Motorola 6845
Encyclopedia
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
and used among others in the Videx
Videx
Videx is a manufacturer of access control products and data collection terminals. Better known in the 1980s as manufacturer of Videx VideoTerm and Videx UltraTerm add-on expansion card for Apple II computers...
VideoTerm display cards for the Apple II
Apple II
The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...
computers, the MDA
Monochrome Display Adapter
The Monochrome Display Adapter introduced in 1981 was IBM's standard video display card and computer display standard for the PC. The MDA did not have any pixel-addressable graphics modes...
and CGA
Color Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....
video adapters for the IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
, in the Amstrad CPC
Amstrad CPC
The Amstrad CPC is a series of 8-bit home computers produced by Amstrad between 1984 and 1990. It was designed to compete in the mid-1980s home computer market dominated by the Commodore 64 and the Sinclair ZX Spectrum, where it successfully established itself primarily in the United Kingdom,...
and the BBC Micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
. Its functionality was duplicated and extended by custom circuits in the EGA
Enhanced Graphics Adapter
The Enhanced Graphics Adapter is the IBM PC computer display standard specification which is between CGA and VGA in terms of color and space resolution. Introduced in October 1984 by IBM shortly after its new PC/AT, EGA produces a display of 16 simultaneous colors from a palette of 64 at a...
and VGA
Video Graphics Array
Video Graphics Array refers specifically to the display hardware first introduced with the IBM PS/2 line of computers in 1987, but through its widespread adoption has also come to mean either an analog computer display standard, the 15-pin D-subminiature VGA connector or the 640×480 resolution...
PC video adapters. It is related to the later 6545 manufactured by MOS Technology
MOS Technology
MOS Technology, Inc., also known as CSG , was a semiconductor design and fabrication company based in Norristown, Pennsylvania, in the United States. It is most famous for its 6502 microprocessor, and various designs for Commodore International's range of home computers.-History:MOS Technology, Inc...
(Commodore Semiconductor Group) and Rockwell
Rockwell
- People :* Dick Rockwell, an American comic strip and comic book artist, nephew of Norman Rockwell* Francis W. Rockwell, a United States Congressman from Massachusetts* Francis W...
(in two variations) and was cloned as the Hitachi
Hitachi, Ltd.
is a Japanese multinational conglomerate headquartered in Marunouchi 1-chome, Chiyoda, Tokyo, Japan. The company is the parent of the Hitachi Group as part of the larger DKB Group companies...
46505 (which was used in Videx
Videx
Videx is a manufacturer of access control products and data collection terminals. Better known in the 1980s as manufacturer of Videx VideoTerm and Videx UltraTerm add-on expansion card for Apple II computers...
's UltraTerm card).
It is also known as the 6845 CRTC or the CRTC6845, meaning "cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
controller".
Although intended for designs based on the Motorola 6800
Motorola 6800
The 6800 was an 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System that also included serial and parallel interface ICs, RAM, ROM and other support chips...
CPU and given a related part number, it was more commonly used alongside various other processors.
Overview
The chip generates the signals necessary to interface with a raster displayComputer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
but does not generate the actual pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s, 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
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
is left to other circuits. Because of this, systems using the 6845 may have very different numbers and values of colors, or may not support color at all.
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 pen
Light pen
A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's CRT TV set or monitor. It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touch screen but with greater positional accuracy...
s and light gun
Light gun
A light gun is a pointing device for computers and a control device for arcade and video games.Modern screen-based light guns work by building a sensor into the gun itself, and the on-screen target emit light rather than the gun...
s 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.)
Because all aspects of video timing are programmable, a single machine can switch between NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
and PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
timings in software. The 6845 can be used to drive monitor
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
s or any other raster display.
Internals
The chip has a total of registersHardware register
In digital electronics, especially computing, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously.The hardware registers inside a central processing unit are called processor 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.
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 214+5 = of memory.
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
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
and the row address to index a table of character graphics in ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
an ordinary text mode
Text mode
Text mode is a kind of computer display mode in which the content of the screen is internally represented in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a...
display is constructed.
Linear framebuffers
As described above, the 6845 is not ordinarily able to provide large linear framebuffers. A design could use only the character address and set the number of rows per character to 1 but it would be constrained to of addressable memory.A solution is found in the Amstrad CPC
Amstrad CPC
The Amstrad CPC is a series of 8-bit home computers produced by Amstrad between 1984 and 1990. It was designed to compete in the mid-1980s home computer market dominated by the Commodore 64 and the Sinclair ZX Spectrum, where it successfully established itself primarily in the United Kingdom,...
, 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.
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
MOS Technology
MOS Technology, Inc., also known as CSG , was a semiconductor design and fabrication company based in Norristown, Pennsylvania, in the United States. It is most famous for its 6502 microprocessor, and various designs for Commodore International's range of home computers.-History:MOS Technology, Inc...
and one variation of the Rockwell
Rockwell
- People :* Dick Rockwell, an American comic strip and comic book artist, nephew of Norman Rockwell* Francis W. Rockwell, a United States Congressman from Massachusetts* Francis W...
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
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
6845.
The 6545 may be set to work in linear 14 bit 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).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
- Motorola 6847, a complete video display generator
- List of home computers by video hardware