Hitachi HD44780 LCD Controller - Architecture

Architecture

These LCD screens are limited to monochrome text and are often used in copiers, fax machines, laser printers, industrial test equipment, networking equipment such as routers and storage devices.

The screens come in a small number of standard configurations. Common sizes are 8x1 (one row of eight characters), 16×2, 20×2 and 20×4. Larger custom sizes are made with 32, 40 and 80 characters and with 1, 2, 4 or 8 lines. The most commonly manufactured larger configuration is 40×4 characters, which requires two individually addressable HD44780 controllers with expansion chips as a single HD44780 chip can only address up to 80 characters. A common smaller size is 16×2, and this size is readily available as surplus stock for hobbyist and prototyping work.

Character LCDs can come with or without backlights, which may be LED, fluorescent, or electroluminescent.

Character LCDs use a standard 16 contact interface, commonly using pins or card edge connections on 0.1 inch / 2.54mm centers. Those without backlights may have only 14 pins, omitting the final two pins powering the light. The pinout is as follows:

  1. Ground
  2. VCC (+3.3 to +5V)
  3. Contrast adjustment (VO)
  4. Register Select (RS). RS=0: Command, RS=1: Data
  5. Read/Write (R/W). R/W=0: Write, R/W=1: Read
  6. Clock (Enable). Falling edge triggered
  7. Bit 0 (Not used in 4-bit operation)
  8. Bit 1 (Not used in 4-bit operation)
  9. Bit 2 (Not used in 4-bit operation)
  10. Bit 3 (Not used in 4-bit operation)
  11. Bit 4
  12. Bit 5
  13. Bit 6
  14. Bit 7
  15. Backlight Anode (+)
  16. Backlight Cathode (-)

The nominal operating voltage for LED backlights is 5V at full brightness, with dimming at lower voltages dependent on the details such as LED color. Non-LED backlights often require higher voltages.

The HD44780 interface allows for two modes of operation, 8-bit and 4-bit. Using the 4 bit mode is more complex, but reduces the number of active connections needed. The chip starts in 8 bit mode, with the instruction set designed to allow switching without requiring the lower four data pins. Once in 4 bit mode, character and control data is transferred as pairs of 4 bit "nibbles" on the upper data pins, D4-D7.

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