[sdiy] Adding reset clock to CD4520

S Ridley spridley1 at gmail.com
Sat Jun 22 16:05:25 CEST 2024


In the stock TKB the 4520B is supposed to be reset by Q3 going high, which
is why Q3 is tied to reset pin.
If you want to add an external reset, you need to OR Q3 and your new reset
pulse - either with an OR gate or maybe a couple of diodes.
Having the inverter out and Q3 out tied directly together is never a good
idea.

Steve

On Sat, 22 Jun 2024 at 14:14, Roman Sowa via Synth-diy <
synth-diy at synth-diy.org> wrote:

> First of all, tying 13 with 15 is pointless. It is there only to reset
> the counter after 4 clocks, making it modulo 4, but since you want only
> 2 LSBs (pins 11-12) there's no difference if it counts to 4, 8 or 16.
> Might have more sense if it was 4518 and not 4520. So cut this
> connection. Like it is now, the inverter's reset pulse forces counter
> output (13) to high level, so if the inverter isn't strong enough,
> external reset will not work. Likeweise if the inverter is stronger than
> the counter the 13-15 link will not work anyway. Long story short - if
> it works, depends on actual chips used, year, maker. Short story shorter
> - bad design.
>
> When you say something works once and then needs power cycle, all I'm
> thinking of in latch-up condition. Could happen if you force output or
> input above supply level. Is that inverter supplied from 12V too? Unless
> the cap in the input is like 10uF or so. I'd put 100n max there.
> Maybe try to replace the inverter with a transistor and leave the poor
> pin 13 alone. You could even make whole reset conditioning circuitry
> with one transistor.
>
> Roman
>
> W dniu 2024-06-22 o 00:01, denshi via Synth-diy pisze:
> > im trying to add a reset to a CD4520 but running into an issue:
> >
> > in the TKB serge uses a CD4520 to handle the vertical clock (cycling
> > between four vertical rows of pots)
> > here is a relevant snippet of the schematic (as found on hyperreal)
> > i copy and pasted the clock input to use as the “vertical reset” and
> > am sending that to an inverter to the bottom half reset - it works,
> > but only once, and then not again until i power cycle. am i missing
> > something simple here?
> >
> > Inline image
> >
> >
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