Motorola 6845 CRT controller

The Motorola 6845 (commonly MC6845) is a video address generator first introduced by Motorola and used in the MDA, CGA and EGA video adapters, Amstrad CPC and BBC Micro. Its functionality was duplicated and extended by custom circuits in the VGA. It is related to the later 6545 manufactured by MOS Technology (Commodore Semiconductor Group) and Rockwell (in two variations) and was cloned as the Hitachi 46505. Motorola (NYSE: MOT) is an American multinational communications company based in Schaumburg, Illinois, a Chicago suburb. ... Green screen driven by a Monochrome Display Adapter The Monochrome Display Adapter (MDA, also MDA card, Monochrome Display and Printer Adapter, MDPA) introduced in 1981 was IBMs standard video display card and computer display standard for the PC. The MDA did not have any graphics mode of any kind... The Color Graphics Adapter (CGA), introduced in 1981, was IBMs first color graphics card, and the first color computer display standard for the IBM PC. The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory. ... The Enhanced Graphics Adapter (EGA) is the IBM PC computer display standard specification located between CGA and VGA in terms of graphics performance (that is, colour and space resolution). ... The Amstrad CPC was a series of 8-bit home computers produced by Amstrad during the 1980s and early 1990s. ... The BBC Microcomputer System was a series of microcomputers and associated peripherals designed and built by Acorn Computers Ltd for the BBC Computer Literacy Project operated by the British Broadcasting Corporation. ... VGA redirects here. ... MOS Technology, Inc. ... Rockwell can refer to: Rockwell International - a defense company in the United States Rockwell Automation - an industrial automation company that descended from Rockwell International Rockwell Collins - a communications and aviation electronics company that also descended from Rockwell International Willard Rockwell - businessman who helped shape and name what became Rockwell International... It has been suggested that Hitachi Works be merged into this article or section. ...

It is also known as the 6845 CRTC or the CRTC6845, meaning cathode ray tube controller. Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT Electron guns Electron beams Focusing coils Deflection coils Anode connection Mask for separating beams for red, green, and blue part of displayed image Phosphor layer with red, green, and blue zones Close-up of the phosphor...

Although intended for designs based on the Motorola 6800 CPU and given a related part number, it has been used alongside various other processors. Motorola 6800 Microprocessor The 6800 is a microprocessor produced by Motorola and released shortly after the Intel 8080 in 1975. ...

Overview

The chip generates the signals necessary to interface with a raster display but does not generate the actual pixels, though it does contribute cursor and video-blanking information to the pixel video (intensity) signals. It is used to produce correctly timed horizontal and vertical sync and provide the address in memory from which the next pixel or set of pixels should be read. The process of reading that value, converting it into pixels, and sending it to a CRT is left to other circuits. A computer display monitor, usually called simply a monitor when the meaning is clear from the context, is a piece of electrical equipment which displays viewable images generated by a computer without producing a permanent record. ... This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ... Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT Electron guns Electron beams Focusing coils Deflection coils Anode connection Mask for separating beams for red, green, and blue part of displayed image Phosphor layer with red, green, and blue zones Close-up of the phosphor...

Interlaced and non-interlaced output modes are supported, as is a hardware text cursor. The sync generation includes generation of horizontal and vertical video blanking signals, which are used to condition the external pixel generation circuits. Also, an internal latch is provided which when triggered will duplicate and retain a copy of the video address so that it can later be read back by the CPU. This is useful for light pens and light guns which can function by sending a pulse to the 6845 when the electron beam passes, allowing a running program to read back the location that was pointed at. (Because of this feature, most computer video adapters using a 6845 included a light pen interface, though it was usually an internal connector on the board itself, not on the outside of the computer, and it was usually undocumented in the user manual.) Interlacing is a method of displaying images on a raster-scanned display device, such as a cathode ray tube (CRT). ... A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with the computers CRT monitor. ... This article or section does not adequately cite its references or sources. ...

Because all aspects of video timing are programmable, a single machine can switch between NTSC and PAL timings in software. The 6845 can be used to drive monitors or any other raster display. The references in this article would be clearer with a different and/or consistent style of citation, footnoting or external linking. ... Television encoding systems by nation PAL, short for Phase Alternating Line, is a colour encoding system used in broadcast television systems in large parts of the world. ... A computer display monitor, usually called simply a monitor when the meaning is clear from the context, is a piece of electrical equipment which displays viewable images generated by a computer without producing a permanent record. ...

Internals

The chip has a total of 18 8-bit registers controlling all aspects of video timings. Only two addresses are exposed to external components - one to select which internal register is to be read or written to and another to access that register. 8-bit refers to the number of bits used in the data bus of a computer. ... In computing, a hardware register is a storage area for hardware I/O (input/output) of different kinds. ...

The 6845 is intended for character based displays. Every address it generates is composed of two parts - a 14 bit character address and a 5 bit row address. Using the full address range RA0-RA4:CA0-CA13 the 6845 can address 2^(14+5) = 512KiB of memory. A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1,024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to...

The character address increases linearly. When the chip signals horizontal sync it increases the row address. If the row address does not equal the programmatically set number of rows per character then the character address is reset to have the same value as it did at the beginning of the current scanline. Otherwise the row address is reset to zero.

If the character address is used to look up a character reference in RAM and the row address to index a table of character graphics in ROM an ordinary text mode display is constructed. Look up RAM, Ram, ram in Wiktionary, the free dictionary. ... Read-only memory (often referred to as its acronym ROM) is a class of storage media used in computers and other electronic devices. ... A text mode program communicates with the user by only displaying text and possibly a limited set of predefined semi-graphical characters, which allow the drawing of rudimentary boxes around portions of text, either to highlight the content or to simulate widget or control interface objects found in GUI programs. ...

Linear framebuffers

As described above, the 6845 is not ordinarily able to provide large linear framebuffers. A design could use only the 14 bit character address and set the number of rows per character to 1 but it would be constrained to 16KiB of addressable memory. A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1,024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to...

A solution is found in the Amstrad CPC, which combines the row address and character address to provide linear scanlines within a non-linear buffer. It maps row address RA0-RA2 to memory address MA11-MA13 and character address CA0-CA10 to memory address MA0-MA10. This has the advantages of easier programming for non-character display and easy smooth horizontal scrolling but can impede smooth vertical scrolling. The Amstrad CPC was a series of 8-bit home computers produced by Amstrad during the 1980s and early 1990s. ...

Differences from the 6545

Although overwhelmingly compatible, a number of small variations exist between the 6845 and 6545.

The biggest difference is that the 6545 may be configured so that it has sole access to the address bus for video memory. Two additional registers are included for setting any address the CPU wishes to read and the chip alternates between outputting addresses for display generation and the display set for CPU access.

Smaller changes are that the MOS Technology and one variation of the Rockwell 6545 lack interlaced output support and all 6545s include an optional address skew, which delays display enable for one character cycle if set. This second feature was incorporated into later variations of the Motorola 6845. MOS Technology, Inc. ... Rockwell can refer to: Rockwell International - a defense company in the United States Rockwell Automation - an industrial automation company that descended from Rockwell International Rockwell Collins - a communications and aviation electronics company that also descended from Rockwell International Willard Rockwell - businessman who helped shape and name what became Rockwell International... Motorola (NYSE: MOT) is an American multinational communications company based in Schaumburg, Illinois, a Chicago suburb. ...

The 6545 may be set to work in linear 14bit mode using a status bit. On the 6845 the same thing requires adjustment of the character height.

Tricks

The 6845 reads the start address for its display once per frame. However, if the internal timing values on the chip are altered at the correct time it can be made to prepare for a new frame without ending the current one - creating a non-continuous break in generated addresses midway through the display. This is commonly used by games to provide one moving area of the display (usually the play field) and one static (usually a status display). In film, video production, animation, and related fields, a frame is one of the many still images which compose the complete moving picture. ... It has been suggested that Multiplayer game be merged into this article or section. ...

Vertical scrolling appears constrained because only the character start address can be set and the row address is always zeroed at frame start, but by adjusting border times it is possible to shift the position the framebuffer is shown on the raster display for increments in between whole characters. With drawing of blank pixels at the screen edges, this can be made invisible to the user creating just the illusion of a smooth vertical scroll.

See also

• MC6847

External links

• HTML documentation of the 6845 registers
• Zipped TIFs of the 6845 data sheets
• Differences of CRTC models
• VHDL implementation of the 6845
• 6845/6545 Pinout
• Synertek 6545 Application Note