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Sharing SRAM device in bettween two bus masters - how was it done back then?
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capt bullshot:

--- Quote from: westfw on November 28, 2018, 12:31:40 am ---The 8051 is neither a microprocessor, nor is it from "way back when."  I don't recall ever seeing an 8051-based display.  Or other multi-master system.  In that age, 8051s were not very available, not easily programmable, and not very cheap compared to real microprocessors; and you'd have needed most of the same external components anyway.


--- End quote ---

Somewhen around 1990 ... 1993, I've designed two 8051 based display systems as the display units for a piece of test equipment. They had one of these early B/W graphic LCD modules (one 640x400, the smaller one 240x128), that had to be driven with pixel clock, pixel data and H / V sync. Basically it had a video memory that was read out by some logic circuitry (all 74HCxxx style ICs), with time shared access granted to the external bus of the 8051. Because of the very predictable timing of the 8051, one could do two memory cycles (one video, one MCU) within one MCU cycle, so both had equal priority access without wait states. Just the same way many 6502 designs did as stated above.

Edit: found a hand-drawn schematic of the controller (posting it just because)
westfw:

--- Quote ---I've designed two 8051 based display systems ... graphic LCD modules ... that had to be driven with pixel clock, pixel data and H / V sync.
--- End quote ---
Neat; thanks for posting.
I've bought a couple of similar surplus displays; spec sheets always show pixel clocks designed to update the display at TV-like frequencies, but I've always wondered how arbitrary those rates were.  After all, the LCD doesn't contain circuitry that is inherently tied to 60Hz...
SiliconWizard:
Yep, there were actually a few simple video terminals based on 8051's AFAIR. Obviously nothing high-resolution, but for simple text terminals that did the trick.
Yansi:
Wow, thank you guys, some very valuable input has been put here.  :-+

I have also learned about the existence of 6845. Had a brief look at it, however it seems, it does not produce the sync mix for composite video. They write in the datasheet, it can be connected to "video processing circuitry to generate a composite video signal. ".  However I could not find any example of such circuitry, that would produce a composite video compliant sync signal, rather than some garbage that "just sometimes works". Do you have any examples of that?

Thx, Y
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