4051 mux problem?

[Harry Bissell]

You can bias the 4051 one diode drop below ground and it will support input
signals down to ground. The advantage of this is that the drive signals can
still be 0-5V logic (or whatever) without losing anything except a tiny amount
of noise immunity in the logic. Prophet V did this...  Harry Bissell

Stewart Pye wrote:

> At 11:31 AM 23/07/99 -0500, Grant Richter wrote:
> >LM324s are not the best choice for this application.
> >Use FET input op-amps like TL084 or LF444.
> >That should reduce your bias current by a factor
> >of 1000.
> >
> >Sample and holds will be adequate for the job.
> >
> >On most micros, a timer can be used to generate
> >an interrupt. This is the interrupt service routine I spoke
> >of. The MIDI input would generate an interrupt of higher
> >priority, so the refresh will hold off until the MIDI
> >routine clears.
>
> That would be nice if the PIC16F84 had interrupt priority. I thaught the
> next best thing was to poll the timer overflow flag. There is only going to
> be a small variation in timer overflow detection time.  <100us for sure.
>
> I was going to use FET input OpAmps but I was trying to keep it single
> supply. I might try -5v on the mux 4051 and see if it helps then I could
> use it for some TL084 as well.
>
> Regards,
> Stewart.
>
>
> >----------
> >> From: Stewart Pye <stew at uq.net.au>
> >> To: Grant Richter <grichter at execpc.com>; synth-diy at mailhost.bpa.nl
> >> Subject: Re: 4051 mux problem?
> >> Date: Friday, July 23, 1999 2:33 AM
> >>
> >> At 12:13 AM 23/07/99 -0500, Grant Richter wrote:
> >> >That would be the current out of the op-amp input.
> >> >Remember, even if they are FET inputs, they still require
> >> >some finite amount of current flow to stay active.
> >> >
> >> >In this case they source current, so the voltage drifts up.
> >> >If they sank current the voltage would drift down to -V.
> >> >
> >> >Typically in this arrangement you would write an interrupt service
> >routine
> >> >that scans a certain number of memory locations into the
> >> >sample and holds. That way, the main code routine only has to write
> >> >values to the RAM buffers and the interrupt routine takes
> >> >care for scanning them out. For MIDI, you would like the service routine
> >> >to refresh all channels within one millisecond.
> >> >
> >> >One basic assumption is that time differences less than a millisecond
> >> >are not perceptible. Possibly wrong, but that is the reason MIDI has
> >> >such a low baud rate. If your triggers are within a millisecond of
> >> >each other, it should have minimum "flam" effect.
> >>
> >>
> >> The MIDI input is the only interupt used. (It's a software serial pot
> >using
> >> the
> >> EXT INT pin).
> >>
> >> -The timer is polled to check if it's ready for an output update,
> >including
> >> bringing a tigger low again.
> >> -Each trigger output is triple buffered using 3*8 bytes of 'TrigOn'
> >flags
> >> (8 triggers) registers.
> >> -The actual output values are stored in 8 'VelOut'     registers.
> >> -When a relevant noteon velocity byte byte is e
> >>
> >> This ensures that almost as soon as the MIDI velocity byte is recieved it
> >> is output to te DAC and MUX. Certainly before another MIDI message will
> >be
> >> recieved. (I have to buy new batteries for my calculator tomorrow!). It
> >> also ensures the pulse width doesn't vary too much, while enabling
> >> reasonably long pulses.
> >>
> >> Thanks Grant,
> >> Stewart
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> >> From: Stewart Pye <stew at uq.net.au>
> >> >> To: synth-diy at mailhost.bpa.nl
> >> >> Subject: 4051 mux problem?
> >> >> Date: Thursday, July 22, 1999 6:58 PM
> >> >>
> >> >> Hi...
> >> >>
> >> >> I've built a midi to drum trigger unit. The output of the micro goes
> >to a
> >> >> DAC and then to a 4051 mux. The mux then goes to an opamp buffer with
> >> >> parrallel cap.(S&H similar to paia midiCV) The problem is:
> >> >>
> >> >> If I send 00h to the dac then switch through each channel of the DAC
> >to
> >> >> clear the triggers so no outputs trigger, they stay low for a while
> >and
> >> >> then start rising which causes oscillation of the drum oscillators. I
> >> >would
> >> >> have thaught that they would stay low.
> >> >>
> >> >> The 4051 supply is +12V and gnd. Could this be the problem? Maybe I
> >> >should
> >> >> use -12V instead of ground. It's not really a problem because I just
> >keep
> >> >> refreshing the mux. I'd really like to know why this is happening
> >though.
> >> >> Any ideas??
> >> >>
> >> >> Regards
> >> >> Stewart.
> >> >
> >> >
> >
> >