4051 mux problem?

[Stewart Pye]

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.
>
>