MC6845

The MC6845 is a Cathode Ray Tube Controller (CRTC) designed by Motorola, primarily used to generate video timing signals and manage video memory addressing for text-mode (character-based) displays. It doesn't generate pixel data itself but works in conjunction with character generators and video RAM to output signals for video display systems.

Basic Function

• Generates horizontal and vertical synchronization signals (HSYNC, VSYNC).
• Addresses display memory in a raster scan pattern.
• Provides cursor positioning.
• Interfaces with character generators to produce alphanumeric displays.
• Can support both text and graphics modes (when combined with additional logic).

It operates primarily by maintaining internal counters for line/row/character scan timing and feeding addresses to video memory.

Clones

• Originally designed by Hitachi as the HD46505 (so this is not a clone)
• Fujitsu MB89321A
• Bulgarian CM607P (Botevgrad factory)

Where It Was Used

The MC6845 (or compatible chips) was used in many early video display systems:

System/Adapter Usage Notes

• IBM Monochrome Display Adapter (MDA) Used the HD6845 (Hitachi's compatible version) to drive text-only monochrome displays.
• IBM Color Graphics Adapter (CGA) Also used a 6845 variant to handle 40x25 and 80x25 text modes and basic graphics.
• Hercules Graphics Card Used a 6845 to enable both text and high-resolution monochrome graphics (720x348), with custom logic around it.
• VideoTerm display cards for the Apple II
• BBC Micro
• Amstrad CPC

References

• https://bitsavers.trailing-edge.com/components/motorola/_dataSheets/6845.pdf (WebArchive)
• https://en.wikipedia.org/wiki/Motorola_6845 (WebArchive)