Passive Matrix Driving

Overview

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Passive matrix driving is the most intuitive form of display circuitry, using simple row-column addressing without active switching elements at each pixel.

Architecture

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         Column Lines (Data)
         C1   C2   C3   C4
          │    │    │    │
    ┌─────┼────┼────┼────┼─────┐
R1 ─┤     ●    ●    ●    ●     │
    ├─────┼────┼────┼────┼─────┤
R2 ─┤     ●    ●    ●    ●     │  Row Lines
    ├─────┼────┼────┼────┼─────┤  (Scan)
R3 ─┤     ●    ●    ●    ●     │
    ├─────┼────┼────┼────┼─────┤
R4 ─┤     ●    ●    ●    ●     │
    └─────┴────┴────┴────┴─────┘
          Pixels at intersections

Operating Principle

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Sequential Scanning

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1. Row Selection: Activate one row at a time
2. Column Data: Apply voltage to all columns simultaneously
3. Pixel Response: Only pixels at selected row respond
4. Repeat: Move to next row, continue cycling

Timing Diagram

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Row 1:  ████____________________████
Row 2:  ____████________________████
Row 3:  ________████____________████
Row 4:  ____________████________████
        ← One Frame Period →

Key Characteristics

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No Storage Capacitor

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• Voltage not maintained between scans
• Light emission only during row selection
• Relies on persistence of vision

PWM for Brightness

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Pulse Width Modulation controls gray levels:

Advantages

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Disadvantages

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Crosstalk Problem

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When one pixel is addressed, neighboring pixels receive partial voltage:

        Selected Column
             ↓
Row OFF ─── ◐ ─── Partial voltage
Row ON  ─── ● ─── Full voltage
Row OFF ─── ◐ ─── Partial voltage

This limits practical display size.

Persistence of Vision

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The eye perceives continuous image if:

• Refresh rate > 60 Hz
• Frame time < 16.7 ms

Human vision integrates rapid sequential images into perceived static display.

Applications

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LCD Displays

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• Simple calculators
• Basic watches
• Small character displays
• Low-resolution graphics

OLED/MicroLED

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Passive matrix principles extended to:

• Small OLED displays
• Wearable devices
• Indicator panels

Comparison with Active Matrix

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Circuit Implementation

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Row Driver

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Sequentially activates each row with scan pulse.

Column Driver

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Applies data voltage pattern to all columns during row selection.

Timing Control

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Synchronizes row selection with column data.

Evolution

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Passive Matrix
      ↓
Super Twisted Nematic (STN)
      ↓
Dual Scan STN
      ↓
Active Matrix (TFT)

The need for higher resolution and faster response led to active matrix development.