Re: 1861 Replacement

Lee Hart

cmdrcosmac wrote:
The design I'm building id Chuck Yakym's Rev E, not the STG design.
Since Chuck Yakym's file area on the Group is under construction I've
the files. The REV E is the video board.
I designed the original using just 4 chips; a 4040, EPROM, and a couple gates. It worked, but the 4040 is a ripple counter, so there were glitches as it advanced to the next state. The EPROM would momentarily look up the wrong address, so there were glitches in the outputs.

Chuck and I worked on improving the circuit. A 74HC4040 is faster, and reduced (but didn't eliminate) the glitches. Chuck used three 74HC163 synchronous counters in his rev.E version; they eliminated the glitches going into the EPROM addresses; but the EPROM itself still has small glitches in its outputs as the addresses change.

I improved it still further in my VIP2K design. I kept the 4040, but added an octal latch on the EPROM outputs, which eliminates the glitches entirely. But the VIP2K board is so tight on space that I couldn't latch all the EPROM outputs (I needed two sections of the octal latch for a /4 counter).

Back to the original question: Yes, this circuit can be configured to do other video formats (other resolutions, separate sync and video, PAL, VGA, etc.) It's basically a state machine, so the clock frequency and EPROM program can be changed for just about any format.

A designer knows he has achieved perfection not when there is
nothing left to add, but when there is nothing left to take away.
-- Antoine de Saint Exupery
Lee Hart, 814 8th Ave N, Sartell MN 56377,

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