An integrated bias approach to LCD segment drive pin assignment - Embedded.com

An integrated bias approach to LCD segment drive pin assignment

Segment LCD drives can be found in everything from watches, home thermostats, portable medical instruments like glucose meters and blood pressure monitors, and even in certain automobiles. To meet the needs of these applications, nearly every MCU vendor provides devices with LCD drive capabilities.

And even as a mature technology, LCD drives continue to evolve, offering developments new and innovative capabilities. With integrated bias circuitry, individual pin muxing, and mapping tools, developers can simplify and accelerate the design process while reduce system component count and cost.

Programmable Resistor Ladders
A traditional LCD drive needs external R or C circuits to create a bias voltage (Figure 1 below ). Almost all MCUs and SoCs can drive up to 4 commons. This usually require 1/3 bias and so takes approximately 4 pins. Developers need to connect an external resistive ladder or capacitive network to these pins to generate the LCD bias.

Figure 1: Traditional LCD segment controller seen in MCU /SoC.

Attempts have been made to integrate this resistor ladder inside the drive device. However, the resistor ladder should be optimized for a given display to achieve the best power and contrast. As a consequence, an integrated ladder will provide compromise performance for a typical display and may not meet requirements for all systems.

Alternatively, new LCD drives utilize a high-value resistor or programmable resistor ladder to optimize power and use a dynamically controlled buffer to solve drive strength issues. This buffer is automatically turned off to save power after the pixel voltage reaches the target voltage, and then a low drive strength output retains the state.

Implemented in this way, developers still have the ability to tune bias circuitry for optimal power and contrast while conserving pins on the device and avoiding the need for as many external components.

Individual Pin Muxing
LCD segment drives commonly have a fixed pinout. This means certain pins are defined as common (COM) and certain pins are defined as various segment (SEG) outputs. In addition, these devices will have certain other MCU functionality locked onto particular pins such as UART and SPI signals.

Depending upon the pinout, developers can encounter difficulties driving the necessary pixels while also keeping the PCB layout and firmware simple, all without losing critical functionality on the MCU.

Figure 2. LCD segment controller seen with integration bias circuitry

Many MCUs and SoCs use an m-to-n multiplexer (Mux) to address segment and signal output routing issues. These muxes support m total pixels and n segment pins (Figure 2 above ). Newer drivers take the approach of putting a mux at each pin.

This allows any pin to be common or segment, giving designers complete flexibility in how to implement different functions. Such an implementation also uses a much smaller Mux ” p to 1 ” where p is number of bias level supported. Also, this logic can be part of the pad logic and not require extra silicon area, so the cost impact is minimized while shrinking the overall footprint of the system.

When coupled with a programmable resistor ladder, developers are able to more easily support higher bias levels with the end result that a system can have more commons or backplanes. Specifically, the resistor ladder allows developers to add more bias levels without using additional pins or sacrificing power while a mux at each pin enables outputting of more bias levels.

Having more commons or backplanes makes it possible to either drive more LCD pixels or drive the same number of pixels with fewer pins. Additionally, with re-locatable LCD pins, the same common signal can be output to multiple pins. This makes it very easy to drive very large displays requiring higher driver strength. Consider using an LCD drive with 52 pins.

With 4 commons supported, the system can drive a maximum of 192 pixels (48 segments x 4 commons). However, if the system supports 16 commons, then for same 52 pins it can drive 576 pixels (36 segments x 16 commons). If one wants to drive fewer pixels then fewer pins are required, which means a designer can use a smaller pin count MCU to lower system cost.

Graphical User Interfaces
Developing the firmware to drive segment LCD displays can be a laborious process since developers need to manually map each pixel to a segment and common intersection, identify the corresponding control bit in the MCU register map, and then write functions to control each register map. When this needs to be done for hundreds of pixels, this can be a very time-consuming and error-prone process, one that often requires days to implement correctly and verify.

With programmable routing, the mapping process can be significantly simplified by tools (Figure 3 below ) that allow developers to drag and drop a variety of LCD objects to create any type of display.

Figure 3: Tools such as PSoC Creator simplify mapping of pixels to segment and common outputs

Once the display is created, each pixel can be dragged and dropped to the intersection of a row and a column for assignment. The software tool then generates the necessary firmware APIs for integration with the main application. In this way, a graphical user interfaces can reduce a couple of days of effort to less than an hour of graphical configuration.

Even established technology such as LCD drives can be a source of innovation, design simplification, and cost savings. New advances in segment LCD drive systems enable designers to conserve pins and eliminate external passive components to generate bias voltage.

The ability to assign any pin as a segment or common output simplifies PCB layout and maximizes use of onboard peripherals, as well as significantly reduce time required to develop segment LCD drive firmware.

LCD drives are also able to drive more commons to either drive many more pixels or use far fewer pins to drive the same amount of pixels. In addition, outputting a common signal to multiple pins increases drive strength and drives larger displays using a single device.

Gaurang Kavaiya is Product Applications Director at Cypress Semiconductor Corp.

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