is cycled on and off several times a second, but the human eye sees a nice even glow.
It is possible to drive a segment display at a lower duty cycle such as 12.5% or less in an effort to decrease the number of connections, but we do not recommend this. Once you drop below the 25% duty cycle, the end user will notice a loss of contrast and the segments will look faded. The cost savings of a few additional pins is low and decreasing the number of pins would only lower the cost by a few pennies.
Calculate The # Of Pins For A Segment Display
To calculate the number of pins for a segment display, follow the instructions below:
- Determine the total number of segments you will need. This example will use 164 segments
- 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.
- The total number of pins =
- Number of pins = (Number of Segments) X (Mux Ratio) + (Number of Backplanes.)
- There are (164 segments) X (1/4 duty cycle) + (4 backplanes).
- Total pins will be (164)X(1/4) + 4 = 46
- 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’
Additional interface types such as direct drive, SPI and I2C can be located in our Journals section of our site.
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