Segment LCD modules with Direct Drive or SPI bus Focus LCDs | Stan

This article on the segment LCD interface is a continuation in our Liquid Crystal Guide for Engineers series and will cover segment LCD modules with direct drive and SPI interfaces.

Intro to the segment LCD interface

Segmented display technology is the most popular custom LCD technology that Focus Displays designs. Customized LCD solutions allow the customer (engineer) to choose the dimensions of the 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 UWVD (Ultra-Wide Viewing Display Technology), FSTN, DFSTN, STN and TN fluids.

Static or direct drive Liquid Crystal Displays

Direct drive or static drive LCD means that there is one segment per pin or connection. This is the second most popular option for a custom glass only LCD with 4:1 mux being the most popular. The segment bus has a duty cycle of 100%; the segment is either on 100% of the time, or off 100% of the time. Advantages of a direct drive versus a display with a lower duty cycle include segments that appear a little darker and better operation in colder temperatures.

This display, like the 4:1 mux option, does not contain a controller/driver chip which reduces unit and NRE (tooling) cost.

Most customers choose a direct drive method if their LCD module contains less than 16 or 20 segments.

NRE cost for a direct drive and a 4:1 mux are the same. The unit cost is very similar with the number of pins being the main item to increase the cost. (A few pennies more for the extra pins)

Serial Peripheral Interface bus or SPI interface

LCD Displays with a SPI interface are becoming more common for segmented LCDs.

SPI which stands for (Serial Peripheral Interface) is also referred to as Four-wire serial bus and SSI (Synchronous Serial Interface); SPI is achieved by integrating a controller chip with the segment LCD module. The chip allows a multi-segment LCD to be driven with few pins or connections.

The SPI bus specifies four logic signals

SCLK: serial clock (output from master)
MOSI: master output, slave input (output from master)
MISO: master input, slave output (output from slave)
SS: slave select

The addition of the controller chip increases the unit and NRE cost. One detail to keep in mind is that every LCD module requires a controller/diver; either on the LCD itself or on your PCB. The increased cost of moving the controller from your PCB to the LCD modules may lower the cost of your PCB.

Multi-segment LCD module with SPI interface

An example of this segment LCD interface would be a custom segment LCD module that contains 160 independent segments. A segment LCD driver chip such as a HT1622 contains 8 coms and 32 segments which can drive 256 segments.

It is possible to design and build the segment display with two (2) HT1622 controllers which would reduce the duty cycle in half. The question then becomes, is it worth the extra unit cost for the lower duty cycle?

In part three we will cover segment LCD displays with an I2C bus.

Focus Display Solutions offers US based Engineer design and support for your segment LCD display.

Ready to start your custom segment LCD design? Call us at 480-503-4295 or click here.