Electronics > Projects, Designs, and Technical Stuff

TTL Counter with tri-state

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obiwanjacobi:
Looking for a cheap option to count 18 bits but with output enable (tri-state).

Have looked at the 74x590 (more than 10,= euro a pop @ Farnell) 74x593 not available.
At Ali and ebay this kind of chip is either not available or pricey as well.

Context:
I am trying to make an address counter for a TTL VGA project and need to be able to access the video memory this counter addresses from the CPU bus. I figured if I could get a counter that I could tri-state I would not need any muxes (or addition dirvers with OE) and would save a couple of precious nanoseconds. I run the pixel clock at 25.175 Mhz and have little less than 40ns to get a pixel out, 15ns are taken by the SRAM I plan to use.

I have the usual counters in stock (74x161 and 74x393) as well as a butt load of 245's...

One option I have considered is using two GAL 22V10's to build the counter. That would actually be a pretty nice fit (also have them in stock - and already use them in the design).

rstofer:
You seem to have some expensive solutions to a $5 problem.  5 chips, $1/each => $5 if I read it right.  Maybe a little more for qty 1 but you gave the qty 10 price.

Here's my approach:  My time is billed at $100/hour, even for hobby work.  A proper $5/solution can be substituted with any other as long as the parts cost + the labor cost is cheaper.  So, even if I had alternate, but useless, chips on hand and they were totally free, I couldn't spend more than 3 minutes thinking about it before I just ordered the proper parts.

Using this labor rate is why I don't do plumbing under my sink while standing on my head.  I hire somebody for that kind of thing.  Same with the landscape service.  And other stuff...  Unless I can do it cheaper at my internal billing rate, I won't do it at all.  Sometimes I don't even care what the outside cost is, I don't climb ladders at my age.

Time has a price, pick a number.

oPossum:

--- Quote from: obiwanjacobi on February 12, 2020, 06:25:41 pm ---Have looked at the 74x590 (more than 10,= euro a pop @ Farnell) 74x593 not available.

--- End quote ---

74HC590 is ~$0.50. Making it work with TTL/NMOS is usually just a matter of some pull-up resistors.

obiwanjacobi:
Did not think to search for HC parts...  :-//

Thanks @oPossum. The 590 has registers to store the count bits - which complicates things a bit.
Can I tie the register- and counter-clocks together to sort of bypass the registers...? Or will the (considerable) delay times make that unstable?

Or does anybody know any other counters without registers but with tri-state outputs?

{I've always wondered how other people do their parts selection in the design stage...? And how do you know where to choose from?}


--- Quote from: rstofer on February 12, 2020, 10:10:38 pm ---You seem to have some expensive solutions to a $5 problem.  5 chips, $1/each => $5 if I read it right.  Maybe a little more for qty 1 but you gave the qty 10 price.

--- End quote ---

I can give you a screen shot of the Farnell page but you probably bill me for looking at it   ;D

cj:

The 74F579 is synchronous 8bit up/down counter with bi-directional 8bit interface so it has tri-state output.
I haven't seen these in other technologies though so it might be hard to find.

CJ

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