[simpits-tech] MCC838 - Obscure decade counter chip

[alex]

Alex
http://www.flyinglow.ca

(Missed the group on my reply - not sure if this might benefit anyone else)

>  -------Original Message-------
>  From: Alan Murray <realclassclown at yahoo.com>

>  Try putting a count on the chip (say 101), then remove
>  the clock leaving the count static..next apply a LOW
>  to the reset line without going through the NAND (take
>  measure of output) then send the reset high again
>  (take measure again)...do you end up with 101? I'm
>  wondering if CD turns out to be an inhibit line of
>  some kind. Count inhibit (maybe), output inhibit (no
>  mention of tri-state)...

Ok. Will try that this weekend (schedule it too full to look at it tonight).

>  Something about it doesn't make sense..how do you have
>  the presets (SD1 etc.) configured?

I will have to double check, but I believe I have the presets set High (or they may be open - I could be wrong, but my gut feeling is that it didn't seem to matter).

>  Motorola scrapped these things for a reason...;)
>  How many do you have?

4 or 5. 2 are wired in already as they work for counting to 10 (0-9). The other 2 will only be used if I can figure out how to change the count. I actually have a lead on a bunch of free 7490's, so this may turn out to be an academic excercise.

>  I wonder if there is a timing issue, something with
>  propagation delay through the NAND while the clock
>  triggers...you might try gating the outputs through
>  flip-flops to align the clock edge with the input
>  edge..your count may go off by one cycle or may synch
>  but you would learn something either way.

Thanks for the tips.

>  Good luck,
>  Alan
>  
>  
>  --- alex <alex at flyinglow.ca> wrote:
>  
>  > >  -------Original Message-------
>  > >  From: Alan Murray <realclassclown at yahoo.com>
>  >
>  > >  A proper truth table in the Motorola
>  > documentation
>  > >  would really help.
>  >
>  > You're not kidding.
>  >
>  > >  Specifically the Q4(not) ties back
>  > >  to the second gate input. When your reset drives
>  > that
>  > >  high, the 2nd gate (essentially a NOR) correctly
>  > >  drives Q2 and subsequently Q3 low which gives you
>  > the
>  > >  output you are getting...
>  >
>  > >  What does the counter do without the NAND gate in
>  > the
>  > >  circuit? Same thing right?
>  >
>  > Actually, no. If the Cd line is left High (No NAND
>  > gate), then Q4-Q1 counts 0000, 0001, 0010... 1000,
>  > 1001 (0-9). And in 2 places where I need the full
>  > decade counter this configuration is currently
>  > working as I need.
>  >
>  > Somewhere I found a snipit of text (buried in a
>  > patent application I believe), that claims that if
>  > SD4-SD1 are high, then driving CD High to Low should
>  > reset the counter. If CD is Low, then driving any of
>  > SD4-SD1 High to Low will cause the corresponding
>  > Q4-Q1 to go High. But without the proper truth table
>  > even better verbage from Motorola, this little
>  > critter is turning out to be quite the little black
>  > box.
>  >
>  > Alex
>  >
>  
>  
>  
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