4046 clock sync issues

[herbstones]

Hi folks, first post!
First of all: i found out about this forum some times ago, just want to say how thankful i am for all the interesting stuff i learned from here!
I'm working on a cmos based sequencer and i'm experimenting a bit with various ideas about external clock synchronization: i want my clock to be obviously adjustable via a dedicated knob and i would like it to synchronize to an external pulsewave only if it is present (via an "ext clock in" jack).
I spent some times looking through the CD4046 PLL chip but only managed to achieve a sort of "hard sync" between the two clocks...(the internal clock resets everytime the external one goes high). Any idea if it's possible to obtain this with a PLL? Am i looking in the wrong direction? Thanks in advance

[synaesthesia]

Hello herbstones and to this forum,

if I got this right, you want to have your own clock generator as well as the ability to use an external clock. Wouldn't the easiest way to implement this be to use a switch to select between internal and external clock?
The switch could be manual or even automatic, using an analog switch (e.g. 4053 triple SPDT) controlled by a cap that integrates the external pulse wave.
Or is there another requirement that you didn't mention yet?

[herbstones]

Yeah i forgot to mention that i would like it to be automatic! No manual switches (that is because i won't like the possibility of the sequencer stopping if the switch is set to "ext" but there's no external clock pulse).
I'm sorry but i can't figure how i could use a mux/demux like the 4053 to obtain this.
Anyway, i'll think about it, just want it to be as simple as possible.
Maybe to make things more clear an example would be Korg's Volca series sync in/out: it'll automatically sync to a pulse if put into the sync in jack, then back to "normal" speed if the jack gets pulled out.

[PHOBoS]

I'd use some kind of switching chip too, could be a mux or something like the CD4066, but since you are working with digital
signals you could do it with some logic as well. I posted an example using NAND gates here. An integrator is indeed a nice
way to control it, probably wouldn't have thought of that myself.

Unless you actually want the internal clk to take over at te same tempo as the external one in which case you'd probably have
to resort to using a microprocessor. (not that it couldn't be done otherwise but that would probably be the easiest way)


btw this is only needed if you want it to switch automatically with the signal always connected. Otherwise you could use a socket
with a switch/normalize connection.

[herbstones]

Thank you for your replies!
Yesterday night i ended up with this circuit:
It's the simplest way that i could come up with to get a steady high voltage to control the 4053 "A" input if there's a clock pulse present at the external clock input. No clock input, "A" goes back to 0V.
Hope the schematic attached helps to explain a bit better how this works. I'd like to know if anyone has other ideas to slim down the thing even further.
I actually kept on thinking about this for so long that i ended dreaming about it... shit

(btw i named the topic after the 4046 because i thought it might be the most easy way to achieve what i wanted, guess i was wrong!)

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[Grumble]

Maybe you should add a pull-down resistor to the ext-clock input, especially if you use a connector here.

[synaesthesia]

Yes, that should work. Schmitt-Trigger inverters are better, but normal inverters should work as well. Definitely add a pull-down at the input.
Mind that in your circuit a permanent HIGH from the external clock will leave the switch selected for the external clock, although there are no pulses coming from it.
You can add a differentiator to avoid that. Like this (here a HIGH to the 4053 would select the internal clock):

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[herbstones]

As soon as i have some free time i'll definitely try this circuit, thanks synaesthesia!
And yes, a pulldown resistor at the input is absolutely necessary.