6845 CRT CONTROLLER PDF

Faegar He hacked what he HaD! The can be used to drive monitors or any other raster display. Using the full address range RA0-RA4: 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 conntroller. The character address increases linearly. But it is good to see that the old CRT is shining again, well done.

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Overview[ edit ] 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.

Because of this, systems using the 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 pens and light guns which can function by sending a pulse to the 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 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.

The can be used to drive monitors or any other raster display. Internals[ edit ] MC pinout. 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.

The 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. If the word size is one byte, as is often the case, the can address KiB. If the word size is 32 bits, e. These limits arise from the combination of the and the design of the external memory connected to it, not from the alone.

It is also significant that each word addressed by the does not have to equal one pixel or one character. As an example, consider the use of the in the IBM CGA, where the word size is one byte and each word represents four or eight pixels in the medium- or high- resolution graphics mode, respectively or one-half character.

In CGA alphanumeric text mode, there are two bytes per character, accessed sequentially by the —the first byte is a character code byte and the second byte is a character attribute byte. 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 the value it had at the beginning of the scanline that was just completed.

Otherwise the row address is reset to zero and the memory address continues increasing linearly. This causes the same sequence of character values to be re-read from the memory for each raster line of each character row, before the advances the memory address to the next character row and repeats the same pattern.

This means that character displays using the , compared to all-points-addressable graphics displays of the same resolution, require much less memory but still require high memory bandwidth on the order of the bandwidth required for graphics. A different video display controller that buffers one whole line of character data internally can avoid this repeated reading of each line of characters from the display buffer RAM, reducing the required memory bandwidth and allowing either slower, less expensive memory chips to be used, more time for a system CPU to access the memory, or a combination of both.

In the s, s, and to a lesser extent the s, memory was expensive, fast memory was especially so, and this was an important concern. Therefore, adding such a character buffer to the was not a cost-effective approach when the chip was introduced. Now that memory is very inexpensive, fast memory included, there is little motivation to reduce the memory bandwidth required by a video display controller, so this is no longer an important engineering consideration.

For low-power handheld devices, which would be the main ones likely to use character displays now, the power used for high-bandwidth memory access would be good reason to reduce the memory bandwidth for display refresh through the use of a line cache in the display controller. 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. The character reference read from memory must be combined with the row address to form the address for the character graphics ROM, with the character reference selecting a set of scan line patterns that forms one character and the row address indexing into that set to select one scan line.

Linear framebuffers[ edit ] As described above, the 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 16 kB of addressable memory. 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. 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 [ edit ] Although overwhelmingly compatible, a number of small variations exist between the and The biggest difference is that the 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 lack interlaced output support and all s 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 The may be set to work in linear 14 bit mode using a status bit. On the the same thing requires adjustment of the character height. Tricks[ edit ] The 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 demos and much more rarely 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.

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The 6845 Cathode Ray Tube Controller (CRTC)

Gardagrel When the chip signals horizontal sync it increases the row address. The sync generation includes generation of horizontal and vertical video blanking signals, which are used to condition the external pixel generation circuits. Smaller changes are that the MOS Technology and one variation of the Rockwell lack interlaced output support and all conttoller include an optional address skew, which delays display enable for one character cycle if set. The two ICs were quite different. Motorola — Wikipedia Leave a Reply Cancel reply Enter your comment here 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 16 kB of addressable memory. 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.

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Motorola 6845

Vular Too slow for any practical use, but good enough to prove the system worked. Notify me of new posts via email. The chip he used is 20MIPs and would likely be faster than the original It was used in a few other machines, e. Motorola If the word size is 32 bits, e. The two ICs were quite different. Therefore, adding such a character buffer to the was not a cost-effective approach when the chip was introduced.

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