| LCD Direct Drive: (February 2016) (Last Update: December 2022) |
LCD screens are popular alaphanumeric display units, typically driven by a dedicated IC such as the HD44780. Microcontroller-based software most commonly employ libraries to do the heavy-lifting as one particular challenge in driving these units directly is that they most often require a 2.7-5V AC signal operating with a frequency of between 30 and 100 Hz to maintain its display. Constant DC current can damage the segments in short order.
The photo to the right depicts my first attempt to wire the 40-pin LCD-S401C71TR from Lumex. As can be determined from its datasheet it is a Static (see below) drive unit making the AC Waveform simpler to create through code. The AC waveforms for Multiplexed panels (ie, 1/2, 1/3 1/4, etc.) require coordinated voltage biasing, typically achieved through external resistor networks. An image of a three-level drive schematic, taken from Zilog's PDF, juxtaposed with a possible resulting AC waveform, appears below.
Progress.
(February 14)The wiring strategy of this first attempt made the circuit appear simpler but complicated the sketch due to the 2-plane architecture of the unit. If I develop a second version, each shift register will drive a single display digit.
Single 595/digitRH, JL, CM, RP, JR, JY
| Resistor Network for Three-Level Multiplex Driving | Resulting Waveform |
|---|---|
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Research Suggestions
A brief overview of LCD composition and DIY possibilities.
The datasheet for the LCD-S401C71TR from Lumex can be found here.
An ATMEL Application Note outlines how to interface a (Static) device using general I/O pins
An ATMEL Application Note outlines the direct driving of an LCD glass using General IO pins.
An LCD Direct Drive Application Note from Zilog. Covers a 1/2 Bias Multiplex drive scheme.
Microchip
Pacific Display Devices
Silicon Labs
An informative tutorial on the Direct Drive of LCD screens is here.
A handy AVR Timer Calculator can assist with determining frequency parameters.
NXP Product Training Module: LCD Drivers (Segment, Character and Matrix)