4051 mux problem?

[Grant Richter]

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.

----------
> 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.
> >
> >